MANY TRIASSIC REPTILES HAVE A CHARMING CRUDENESS ABOUT their appearance, where fairly conventional reptilian bodies and limbs are married to a distinctive, often bizarre head and neck anatomy. This can appear as if evolution was somehow rushed, failing to adapt the entirety of these animals to specific lifestyles and instead just changing a few bare essentials. There may be a nugget of truth to this. Many Triassic animals were rapidly filling ecosystems emptied by the Permian mass extinction, and without significant competition from established species, they may not have needed to optimize their anatomy to a given lifestyles as far as later occupants of the same niches did. But appearances can be deceiving, and we should not assume that these animals were somehow “inferior” to later forms. Many of these Triassic reptiles, despite their seemingly “crude” appearances, belonged to long-lived and widespread lineages and held their own against more sophisticated-looking reptiles of the later Triassic.
Among these unusually proportioned species were the erythrosuchids. These giant-headed reptiles enjoyed twelve million years of evolutionary history through the Early and Middle Triassic world, their fossils remaining undiscovered only in North America and Antarctica. They are most closely related to the archosaurs, though they lack close affinity with any living reptile lineage. Many species are known from relatively complete skeletons, and we have a good idea of their overall proportions and skeletal anatomy. Some species were large, reaching nearly 5 m in length, though others—like Garjainia madiba, shown here—were just over 2 m long. Their most characteristic feature is their enormous heads and jaws, the function of which is betrayed by their sharp, recurved and predatory teeth. Erythrosuchid heads were probably not as encumbering to their owners as they look. Erythrosuchid snouts were air-filled and narrow, so only the back of the head—which housed massive muscles to power their mighty jaws—was especially broad and heavy. Thus, their heads were probably lighter than they appear, and we can read their anatomical indications of expanded neck muscles and powerfully built forelimbs as mechanisms to wield their oversize jaws with fine control. What we know of their limbs suggests that these were long and robust, and erythrosuchids may have been capable of semierect postures instead of a lizard-like sprawl.
Exactly how these remarkable animals made a living remains an area of investigation. Historically, the large heads of erythrosuchids were thought to dictate aquatic habits, it being reasoned that water was essential to supporting their front-heavy anatomy. In this idea, their heavy, thick-walled limb bones would help them sink and submerge in swamps and rivers. But more recent research is favoring terrestrial habits, noting features of weight reduction in their skulls, pointing out their lack of swimming or wading adaptations, and explaining their heavy limb bones as being reinforced for carrying large bodies over land. Curiously, erythrosuchids are often the largest animals of their respective environments, an unusual occurrence among purely terrestrial predators. Might this indicate that they hunted aquatic prey, at least on occasion? More research is needed to clarify the roles of these animals in Triassic ecosystems.
One of the most interesting attributes of erythrosuchids is their growth rates. They grew much faster than living lizards or crocodylians, and at a speed more comparable to many dinosaurs, early birds, and the flying pterosaurs. These elevated growth rates are thought to reflect a fast, “warm-blooded” metabolism and are associated with a number of other anatomies: fully upright stances, insulating fur or feathers, and large brains. But the rapidity of growth in erythrosuchids indicates that fast growth appeared much earlier in reptile evolution, and before the development of those associated features.
Atopodentatus, an Early Underwater Herbivore (Triassic)
THE TRIASSIC IS WELL KNOWN FOR ITS PECULIAR MARINE REPtiles, but few rival the strangeness of the recently discovered Atopodentatus unicus. This species is represented by skeletons from Middle Triassic rocks of China that represent almost all of its bony anatomy. Despite this relative wealth of data, the relationships of Atopodentatus to other marine reptiles are not well understood. Provisional analyses suggest affinities with the sauropterygians, a major clade of marine reptiles that includes the nothosaurs, placodonts, and plesiosaurs. However, general uncertainty about the relationships of marine reptiles to one another, and the relatively recent nature of the discovery of Atopodentatus, mean further work is needed to confirm this hypothesis.
Atopodentatus is a genuinely bizarre animal, an evolutionary collision of seal, crocodile, and vacuum cleaner accessory. At 2.75 m long, it was a moderately sized marine reptile, but its skull was just 12 cm long—less than 5 percent of its body length. This was mounted atop a relatively long neck; a long, deep body; and a powerfully muscled tail. Unlike some marine reptiles, its tail skeleton lacks obvious hallmarks of a fin or a paddle, and Atopodentatus may have relied on its limbs, as well as its tail, to propel itself through water. Both sets of Atopodentatus limbs were stout and paddlelike, with broad, long fingers and toes. However, its limbs were also capable of flexing at their knees and elbows, and the pelvis was strongly attached to the spine. These features indicate Atopodentatus was probably capable of walking on land, though its weak wrists and ankles indicate limited capacity to support its weight out of water. Perhaps it spent much of its time swimming, and left water only to rest, to escape danger, or—if it was incapable of giving birth at sea—to lay eggs.
The most unusual part of Atopodentatus is its tiny skull. Its jaws were lined with hundreds of tiny, peg-like teeth that were, when Atopodentatus was first discovered, thought to line the front of a downturned face in two vertical rows: imagine an unhappy reptile with a zipper on its face and you’re not far off initial interpretations of this species. Subsequent discoveries showed that this bizarre interpretation was based on a broken skull, and undamaged fossils revealed that the jaws actually formed a T-shaped muzzle with a wide set of dental “combs” along the front jaw margins. Further teeth lined the cheek region, and the upper internal surface of the mouth was covered with minute denticles. Its bite—judging by the available space for jaw musculature—was relatively weak, although the musculature associated with opening the jaws was expanded, and the lower jaw was robust. This peculiar arrangement of features is thought to represent a sophisticated mechanism for aquatic herbivory, where the wide dental “combs” scraped or sheared algae from underwater surfaces, and powerful opening motions of the jaws sucked the algae into the mouth. The plant matter was then separated from the water by the internal denticles and cheek teeth, before the sieved water was expelled from the side of the mouth.
Herbivory is relatively rare among marine tetrapods. Perhaps the only other known Mesozoic marine reptile to live exclusively off plant matter is the placodont Henodus, a turtle-like creature that cropped and filtered algae using unusual dentition and baleen-like structures along the side of its jaws. Today, marine iguanas (Amblyrhynchus cristatus) and certain sea turtles graze on algae during underwater dives, while certain mammals, such as sirenians (manatees and dugongs), live off algae and marine grasses. Fish have had far greater success as aquatic herbivores, with Cenozoic groups like parrotfishes and surgeonfish developing means not only to remove algae from underwater surfaces but also to extract microscopic plants living within rocky sediments. This has shaped marine ecosystems in limiting algal growth and facilitating development of grazing-resistant communities, including modern-grade coral reefs. Evidence of these habitats first appears in Miocene sediments, alongside fossils of these remarkable fishes.
Morganucodon and the Dawn of Mammals (Triassic)
OF THE SYNAPSIDS THAT SURVIVED THE PERMIAN EXTINCTION event, only the dicynodonts and a group yielding our own ancestors, the cynodonts, conducted substantial evolution in the Triassic and beyond. The cynodonts, a group originating in the Permian, possessed many features we associate with true mammals: a heightened metabolism, the start of the transformation of the posterior lower jawbones into the inner skeleton of the mammalian ear, and—in some later groups—the probable development of fur and whiskers. Direct evidence for the earliest appearance of fur remains elusive, but sensory pathways in some cynodont skulls match those of whiskered mammals more than those of hairless, nonmammalian species. If correctly interpreted, this implies the development of sensitive facial tissues and whiskers in some particularly mammal-like cynodonts. Our understanding of cynodont evolution, and the discovery of hairlike structures in a coprolite (fossil excreta) indicate that this development took place in the late Permian.
Morganucodon watsoni represents a stepping stone between cynodonts and the true mammals. Technically speaking, Morganucodon is a mammaliaform rather than a true mammal: a very mammal-like creature but not part of, or closely related to, any living mammal group. Numerous Morganucodon bones have been found in ancient fissure fill deposits in Triassic/Jurassic rocks of today’s United Kingdom, allowing experts to reconstruct aspects of its skeleton from numerous individuals. Morganucodon was the first mammaliaform known from such extensive fossil material and was a very welcome discovery given that, until it was found in the mid-twentieth century, paleontologists knew Mesozoic Mammaliaformes largely from isolated bones and teeth.
With a body length of 10–13 cm (excluding the tail), Morganucodon would have resembled a small rodent in life, though its bowed limbs would contrast with the rats and shrews it is often compared to. Its skull was stoutly built, with expanded cavities to anchor jaw muscles. These, and its sharp, piercing teeth, imply a diet of hard-shelled invertebrates and other small animals. Its ear had yet to make the full migration from the lower jaw to the skull, so its ears would have been set lower on the head than we’re accustomed to in living mammals. It is not yet clear whether Morganucodon had ear pinnae—those conspicuous skin and cartilage structures that characterize our mammalian ears. Monotremes (the echidna and platypus, egg-laying mammals which represent the most ancient grade of mammal evolution to survive to modern times) have very small pinnae or none at all, and exceptionally preserved fossils of the aquatic Cretaceous mammal Castorocauda lutrasimilis also lack conspicuous ear tissues. Might these examples suggest that, as shown in this illustration, Morganucodon lacked ear pinnae? Possibly, but we need more than three lines of anatomical evidence to know for certain.
Scurrying away with a kidnapped Morganucodon puggle is a small lizard-like species known as Clevosaurus bairdi. This is not a lizard, however, but a sphenodontian: a relative of the modern tuatara. Sphenodontians and lizards share ancestry on the same branch of the reptile tree, and both arose in the Triassic. Sphenodontians were the more successful group to begin with, however, spreading all over the world and diversifying into a number of lifestyles. It was only in the Jurassic that lizards started to attain their modern level of distribution and diversity. The once-mighty Sphenodontia survives today only as the tuatara, all species of which are found in New Zealand, while lizards occur on all continents except Antarctica.
The Great Crinoid Barges (Jurassic)
HUGE MONSTERS OF WRITHING, FLOWER-HEADED TENTACLES once roamed Jurassic seas. Cruising the waves like Lovecraftian barges, they hung ropelike structures—some over 20 m long—with 80-cm-wide dragnets sweeping the sea clean of organic matter. After years of drifting, the ever-increasing mass of these ropelike creatures surpassed the floatation capability of their vessel and they sank to the seafloor, starving to death in the still, deep sea, or else suffocating in oxygen-free bottom waters. On occasion, these behemoths would be preserved as fossils, the tendrils of some Early Jurassic German examples extending over an area of five hundred square meters. Their remains are, without doubt, some of the most spectacular fossils in the world.
Closer inspection of these fossils shows that they were not single, multitentacled organisms but collectives of several species. At their core were large pieces of driftwood, mostly pieces of tree trunks that are, in the biggest specimens, many tens of meters in length. Driftwood in the modern day can float for several years, but it is often sunk through the actions of burrowing mollusks. Such organisms had yet to evolve in the Jurassic, however, so driftwood may have remained buoyant for far longer. As the logs floated, they accumulated bivalves (clams) across much of their surface, between which anchored gigantic crinoids (sea lilies). Crinoids are a type of echinoderm, the same group that includes starfishes and sea urchins. As adults, echinoderms are characterized by fivefold body symmetry; skeletons made out of numerous calcareous plates, spines, and segments; and the presence of tiny “tube feet”—minute structures, projecting from skeletal pores, that are used in walking and feeding.
Crinoids are among the most ancient of the echinoderm groups. Their fossil record dates back to at least the Ordovician Period, and a Cambrian origin is predicted by some scholars. They can be grouped into two major morphs: stalked crinoids, which position their flowerlike feeding cones in the water column by standing on the seafloor or anchoring themselves to submerged objects; and free-swimming species that lack stalks and move through water by beating their arms. Today the free-swimming species dominate crinoid diversity, but the stalked variants were much more common in the Paleozoic and Mesozoic Eras. Indeed, fossils of stalked crinoids are so abundant in Carboniferous marine sediments that their skeletons form thick limestone deposits. Paleozoic crinoids were particularly diverse in both taxonomy and lifestyle, but both attributes were reduced significantly during the Permian mass extinction. Stalked crinoids bounced back from this catastrophe in the Mesozoic and are common fossils from this time, but they never regained a diversity akin to that found in their Paleozoic heyday.
The giant crinoids that anchored themselves to Jurassic driftwood, known as Seirocrinus subangularis, anchored themselves as larvae and grew to huge size. How quickly they grew remains unknown, but it is suspected that the biggest stalks took many years to attain their great size. As anchoring space on the driftwood became harder to find, smaller crinoids would simply anchor themselves to their larger brethren—stalks growing on stalks. Crinoids are filter-feeders reliant on water currents to bring food particles into reach of their arms, and some living stalked species walk across the seabed looking for suitable foraging areas, crawling along using short tentacles emerging from their stems. Seafaring species like Seirocrinus could feed continuously, however, the motion of the driftwood trawling their feeding apparatus through the water. With relatively little nutritious soft tissue in their bodies, these giant crinoids might have been relatively ignored by predators, but their lives were not without danger. Some Seirocrinus fossils show that they lost their feeding apparatus entirely, their dead, headless stalks trailing in the sea as the barges sailed on.
OUR PREVIOUS MEETING WITH A MEMBER OF THE ICHTHYOSAUR lineage featured a Triassic species still bearing features of terrestrial animals, and thus someway off deserving the title of a true “fish lizard.” Later ichthyosaurs, like the European Middle Jurassic species Ophthalmosaurus icenicus, had fully earned this name, however. Advanced ichthyosaur features included a long torso with a deep chest, limbs transformed into stiffened flippers (larger at the front, smaller at the back), a large tail fin, and a soft-tissue dorsal fin on their backs. This last feature is demonstrated by well-preserved Jurassic ichthyosaur fossils, which record soft-tissue body outlines as dark stains surrounding the skeleton. The nature and reliability of these stains has been historically controversial: are they the remnants of microbes that once covered the tissues of the decaying animal, or genuine ichthyosaur bodies? The authenticity of the preserved outlines has also been questioned, in that components like dorsal fins have been interpreted as wayward body tissues dislodged during decay. Such concerns were further complicated by the tendency of nineteenth-century fossil preparators to “clean up” specimens, removing untidy edges or even plastering over specimen details to make the fossils more aesthetically pleasing. This practice has been an issue not only for ichthyosaur fossils: many specimens collected in the early days of paleontology were “improved” for aesthetic reasons, often to the later detriment of those trying to interpret a specimen’s real anatomy.
Continued study of ichthyosaur fossils has demonstrated that their dorsal fins and body outlines are genuine records of their tissues, and they provide us with a lot of data about their life appearance and hydrodynamics. Alas, such exceptional remains are only associated with a handful of species, so the nature of fin and body shapes across ichthyosaur evolution remains poorly understood. Given the varied skeletal proportions and body sizes of ichthyosaurs, some variation in form might be expected. Some Triassic ichthyosaurs were relatively eellike, with low tail fins and flexible bodies. Later forms, including Ophthalmosaurus, attained a “thunniform” (tunalike) body plan that permitted particularly efficient swimming. Like modern whales and fast-swimming fish, only the end of the thunniform ichthyosaur tail was flexible, an adaptation to maximize thrust generated by the tail while swimming. The tail fin itself was tall and crescent-shaped, and thus capable of generating considerable forward momentum. The rest of the body was deepened and broadened, achieving a body form optimized for aquatic locomotion.
Collectively, these adaptations would have made ichthyosaurs fast, powerful swimmers. Their teeth and stomach contents suggest a diet of fish, squid, belemnites (relatives of squid and octopuses with substantial internal skeletons), and—in larger species, at least—other marine reptiles. In this respect we can analogize them with living toothed whales, such as dolphins and orcas. Their commitment to marine life was so great that they had no ability to walk on land and they gave birth to live young at sea, an adaptation precluding the need to leave water to lay eggs. The occurrence of up to eleven embryos in one pregnant ichthyosaur fossil suggests they gave birth to many offspring at once. Such strategies are used among living animals with high juvenile mortality rates, implying that survival as a juvenile ichthyosaur may have been fraught. Perhaps baby ichthyosaurs relied more on the odds of their brothers and sisters being eaten than on having nurturing parents to help them survive.
Ophthalmosaurus is characterized by the largest set of eyes of almost any animal. At 23 cm across, its eyeballs were second only to those of the modern giant squid, Architeuthis dux. When looking at Ophthalmosaurus, however, we would have seen only a portion of their eyes, as most of the eyeball was hidden behind bone and skin. But even with only partial exposure, their eyes would have been remarkably sensitive to light, and this likely permitted Ophthalmosaurus to find prey in deep or other low-light conditions where other animals would have been rendered sightless.
Anchiornis: A Dinosaur That Was Almost a Bird (Jurassic)
MOST OF US ARE FAMILIAR WITH THE IDEA THAT THE EVOLUTION of birds had something to do with dinosaurs. Less well known is the exact nature of the relationship between these groups and the extensive fossil record that entirely blurs any hard boundary between them. Thousands of feathered dinosaur fossils—many of them from China—document the evolution of birds from theropod (predatory) dinosaurs, with birdlike features especially obvious in the clade that we call Paraves. Mesozoic paravians were feathered animals that include species like Deinonychus and Velociraptor among their anatomically and ecologically varied forms. For much of the Mesozoic, birds were just one type of two-legged, feathered animal among many, and it’s likely that time travelers to Mesozoic settings would struggle to distinguish “true” birds from several types of nonavian dinosaur. This brings home the real fact of bird origins: birds are not related to dinosaurs; birds are not something to do with dinosaurs; birds are dinosaurs. It’s sometimes queried whether birds “stopped” being dinosaurs once they began evolving independently from other theropods, but this cannot be so. Birds can no more stop being dinosaurs than we can stop being mammals. This means that every bird we see today—even a familiar or comical bird like a pigeon, a chicken, or a parrot—is a living dinosaur, and we can no longer consider dinosaurs to be extinct. To the contrary, nearly ten thousand dinosaur species live on Earth today—far from being extinct, dinosaurs are one of the most diverse vertebrate groups of modern times!
Many features we associate with modern birds are actually rooted in their dinosaur ancestry, and in some cases rooted even more deeply within their archosaurian origins. These include their hollow bones, their large brains, a body covered with feathers (or early versions thereof), and even anatomical minutiae like wishbones. The acquisition of avian characteristics was not a sudden burst of evolution specific to one type of dinosaur, but a gradual accumulation of features that produced many birdlike creatures. Animals that might qualify as “the first birds” appear about 165 million years ago in the Middle/Late Jurassic, though distinguishing fossils of the first “true” birds from “very birdlike dinosaurs” is increasingly difficult. Evolution is a continuum rather than a neat series of nested categories, and when our fossil record is relatively complete we can struggle to find obvious boundaries between our taxonomic groups. So blurred is the evolution of birds from nonbird dinosaurs that any distinction between them is now almost arbitrary.
What sort of dinosaurs did birds evolve from? Anchiornis huxleyi is a small paravian from Jurassic deposits of China and is a fairly typical “dino-bird.” It is known from dozens of exceptionally preserved fossils that preserve soft tissues along with entire skeletons, some of which even preserve details of feather coloration (reflected in the illustration opposite). Anchiornis was a long-limbed bipedal animal with a delicate, short-snouted skull; a relatively large brain; and small, sharp teeth. It was covered—literally head to toe—in feathers of various kinds, with simple downy feathers across much of the body and vaned feathers on its arms, legs, and tail. The feathers on the limbs were especially long and give the impression of Anchiornis having four wings, a condition mirrored in several other paravian species. It retained a three-fingered hand, but the second and third digits were married together by soft tissue, a precursor to the bony fusions between the second and third fingers of later birds. Anchiornis had abandoned the heavy, muscular tail typical of reptiles for a lightweight structure composed of slender vertebrae. As a small, lightweight animal, it was well suited for rapid movement through the forests in which it lived, most likely in pursuit of a diverse, omnivorous diet. Anchiornis was probably incapable of flying, however, its wings being too small to sustain flight and its chest skeleton lacking adequate space for flapping muscles. Not all paravians were flightless, though: multiple lineages experimented with different wing apparatuses and flight mechanics, of which the modern avian approach (two feathered wings and flapping flight) is just one configuration.
BRONTOSAURUS IS A NAME THAT MANY OF US ASSOCIATE WITH historic aspects of paleontology: the Golden Age of nineteenth-century dinosaur collecting, twentieth-century imagery of swampbound reptiles, and the 1903 suggestion that this most famous dinosaur was likely just a type of Apatosaurus. This idea saw the term “Brontosaurus” fall out of scientific use for over a century, although nostalgia for the name meant it was never entirely abandoned. Even Knight, in this book’s 1946 predecessor, featured Brontosaurus forty-three years after scientists “officially” abandoned the name. Happily for those with a penchant for vintage dinosaurs, a careful analysis of Brontosaurus and Apatosaurus fossils published in 2015 showed that they were not quite as similar to each other as once proposed, and we can again distinguish Brontosaurus as a valid type of dinosaur.
Brontosaurus is a sauropod, the group of long-necked dinosaurs that includes the largest land animals of all time. The exact length and magnitude that these animals could obtain is a point of contention, as the fossils of the largest individuals are invariably incomplete, and estimating their weight is fraught with challenges. However, lengths of 30 m or more, and body masses exceeding 50 tonnes, are conservative assessments of their maximum size. Only the largest whales can best sauropods in body size, and they have the benefit of water to support their mass. As land animals, sauropods had to rely on spectacular weight-saving anatomies to grow to such sizes. Chief among these were extensive air sacs in their bodies and skeletons, features that helped their tissues grow to huge sizes without adding much additional mass. Most dinosaurs bore air sacs in their bodies, but no others capitalized as much as sauropods on their utility to permit large size.
Today, we know that Brontosaurus is not the round-faced, swamp-dwelling animal many of us remember from our childhoods. Rather, Brontosaurus belongs to a generally gracile group of sauropods known as diplodocids: animals characterized by their proportionally long necks; exceptionally long tails with whiplike ends; relatively narrow bodies; and slender, horselike skulls. It would have consumed huge quantities of plant matter, using jaws lined by relatively simple, peg-like teeth. The guts of sauropods were so long that plant tissues could be broken down effectively within the stomach and intestines, thus negating any need for sauropods to chew their food. This meant that sauropod skulls didn’t need robust teeth or jaws and were light enough to be perched atop long necks, giving them tremendous reach to harvest food. But despite appearances, sauropod heads are not atypically small. Investigations into animal scaling show that the skulls of land-based herbivores generally become proportionally smaller as their bodies get larger, and we are simply seeing this relationship expressed to an extreme degree in these giant reptiles. As with all diplodocids, Brontosaurus seems anatomically suited to rearing up on its hind limbs, perhaps to reach higher foliage or intimidate other animals.
Brontosaurus and its close relative, Apatosaurus, are known for their particularly massive neck vertebrae, the significance of which has long gone without explanation. Recent work has found that much of their unusual vertebral anatomy—including truly massive, looping neck ribs and the presence of prominent knobs along their underside—could be consistent with a role in combat. Might Brontosaurus and kin have used their necks as a thunderous bludgeoning and wrestling apparatus? Neck-based combat might seem strange because our own necks are short and vulnerable structures, but modern giraffes and seals use their necks in various types of aggressive acts, both as clubs and as wrestling aids. A roughly analogous behavior may have been occurring, scaled up many times over, in the Jurassic with Brontosaurus. It’s hard to think of a take on this famous dinosaur that is more divorced from its classic, swampbound visage than this.
OUR MESOZOIC ANCESTORS, SURROUNDED AS THEY ARE BY SPECtacular fossil reptiles, often register as little more than a footnote on the radar of popular science. It seems the charisma of dinosaurs, marine reptiles, and pterosaurs is enough to suppress our typical instinctive interest in our own evolutionary history, of which the Mesozoic years were a major and defining part. Since the earliest days of paleontology, Mesozoic mammals have been relatively poorly known, being represented mostly by individual teeth and bones and only rarely by more complete remains. But our understanding of their diversity and ecological breadth has improved tremendously in recent decades, thanks to new fossils comprising not only complete skeletons but also soft-tissue features such as hair and ear cartilage, and even dietary remains. These fossils confirm long-held views that Mesozoic mammals were primarily small-bodied (the largest known Mesozoic species is about the size of a European badger), but they also show a far more ecologically diverse group than previously realized.
Undisputed members of Mammalia—as opposed to the Morganucodon-like animals we met previously, which are not universally accepted as “true” mammals—appeared in the Jurassic Period and quickly diverged into the major divisions of modern mammals. The egg-laying monotremes, the pouched marsupials, and the placentals (our own group, which gives birth to relatively developed live young) were thus all in existence by the Jurassic–Cretaceous boundary. By the end of the Cretaceous, these lineages had diversified further into the earliest members of the major mammal types we know today, as well as a number of lineages that existed in only the Mesozoic and the early Cenozoic.
The archetypal Mesozoic mammal was something like Durlstodon ensomi (here in left foreground) or Durlstotherium newmani (right and center foreground) in appearance: adaptable, small-bodied creatures suited to life among low vegetation and leaf litter, primarily pursuing insects and nutritious plant matter to fuel their furry, high-metabolism bodies. It’s a mistake, however, to believe the common suggestion that dinosaurs and other predatory reptiles would have been too large to concern themselves with our ancestors: many smaller dinosaurs, including many paravians, were fox-like predators that likely pursued small mammal prey. Predation pressure, and perhaps some other factors, seems to have enforced a protracted period of nocturnality on Mesozoic mammal evolution known as the “nocturnal bottleneck”, and we still see evidence of this adaptive shift in mammal anatomy today. Mammals, including humans, have generally poor color eyesight and are relatively susceptible to eye damage from ultraviolet light, but have contrastingly excellent senses of hearing and smell, as well as supreme tactile abilities on account of hair and whiskers. Broadly, we can view this as lessening our reliance on light and vision in favor of senses that are light-independent. We also have a unique type of fat—brown adipose tissue—that excels at rapidly warming us in cold conditions, as well as the thermal advantages of fur and high-energy metabolisms. Both are well suited to activity during cold nighttimes. Nocturnal habits are common enough in living mammals for us to assume that day-based behavior is an “advanced” trait that developed as we explored the adaptive potential of the mammal-dominated Cenozoic Era.
We should not assume that the diminutive forms and moonlit scurrying of Mesozoic mammals makes them uninteresting creatures. Our occupancy of the night was as much about exploiting nocturnal opportunities as it was about avoiding dinosaur predators. We now know that some Mesozoic mammals were not just ground-based insectivores: they were also gliders, swimmers, burrowers, herbivores, and even dinosaur predators. Mammals did not spend the Mesozoic on the bench, waiting for an opportunity to diversify once dinosaurs disappeared: our earliest evolution was actually complex and inventive, and already hinting at the adaptations and habits that mammals would enhance further when opportunities arose in the Cenozoic.
Yutyrannus, the Feathered Tyrant (Cretaceous)
WHILE NO DOUBT REMAINS THAT MANY SMALL, BIRDLIKE THEROpod dinosaurs were feathered, much remains to be learned about the evolution of feathers among other dinosaurs. Fossils show that many dinosaurs had scaly skin over all or at least most of their bodies, but new discoveries have revealed feather-like structures and fibers on dinosaurs many evolutionary miles from the dinosaur–bird lineage. Presently, the fossil record is sufficient to hint at a complex picture of feather evolution in Dinosauria without being detailed enough to firmly answer when feathers and their anatomical precursors first appeared, how many times they evolved, and how they varied in response to factors like body size, habitat, and climate.
Currently, the largest dinosaur we know of with feather-like structures is the Early Cretaceous Chinese tyrannosauroid Yutyrannus hauli. Yutyrannus was 9 m long and weighed more than 1 tonne. Although far from the largest predatory dinosaur known (the biggest dinosaur carnivores exceeded 14 m in length, with body masses surpassing 6 tonnes), it is much bigger than any other known feathered dinosaur and demonstrates that fibers or feathers could be retained in large dinosaur species. Several Yutyrannus specimens show long, dense filaments across their bodies, including parts of the neck, upper arm, torso, and tail. With such a broad distribution, it’s likely that most of Yutyrannus was covered in some sort of feather-like structure. Yutyrannus is part of the lineage that would eventually give rise to the famous tyrannosaurid dinosaurs, including Tyrannosaurus, Tarbosaurus, and Albertosaurus. Curiously, the little skin data we have from these Late Cretaceous tyrannosaurids suggests a reduction of feather-like features: scales are known to have occurred on their faces, necks, and bellies; over their hips; and on their tails. We cannot be certain that these large, advanced tyrannosaurids were entirely devoid of filaments and fibers, but they do not seem to have been as fluffy as Yutyrannus or their other theropod ancestors. Does this imply that Yutyrannus is close to the body size limit for extensive feather-like coats, perhaps because larger dinosaurs risked overheating under a dense covering of fluffy skin? Or might another factor, such as climate, account for this difference? We are learning that many locations and time periods in the Mesozoic were not subjected to the warm greenhouse climates once thought to be ubiquitous throughout dinosaur evolution, and this further complicates our attempts to understand the adaptive significance of dinosaur feathers. The Lower Cretaceous forests that Yutyrannus called home had an average annual temperature of 10°C, far cooler than the Late Cretaceous floodplains and woodlands occupied by Tyrannosaurus and kin (annual temperature averages of 18°C). Is this enough of a difference to imply that climate, as well as body size, was influencing the evolution of dinosaur skin and feathering? It’s a possibility, but we need more fossils to know for sure.
Early Cretaceous tyrannosauroids such as Yutyrannus were just one type of predatory dinosaur among many, but by the end of the Cretaceous, tyrannosaurs were the dominant large-bodied land carnivores. All tyrannosauroids are characterized by their particularly strong skulls and elevated bite forces, but only the Late Cretaceous species have those famously short arms. Despite their length, tyrannosaurid arms were not weak: they were robustly built and powerfully muscled, and they may have been useful for gripping other animals, such as prey items or mates during copulation. Most tyrannosauroids had long legs, and the longest are found in the tyrannosaurids: despite their size, they were likely relatively swift, agile animals. The tyrannosaurids were bone-crushers, making them the only theropods with jaws and teeth powerful enough to break into large dinosaur skeletons. That they routinely injured each other, as well as their prey, is recorded in bite marks and other facial wounds preserved on their fossil skulls. This behavior is also evidenced in other large theropods, so it may have been a common means for big dinosaur predators to interact socially or to settle disputes.
Flowers and Insect Pollination (Cretaceous)
MESOZOIC FLORAS WERE MOSTLY DOMINATED BY GYMNOSPERMS, the seed-bearing plant group that includes conifers, ferns, and cycads. Vast forests and plains of these plants existed throughout the Mesozoic, supporting the evolution of some of the largest herbivorous animals of all time. To modern eyes, these landscapes would have looked overwhelmingly green, as they lacked the color associated with the flowering plants—the angiosperms—until the Cretaceous Period.
Our cultural association of flowers with romance and delicacy might lead us to imagine their evolution as equally tender and gentle, quietly blossoming color and fragrance into Mesozoic scenery. In fact, angiosperms rose aggressively and rapidly, transitioning from holding minor roles in Early Cretaceous floras to being the dominant plants of terrestrial communities by the end of the Mesozoic. Their sudden abundance had significant effects on Mesozoic biospheres and climates. By virtue of their productivity and an increased capacity to transmit groundwater into the atmosphere, angiosperms enhanced the planet’s capacity for rainfall, which, in turn, lead to greater erosion rates and more nutrients washing into marine ecosystems, heightening marine productivity. Today, something like 350 thousand species of angiosperms exist on Earth. Forget about living in the Age of Mammals: we live in the Age of Flowers.
We have much to learn about the early evolution of angiosperms. When they first appeared is a matter of contention, but they were probably—according to our evolutionary models and some intriguing pollen-like fossils—existent in the Triassic, if not before. Genuine flowers seem to have first appeared in the early Cretaceous, although we might note that angiosperms were not alone in experimenting with flowerlike structures at this time: several non-angiosperm lineages were evolving bright or pungent structures that may have served similar reproductive purposes. Why angiosperms had such sudden success in the Cretaceous has been a long-held mystery, but current research points to increased productivity as their chief advantage. Their enhanced capacity for growth seems to be related to a microscopic, seemingly trivial component of their anatomy: small genome size. Smaller volumes of genetic material equate to smaller cells, and this allows for tighter packing of veins and pores into leaves. This is the plant equivalent of having a supercharged lung and circulatory system, and it may have given angiosperms a physiological advantage over their competitors.
Flowers may also have played a role in angiosperm success. Flowers distribute pollen—the plant equivalent of sperm—on animals that interact with them, and if animals can be coaxed into routinely interacting with the same flower species—via attractive colors, shapes, odors, or foodstuffs—plants can transform their visitors into reliable, efficient agents of fertilization. This is a much more dependable reproductive system than wind carrying pollen through the air, where fertilization depends on fortuitous gusts carrying pollen grains to receptacles on other plants. Insects acted as pollinators for plants well before the rise of angiosperms (exactly when they adopted this role is contested; there are indications that this relationship may have begun in the Paleozoic), but flowering plants ran further with this relationship than any other plant group. Fossils show that early flowers were generalist in structure and open to many types of pollinator, but by the Late Cretaceous many flower species were selective about their insect partners, requiring specialized feeding apparatuses to access their nectar. The close evolutionary kinship and codependence we see between certain plants and insects in modern times is a continuation of a very ancient phenomenon.
By the end of the Cretaceous, insect pollination was the dominant mechanism of plant reproduction, as it remains today. Indeed, for all our technological innovation, we still rely on insects to fertilize our crops. It’s easy to overlook bees, moths, butterflies, and hoverflies busying themselves among our plants, or even to regard them as pests and annoyances, but the pollinating services these tiny creatures provide are absolutely essential to our survival.
Cretoxyrhina and Pteranodon (Cretaceous)
WE TEND TO PORTRAY THE MESOZOIC ERA AS THE TIME OF GREAT sea reptiles—ichthyosaurs, plesiosaurs, mosasaurs, and so on. While there is little doubt that these animals were important to Mesozoic marine ecosystems, we should not overlook the significance of a more familiar, and yet much more ancient, group of predators that flourished alongside them: the sharks.
Sharks are one of the greatest successes of vertebrate evolution. Their fossil teeth are found in abundance all over the world through the last four hundred million years of the geological record. Fossils of their cartilage skeletons are much rarer, however, and only a few sites of exceptional preservation provide more complete insights into ancient shark anatomy. The chalk sediments deposited by the Western Interior Seaway, a shallow marine sea that bisected North America in the latter half of the Cretaceous Period, are one rock type that yields them. The shark fossils here are excellent and abundant, and they leave no doubt that sharks were an important component of an ecosystem also occupied by plesiosaurs, mosasaurs, and predatory bony fish.
Shark fossils of particular interest from the Western Interior Seaway are those evidencing their biting of other animals. Paleontologists go to great length to reconstruct the diets of fossil animals, and this is aided enormously by bite marks or imbedded teeth in the bones of prey species. Western Interior Seaway shark teeth are frequently preserved in close association with the ancient carcasses of virtually all large vertebrates from this environment, and their bite marks, tooth gouges, and embedded teeth are frequently found on fossil bones. It seems that few animals in this inland sea escaped the jaws of sharks, and we know that they even turned cannibalistic on occasion: sharks eating other sharks. The 2–3-m-long shark Squalicorax falcatus has left particularly pervasive evidence of its foraging habits: its teeth and feeding traces are associated with so many carcasses that it was surely something of a scavenger, eating anything it could find regardless of animal type or size. A larger shark, Cretoxyrhina mantelli (shown here), roamed the same waters. At 6–7 m long, it was a top carnivore of the Western Interior Seaway, and fossil evidence directly shows that it dined on even relatively large marine reptiles.
Among the rarest quarry of Cretoxyrhina was Pteranodon, a flying reptile famous for its large, backward-pointing cranial crest; toothless jaws; and wingspan of up to 7 m. In fact, most Pteranodon individuals were much smaller than this, with wingspans of 3–4 m, and they had much smaller cranial crests. It is thought that these smaller morphs were females, and the males were larger and full-crested. The bones of Pteranodon were, like most of its kind, occupied by bony air sacs, so that the bone walls were just a millimeter or so thick. This made for a lightweight and flight-adapted skeleton, but it rendered pterosaur bones highly vulnerable to damage once their owners died. Evidence of carnivorous acts on pterosaur bodies is therefore rare, but we know that pterosaurs were eaten by fish, dinosaurs, ancient relatives of crocodylians, as well as sharks. Both Squalicorax and Cretoxyrhina are known to have eaten Pteranodon, with our limited data set showing Squalicorax dining on pterosaur meat more regularly than Cretoxyrhina. Unfortunately, bone-imbedded teeth and bite marks rarely give clues about whether animals were being preyed upon or scavenged, but both of these shark species would vastly outweigh the largest Pteranodon and probably could have easily overpowered a waterborne individual. We know that Pteranodon ate fish, and they may have routinely entered water to catch them. Though pterosaurs were likely strong swimmers and adept at taking off from water, this behavior would place Pteranodon in the same habitat as these dangerous fish. Unless they were alerted to these predators and quickly became airborne, Pteranodon may have been vulnerable to attacks from swimming carnivores.
Mighty Zuul, Destroyer of Shins (Cretaceous)
AVOIDING THE SHARP END OF A PREDATORY SPECIES IS A PRESsure that most animals face at one time or another. Common adaptive responses include being fast enough to escape pursuit, being strong enough to fight off attackers, or simply being so awkward and difficult to subdue that the predation effort is not worth the reward. Great body size is one mechanism of being awkward to tackle—most predators know their weight class and won’t punch far above it—but deterrents such as armor and spikes achieve the same goal without the pressure of becoming the largest creature in a given habitat. Armored animals have developed numerous times in all vertebrate groups, and they occur in a surprising variety of niches. Though armor is heavy and slows animals down, speed need not be a concern for animals that eat plants, raid insect nests, or restrict their rapid locomotion to short bursts. These animals can take their time with whatever they encounter, safe in the knowledge that their defenses will frustrate or deter hungry predators.
Among the most spectacular armored animals of all time are the ankylosaurian dinosaurs. They are, along with the famous stegosaurs, part of the armored herbivorous dinosaur group known as Thyreophora. Ankylosaurs were common members of dinosaur faunas during the Cretaceous, and they developed amazing defensive structures with spines, knobs, plates, and tail clubs. The entire group is made up of wide-bodied, huge-bellied, and low-slung animals that must have trundled around at relatively low speeds, using their beaked jaws and small, coarsely serrated teeth to graze plant material. We recognize at least two categories of these dinosaurs: ankylosaurids and nodosaurids. Nodosaurids are characterized by their large shoulder spikes and narrow faces, while ankylosaurids have substantial tail clubs, heavily armored faces, and broad muzzles. It is thought that having narrow beaks enabled nodosaurids to feed selectively, while having wide beaks allowed ankylosaurids to grab mouthfuls of whatever vegetation they happened across. Enlarged nasal cavities indicate that ankylosaurids had an enhanced sense of smell, but the complex layout of their nasal apparatus suggests their noses had other functions as well, such as augmenting vocalization or controlling heat and water exchange.
The function of ankylosaurid tail clubs has drawn much attention. Like the rest of their armor, the clubs are made from bones that grow within the skin and were likely covered with thick scales and cornified sheaths in life. Tail clubs often vary in size and shape within ankylosaur species, implying that their main use may have been to settle contests between individuals of the same species instead of deterring attack from carnivores. Perhaps ankylosaurids primarily used their tails to batter each other in contests for mates or resources? Of course, this does not rule out additional roles in predator defense, and studies show that the largest tail clubs may have been capable of smashing large bones when swung at attackers. Indeed, the species name of the large-clubbed ankylosaurid depicted here—Zuul crurivastator—means “destroyer of shins.”
Well-preserved ankylosaurid remains easily rank among the most fantastic of all dinosaur fossils. Their densely armored backs give the impression of seeing skin rather than bones, so especially well-preserved and complete specimens resemble sleeping animals, not long-dead fossils. Excellent preservation of their bodies is facilitated by a quirk of ankylosaurid fossilization, whereby their tough carcasses were capable of remaining intact and drifting far out to sea before sinking and fossilizing. Marine settings are often more conducive to fossilization than terrestrial environments, allowing these amazing animals to be gently buried in fine muds to emerge, as sleepy-looking skeletons, millions of years later.
Giant Sea Lizards and the Mesozoic Marine Revolution (Cretaceous)
AT FIRST GLANCE THE MOSASAURS, A CRETACEOUS GROUP OF aquatic reptiles, seem alien enough for us to assume that their origins lie among some long extinct, exotic lineages. In fact, details of skull anatomy show that these paddle-limbed reptiles were members of the lizard group, and were most likely close relatives of snakes and monitors. Like modern lizards, they had scaly skin, bore forked tongues, and had additional rows of teeth on the upper surface of their mouths. But unlike modern lizards, these reptiles could grow to enormous sizes, some even becoming whale-sized creatures of 15–17 m body lengths. They occupied predatory niches in Late Cretaceous seas at a time when other marine reptiles—such as plesiosaurs and ichthyosaurs—were in decline. Gut content, as well as studies of mosasaur skull mechanics, suggests they were powerful carnivores that essentially ate whatever they wanted. Half-digested bones of birds, sharks, bony fish, and other marine reptiles have been found within their fossilized guts. Ammonites were also common mosasaur prey, with some mosasaur species being specifically adapted for crushing mollusk shells. That ammonites could escape these predators is evidenced by healed tooth marks left in their shells—wounds that have been linked incontrovertibly to mosasaur dentition. Globidens dakotensis, the 6-m-long mosasaur from North America shown here, apparently specialized in such prey. Unlike the sharp, pointed teeth of mosasaurs that were adapted for gripping slippery, fleshy prey, Globidens teeth were blunt and subspherical, and ideal for crushing shells.
Mosasaurs have long been regarded as very lizard- or crocodylian-like in appearance and swimming behavior, moving through water via strong lateral undulations of their bodies, and propelled with a frill lining the top of their tails. But recent discoveries have forced reinterpretation of both their life appearance and their swimming mechanics. At least some mosasaurs bore a well-developed tail fin, more like that of an oceangoing shark than a crocodylian, and their bodies were chunky, streamlined, and mostly stiffened, making oceanic fish or whales better functional analogues than any living reptile. These features present mosasaurs as far more aquatically adapted than historically envisaged, and we should imagine them as the lizard equivalent of ichthyosaurs or whales rather than overgrown monitor lizards paddling out to sea. Rare fossils of extremely young mosasaurs suggest that they were born at sea and did not hatch from eggs on land, another indication of how far mosasaurs took their aquatic adaptations.
We take it as a given that animals such as Globidens were able to smash open shellfish to access the flesh within, but the capacity to bypass the defenses of shelled invertebrates was a major Mesozoic innovation that transformed marine life. This widespread evolution of crushing, drilling, and rasping apparatuses is known as the “Mesozoic marine revolution,” and it allowed certain vertebrates (fish and swimming reptiles) and invertebrates (snails and decapod crustaceans) to predate species that were previously inaccessible inside shelly homes. This forced rapid adaptation, or else quick evolutionary demise, for the affected prey species. Throughout the Mesozoic animals responded to these newly equipped predators by strengthening their shells, finding new ways to take refuge, or shifted to safer habitats, and lineages that couldn’t adapt in this way diminished in diversity and abundance. Brachiopods and the stalked crinoids were among those that struggled to combat shell smashers, drillers, and raspers, so they retreated into deeper, quieter habitats where predation pressures were lessened. Snails and bivalves responded more readily by thickening their shells, growing antipredator defenses such as spines, developing burrowing habits, and evolving quick escape tactics. This revolution played a major role in transforming shallow sea faunas from older Paleozoic-style ecosystems to more recognizable modern marine communities.
A Colossal Ammonite (Cretaceous)
MANY OF THE SPECIES WE’VE ENCOUNTERED THUS FAR ARE known from rare fossils, maybe even single specimens. They’re the sort of creatures we dream of finding when traversing fossil outcrops, but only a tiny fraction of us are so lucky. This does not apply to the subject of our next painting, however: an ammonite. Remains of these animals are extremely common in Mesozoic marine beds, and in some localities it’s easy to collect dozens or even hundreds in just a few hours. The abundance, extraordinary diversity, and rapid evolution of ammonite species makes them useful fossils for dating Mesozoic rocks. Many ammonites are characteristic to a time interval of one million years or less, so if we can confidently identify an ammonite species, we obtain a precision date for the rock it was found in.
Ammonite fossils are calcium carbonate shells coiled in a single plane, and it’s this coiling that reveals their relationship to other animals. They are part of Mollusca, and specifically a type of cephalopod—the group that today contains octopuses, squid, and nautilids. Cephalopods are tentacled, free-swimming, intelligent mollusks that have an ancestry extending into the Cambrian. Shelled forms, like nautiloids and ammonoids, were common throughout the Paleozoic and the Mesozoic, but modern cephalopod diversity is dominated by shell-less forms. Only a few living species of nautilids have shells, of which Nautilus pompilius is the most famous.
Like nautilids, ammonite animals lived within their shells. Their bodies inhabited a chamber at the large end of the spiral, and the preceding chambers were filled with air or flooded with fluid as a means of buoyancy control. Adjusting the ratio of fluid to air in these spaces makes shelled cephalopods negatively or positively buoyant, allowing them to effortlessly ascend or descend through the water column. But though nautilids and ammonites share some basic principles of shell anatomy, ammonites frequently differed from Nautilus in form and habits. Ammonites are actually more closely related to octopuses and squid than nautilids, and they are distinguished from living and extinct nautiloids in several ways. Modern nautilids are deep-sea creatures with eighty to ninety tentacles, leathery hoods covering the opening of their shells, simple eyes, and—unlike squid or octopuses—no ability to project clouds of ink when startled. Ammonites, in contrast, often lived in shallow waters, used part of their jaw apparatus to close their shells, and were equipped with defensive ink sacs. They may—as closer relatives of squid and octopus—have also borne fewer tentacles (squid and octopus have ten and eight tentacles, respectively) and had better eyesight than nautilids. The precise form of the ammonite animal remains unknown (a somewhat surprising fact, given the millions of ammonite shell fossils we have), but there is no reason to think they were Nautilus clones.
The lifestyles of ammonites remain mysterious. Their jaw apparatus was a parrotlike beak that seems suited for eating soft prey, and some ammonite fossils contain the remains of their last meals—small planktonic animals, such as crustaceans. But how they obtained their prey, and where and how they preferred to live, is not easily predicted, a conundrum made all the more complicated by the staggering diversity of their form. In addition to the familiar coiled shape, ammonites can be intricately ornamented, uncoiled, twisted and knotted, and all sorts of variations between. It is thought that vast size differences existed among ammonite genders, with smaller, more elaborate males being dwarfed by larger females, and we have no idea how this might have influenced ecology. Size differences are also extreme between species: some shells never exceed a few centimeters across, while Parapuzosia seppenradensis—a species from Late Cretaceous Germany (shown here)—reached 2–3 m across the shell and probably weighed well over 1 tonne. There is likely not one answer to the question of ammonite habits, their different forms surely reflecting contrasting adaptations to varying lifestyles.
Giant Flying Reptiles (Cretaceous)
IF WE’RE EVER ABLE TO RESURRECT SPECIES FROM MILLIONS OF years ago, the pterosaurs—the first vertebrates to achieve powered flight—are surely at the top of our wish list. Using a unique membranous wing anatomy where the fourth finger—the equivalent our ring finger—supported much of the airfoil, pterosaurs flew through most of the Mesozoic after evolving, under currently poorly understood circumstances, in the Triassic Period. Their relationship to other reptiles has long been mysterious on account of their geologically oldest fossils being true, bona fide pterosaurs without obvious anatomical links to other Triassic reptiles. Decades of interrogating their fossils, however, have enabled researchers to peel back the outwardly unusual anatomy of pterosaurs and find features linking them to archosaurs, with dinosaurs likely being close relatives.
For much of their research history, pterosaurs were regarded as evolutionary failures: reptilian gargoyles that struggled to walk, could barely fly, and just kept the sky warm until superior fliers—birds and bats—could take over their niches. This view has been overturned in recent decades as researchers have gained greater understanding of pterosaur anatomy and biology. Their lanky proportions reflect expanded, air-filled bones that probably linked with an efficient, birdlike lung system. Their fossil footprints show that, far from having the terrestrial capability of a collapsed tent, pterosaurs strode and even ran confidently. Many species probably spent a lot of time this way, searching for food on the ground. Other lineages were adapted to chase aerial insects, scavenge carcasses, to swim after or glean fish from the seas, or waded through shallow waters, using a variety of jaw and tooth shapes to detect and strain food. Large shoulders with expanded spaces for flight muscles indicate a capacity for powerful flapping and genuine powered flight, not simple gliding. Our perception of their diversity continues to expand too, with a glut of new discoveries—particularly in South America and China—revealing unusual, fascinating new species. These same localities have yielded the first pterosaur eggs and embryos, which show that pterosaur hatchlings were so similarly proportioned to their parents that it’s near certain they could fly very soon after hatching.
But perhaps the pterosaurs most sorely lost to Deep Time are the giants. Cretaceous pterosaurs routinely achieved wingspans of 3–6 m, comparable to the largest birds that have ever existed, and a number of species became even larger: behemoths with 10-m wingspans and 200–350-kg body masses. These animals belonged to a very successful, globally distributed group of toothless pterosaurs known as the azhdarchids (shown here). The fossil record of giant azhdarchids is relatively poor compared to that of their smaller cousins, but every component of their anatomy is consistent with flying habits, and flight models predict superior soaring skills that would enable them to travel thousands of kilometers with ease. The biggest pterosaurs were capable of surpassing the size of the largest birds because of their more powerful takeoff mechanism, which incorporated power from all four limbs, rather than just their legs. All animal flight begins with a leap rather than a flap and, as takeoff is the most strenuous part of flight, the power available for this initial bound into the air is a major factor in determining the maximum size of a flying species. Launching quadrupedally is so efficient because it uses the largest muscles in the body, the flight muscles of the wings, to primarily power the takeoff leap, whereas bipedal takeoff—as practiced by birds—is entirely reliant on the lesser muscles of the hind limb. Many bats use the same quadrupedal launch mechanic as pterosaurs, but as mammals they lack the expansive body air sacs and hollow bones that are probably also essential to becoming aerial giants. Pterosaurs combined their large, lightweight bodies with an efficient, powerful launch strategy to become the largest flying animals of all time. We can only imagine what it would be like to see these marvelous animals flying overhead.
Deinosuchus, an Enormous Alligatoroid (Cretaceous)
CROCODYLIANS ARE OFTEN PRESENTED AS LIVING FOSSILS OR even modern dinosaurs, neither of which is true. Crocodylians and dinosaurs are both archosaurs, but they have not been part of the same evolutionary line since the Triassic Period—over two hundred million years ago. The crocodylian branch of Archosauria is known as Pseudosuchia, and it has a significant role in reptile evolution. Pseudosuchia was once far more diverse than it is today, including reptile types that vied with other species on land and in water for predatory and herbivorous niches. Our modern crocodylian group was a surprisingly late addition to this roster, appearing only toward the end of the Late Cretaceous Period. Crocodylians would eventually dominate pseudosuchian diversity, and in the modern day they represent the last of this great reptile line. But the semiaquatic habits of living species are just one of many lifestyles practiced by pseudosuchians in their long history, and far from being living fossils, modern crocodylians are the tip of a huge evolutionary iceberg.
Among the first, and surely most spectacular, of the true crocodylians was the giant Late Cretaceous alligatoroid Deinosuchus. Two species of this animal are generally recognized as having lived around the coasts and estuaries of the North American Western Interior Seaway. The eastern species, D. rugosus, grew to 8 m long, a little larger than the longest-living crocodylian (the saltwater crocodile, Crocodylus porosus). The western species—D. riograndensis—attained lengths of 10 m or more, making it one of the largest pseudosuchians to ever have lived. Only the Cretaceous crocodyliform Sarcosuchus imperator and Miocene caiman Purussaurus brasiliensis challenge Deinosuchus in size, though the lack of complete skeletal remains for any of these species precludes determination of the true record holder.
The appearance of Deinosuchus is often mischaracterized in art, both reflecting assumptions that Deinosuchus was just a scaled-up crocodile as well as in earlier, erroneous skull reconstructions based on fragmentary material. Often restored with a triangular skull and nothing but conical teeth, in fact Deinosuchus had a much broader skull with sophisticated dentition: conical, piercing teeth at its jaw tips and crushing teeth further back. All its teeth were covered with thick, wrinkled enamel that reinforced them against powerful biting. A further distinction from living crocodylians are the large, bulbous, and deeply pitted scutes along its back. These were embedded in tough skin and muscle, and they served to both armor the back and strengthen it during terrestrial locomotion, a scute function also seen in living crocodylians. So characteristic are Deinosuchus teeth and scutes that they are identifiable even when discovered in isolation, a happy fact given that they represent the overwhelming majority of this animal’s fossil record. A complete picture of Deinosuchus proportions remains elusive thanks to its remains being damaged and scattered by storms prior to fossilization, a process that has left us with scant remains and little in the way of associated skeletons.
As an alligatoroid, Deinosuchus is closely related to living alligators and caiman. Like these species, it seems to have had very high biting strength. Large turtles seem to have been among its most common prey, and the fossil bones of these shelled reptiles are often riddled with Deinosuchus tooth marks. Deinosuchus evidently crushed their shells with some force, its teeth often bearing cracks and chips from powerful chewing and smashing activity. Some turtle shells show evidence of healing from their Deinosuchus attacks, indicating that Deinosuchus preyed upon live animals and was not merely scavenging dead ones. Popular culture has a penchant for the idea of Deinosuchus ambushing shore-based dinosaurs, and rare fossils hint that events of this nature may have happened. Indeed, one dinosaur limb bone is known that shows extensive Deinosuchus-induced damage, its round cross-section being mashed and chewed into a roughly square one. It was a formidable animal indeed that could use dinosaur bones as chew toys.
THE HORNED DINOSAURS, OR CERATOPSIANS, WERE AMONG THE most spectacular of all dinosaurs—no mean feat considering the number of iconic and charismatic species in the dinosaur evolutionary canon. Ceratopsians arose in the late Jurassic Period and they were represented by small bipedal dinosaurs distributed across Europe, Asia, and North America until the Late Cretaceous. As the Mesozoic drew to a close, ceratopsians developed into an astounding diversity of primarily North American, large-bodied quadrupedal forms, all of which have fantastic cranial frills, bosses, or horns. Skulls are the most common fossils of these larger species, and their ornamentation has proved very useful for distinguishing different species. By the close of the Cretaceous, horned dinosaurs were abundant herbivores in some dinosaur ecosystems, their fossils being far more common than those of other dinosaur species. Bone beds containing the remains of numerous individuals are known for several species, which implies that at least some horned dinosaurs engaged in gregarious behavior. Their apparent sociality and horned faces make it difficult not to imagine large horned dinosaurs as a dinosaurian take on cattle.
Triceratops is not only the most famous ceratopsian, but also one of the most famous extinct animals of all. A pair of brow horns discovered in in 1887 was the first evidence of this genus. Partly as a result of confusion about the age of the rocks the horns were found in, these were initially thought to belong to a strange extinct bison, but the true nature of Triceratops as one of the last of the horned dinosaurs soon emerged. Today, it is regarded as one of the best-represented dinosaur species, with many dozens of skulls and skeletons known, ranging from small juveniles to gnarled, mature adults. Once thought to contain multiple species, only two Triceratops species are now recognized. T. horridus is shown here.
At 9 m long and with an estimated body mass exceeding 6 tonnes, Triceratops was among the largest of all horned dinosaurs. Its skull is characterized not only by the three large horns that give us its name (“three-horned face”), but also by a relatively short and simple frill. Several functional explanations for ceratopsian horns and frills have been proposed, the most common being defense and sociosexual display. Triceratops growth series show that, compared to adults, juvenile individuals had much smaller horns and stunted frills. Given that juveniles are far more vulnerable to predation than adults, this casts doubt on a function purely related to predator defense. The development of fully realized horns and frills in adults is consistent with a sociosexual role: it’s adults, after all, that tend to be most interested in competition for territory and reproductive rights. Healed wounds on Triceratops skulls, which correlate precisely to simulated “horn lock” combat between mature individuals, are direct evidence that Triceratops horns had a role in contests between individuals. Evidently their facial ornamentation was not just for show, and, of course, if Triceratops could battle each other with their horns, they might have used those horns against predatory species as well.
The life appearance of Triceratops is unusual compared to that of other horned dinosaurs. Some ceratopsians had a series of scales on their faces, but the faces of adult Triceratops seem to have been entirely covered in a sheathlike skin, possibly akin to that covering bird beaks or cattle horns. Perhaps this protected them against facial injury? Skin impressions from Triceratops bodies are unusual, too. The skin of ceratopsians is typically a mosaic of small scales punctuated with occasional larger oval scales. Triceratops, in contrast, was covered in relatively large polygonal scales, some with peculiar projections in their center. The significance of these is unclear—were they low spikes, apertures for a large bristlelike filament, or something else entirely? At least one horned dinosaur, the Early Cretaceous Psittacosaurus, had a row of bristles along its tail, an entirely unexpected structure in a dinosaur group with an extensive record of scaly skin. Perhaps, as better soft-tissue fossils of Triceratops are found, further surprises may lie in store for our appreciation of its appearance.
The Cretaceous–Paleogene Extinction
THE MOST FAMOUS EXTINCTION EVENT IN HISTORY OCCURRED sixty-six million years ago and brought the Mesozoic Era to an end. This was the mass extinction that extinguished approximately 75 percent of plant and animal species, including the nonavian dinosaurs, ammonites, pterosaurs, and the vast majority of marine reptiles. The removal of so many species meant the post-Mesozoic world was radically different from the world that preceded it: fish, mammals, and birds diversified dramatically as they filled niches once occupied by reptiles and other Mesozoic forms. The K/Pg ([K]retaceous/Paleogene) extinction event was not, as it’s often stereotyped, just the event that wiped out the non-bird dinosaurs: it was another radical reshuffling of Earth’s biosphere.
Exactly how the K/Pg event played out remains the subject of investigation. Most of us are familiar with the fact that an asteroid was involved, but this may have been only one factor among many. Several major stresses were affecting life on Earth at the end of the Cretaceous, including falling sea levels (thus reducing the amount of shallow seas, environments where life thrives) and huge fissure eruptions in the Indian subcontinent—the remnants of which we call the Deccan Traps—were releasing enough gases and dust to impact climate and perhaps limit sunlight access. The latest Cretaceous fossil record is less than exemplary, so scientists are still trying to establish how life was impacted by these stresses. It remains controversial whether some groups, such as dinosaurs, were already suffering before the very end of the Cretaceous, or if the K/Pg event struck in their prime.
The final event of the Cretaceous was the impact of an asteroid, some 10 km in diameter, into what is now the Yucatán Peninsula of Mexico. The impact created the 180-km-wide Chicxulub crater and left a layer of iridium-rich clay across the planet. Iridium is rare on Earth but common in asteroids, so it’s likely that this distinctive sediment layer records fallout from an enormous impact of extraterrestrial origin. These geological phenomena coincide perfectly with the disappearance of characteristic Mesozoic fauna and flora from the rock record to suggest that, whatever detriment volcanism and sea-level fall had already delivered to Cretaceous organisms, the Chicxulub impact was the actual curtain call for Mesozoic life.
Geological data suggests that the asteroid collision was one of the grandest, most terrifying moments in the history of life on Earth. The immediate effect was an explosion equivalent to ten billion Hiroshima-grade atomic bombs. Organisms living within one thousand kilometers that were not killed by the initial blast would have encountered massive energy output that triggered wildfires, earthquakes, the collapse of the Yucatán continental shelf, and tidal waves hundreds of meters tall. Organisms even five thousand kilometers away would have been showered with dust and particles ejected from the impact site, burying landscapes in debris layers up to 10 cm thick. Around the world, particles hot enough to start local fires or kill exposed organisms rained down after having been being blasted into space, and tsunamis ravaged coastlines. As these violent events subsided, the long-term effects of the impact began, enhancing the stresses already catalyzed by the Deccan Traps eruptions. The impact had vaporized rocks rich in carbon and sulfur, turning rainfall acidic and filling the atmosphere with sunlight-scattering aerosols. These aerosols reflected sunlight away from Earth’s surface and began to cool the planet, an effect worsened by sun-blocking dust blown into the atmosphere during the impact. With Earth’s surface receiving only weak sunlight, the entire planet cooled by 10°C. Earth remained dim and cold for decades, weakening ecosystems that were adapted for receiving plentiful solar energy. Only plants adapted to low light and detritus-based food chains thrived in this interval, while forests, complex marine ecosystems, and large animals disappeared from the planet. Recovery to pre-extinction diversity took, in some ecosystems, up to three million years.
House Mice and Other Modern Evolutionary Winners (Holocene)
WE HAVE ALREADY SEEN, COURTESY OF MORGANUCODON AND some Mesozoic mammals, that a fairly generalized small-bodied form was a proven success for mammal-line animals early in their evolution. Rodents—the most speciose modern mammal group (2,277 living species, comprising about 40 percent of all mammal diversity)—show that this body plan remains useful today. Rodents are extremely adaptable and have colonized most continents in great numbers; Antarctica is the single holdout against their spread. Though most rodents are small—rats, mice, hamsters, squirrels, guinea pigs, and so on—some living examples are the size of large dogs (beavers, porcupines, and capybaras). In the past, rodents experimented with being even bigger, including South American forms as large as rhinoceroses, and beavers the size of humans. They are undeniably one of the greatest successes of mammal evolution, true champions of the post-Mesozoic “Age of Mammals.”
Rodents are specialist chiselers with two pairs of continuously growing incisor teeth and powerful jaws. This enables them to gnaw at foodstuffs that would be too tough for other animals to eat, as well as to manipulate their environment: chewing into structures to create dens or nests, or felling and gathering robust building material for their own architecture. Their characteristic jaws and teeth make their fossils highly identifiable, and we can trace the evolution of the four major rodent groups into rocks of the Paleocene Epoch. Some disagreement exists, however, over whether these fossils represent the first rodents. While fossils suggest that rodents arose in the aftermath of the Cretaceous mass extinction, genetic data predicts a slightly older origin, just before the end of the Cretaceous. In either eventuality, rodents were well equipped to capitalize on the potential presented in early Cenozoic ecosystems, and today they are among the most abundant mammals on the planet. As fast breeders adapted to giving birth to large numbers of offspring, rodent populations increase rapidly and spread quickly. The oldest fossil rodents indicate an origin in China and Mongolia, and within a few million years they had spread to Europe and North America. By the middle Eocene they had rafted over the Atlantic Ocean from Africa to South America (then isolated from other continents), and they finally reached Australasia within the last few million years.
Though many rodents are endangered, a handful of species are so well adapted to living alongside humans that they are considered pests: species that parasitize our settlements by stealing food, infesting our buildings, and causing health concerns. But this does not reflect a nefarious or intentioned assault on humanity: species such as house mice (Mus musculus, right) are simply exploiting our way of life in the same way they would approach any evolutionary scenario, making the most of circumstances because they can. Indeed, this is the reality of all “pests”: they are not species that we happen to share environments with, but those that directly benefit from the way we live. By shaping the world with extensive agriculture and urban settings, we have created conditions conducive to the likes of rodents, pigeons, and weeds, and they propagate because of us, not in spite of us. Our pests have followed us as we’ve colonized the globe, stowing away on ships or following the development of farms between towns and cities. Once restricted to regions of Asia, house mice and brown rats (Rattus norvegicus) now live worldwide, making them major nuisance to humans as well as invasive species that endanger indigenous wildlife across the globe. Invasive rodents can spell disaster for foreign ecosystems: their breeding potential and adaptable feeding habits often overwhelm native small mammals (including other rodents) and spell doom for species such as ground-nesting birds, which are ill-equipped to deal with them. We may regard pests and their ecological impact as operating independently of humanity, but they are really an extension of our own success. They stand on our shoulders to gain an evolutionary boost, and they capitalize on the chances we have provided for them.
Gastornis, a Giant Land Waterfowl (Eocene)
THOUGH MANY DINOSAURS BECAME EXTINCT AT THE END OF THE Cretaceous Period, one lineage continued to flourish throughout the Cenozoic: the birds. Today, they are some of the most speciose and charismatic of all animals, a group of feathered theropods that lives on every continent and in most major habitat types. The Age of Dinosaurs did not end with the K/Pg extinction; it was simply retooled into an all-avian spin-off.
The roots of modern bird diversity extend into the Late Cretaceous, with many survivors of the K/Pg event representing early members of present-day bird groups. Birds underwent an explosion of diversity early in the Paleogene Period. At this time, most birds were adapted to life in open settings, likely reflecting a scarcity of extensive forest environments in the aftermath of the K/Pg extinction. Many Cenozoic avians would have looked unusual to us today, taking on dramatically different anatomies, body sizes, and lifestyles compared to their closest living relatives. This includes the famous Paleogene bird Gastornis, a human-sized flightless bird that lived across Asia, Europe, and North America in the guise of several species. Skeletons of this heavyset bird look remarkable to modern eyes, being comparable in size to ratites (the group that includes ostriches and emus) but having robust, bulky features that are difficult to place among living species. The New Zealand flightless rails known as takahe (Porphyrio hochstetteri) are perhaps our best, though still crude, anatomical analogues. Gastornis has no close relatives among the ratites or rails, however, and is probably an early offshoot of the waterfowl branch—the Anseriformes—making swans, geese, and ducks its closest living relatives. Early reconstructions of Gastornis assumed a ratite-like appearance with long shaggy feathers, but their affinities to waterfowl imply a tidier appearance with neat vaned feathers. The discovery of a giant feather from a Gastornis site in the United States adds credence to this interpretation.
The lifestyle of Gastornis has been the focus of several studies. It was clearly better suited to walking than running, on account of its stout limbs and hooflike claws, and its proportionally large head implies a strong jaw for powerful biting. Computer modeling predicts a high bite strength and excellent stress distribution across its skull during feeding, but opinion has been split over how to interpret this. Was Gastornis a powerful carnivore, killing prey with forceful bites and maybe cracking open their bones like an avian hyena, or was it an herbivore adapted to eating tough plant material? Further studies, which include analysis of Gastornis jaw musculature, jaw shape, running speed, claw morphology, and bone chemistry, point to the herbivorous hypothesis being more likely. Rather than seeing Gastornis as the devourer of diminutive early horses, as it’s portrayed in many paleoartworks, we should envisage it using its powerful beak to eat tough vegetation and nuts. In this respect, the lifestyle of Gastornis was similar to that of mihirungs (dromornithids), a closely related extinct group of large ground birds that lived in Australia from the Oligocene to Pliocene, but different from the predatory phorusrhacids, which were formidable, carnivorous, and often giant terrestrial birds of North and South America.
Fossil beds composed of large, broken fossil eggshells are known from southern France that might, based on their size and geological age, represent Gastornis nesting sites. The abundance of these shells implies colonial nesting behavior localized to the same region from generation to generation, creating a remarkable mental image of possible Gastornis reproductive behavior. Growth rings in Gastornis skeletons suggest that they grew more slowly than most modern birds, taking several years to attain their full height of 1.5–2 m. We can assume, based on the reproductive strategies of living fowl, that Gastornis chicks would be precocial, likely able to follow their parents and feed themselves.
Onychonycteris, an Early Bat (Eocene)
THE RADIATION OF BIRDS WAS NOT THE ONLY MAJOR EVOLUTIONary event occurring among flying animals in the early Paleogene. Bats, the only known mammalian foray into powered flight, also appeared at this time. The bat group, Chiroptera, contains approximately twelve hundred species and represents about 20 percent of modern mammal diversity—only rodents are a more speciose lineage. Their ability to fly has allowed bats to spread to almost every landmass on the planet, with just the Arctic, the Antarctic, and a handful of remote islands free from their presence. The largest bats are sizeable animals with wingspans of 1.7 m, while the smallest vie for the title of most diminutive living mammal, having wingspans of just 15 cm, and body masses under 3 g. Though superficially rodent-like in appearance, they are not closely related. Bats actually stem from the same branch of mammal evolution that houses the carnivorans and hoofed mammals.
Fossils recording the early evolution of bats are extremely rare. As with pterosaurs, our fossils representing the earliest stages of bat evolution are already entirely batlike, including a full suite of flight adaptations. Species representing previous stages of their evolution, which perhaps included flightless climbers as well as gliding forms, remain elusive. We can at least be thankful that several of our oldest bat fossils, from rocks deposited fifty-five million years ago, are well preserved, complete skeletons. The bat fossil record is otherwise largely formed of isolated teeth and jawbones, so these excellent Eocene fossils give us important insights into how bat anatomy and flight abilities have developed since the beginning of their evolutionary history.
All bats have membranous wings supported by long fingers. Their finger bones are slightly pliable on account of having reduced mineral content, permitting their wings to adopt especially aerodynamic shapes throughout the flap cycle. The membranes on the wings are more than just stretched skin: they contain sheets of muscle that allow the bat to control membrane stiffness in each flight stroke. These aspects mean that bats are not merely “rats with wings” but sophisticated, highly evolved aeronauts with more control over wing shape than birds and (probably) pterosaurs. They are supreme aerial acrobats, a fact exploited extensively by microbats, a (possibly artificial) group of flying insectivores numbering over one thousand species. They catch flying insects either in their jaws or with a membrane between their legs, using echo-location to find their way through dimly lit settings. Note that, while not all bat species can echolocate, none are blind.
Onychonycteris finneyi, a small (wingspan of 220 mm) fossil bat from Eocene rocks of Wyoming, United States, is among the oldest known bats and the earliest “grade” of bat evolution yet represented in the fossil record. Though undeniably bat-like in form, it retains a number of features from its ancestors that were lost in later bats, including claws on all its fingers (living bats have just one or two wing claws) and relatively long legs. Its wings have proportions typical of gliding mammals, though the presence of augmented shoulder and forelimb anatomy shows it was capable of true flight. Similarly proportioned living bats alternate between flapping flight and gliding flight, and it is possible that Onychonycteris did the same. Its jaws and teeth suit those of an insectivore, but its ear anatomy is unusual compared to both other fossil bats and modern species: perhaps this indicates an inability to echolocate. We probably need superior fossils of Onychonycteris to be confident of this, however, meaning a long-standing question about bat evolution—flight first, or echolocation first?—remains unanswered. The long limbs and short wings of Onychonycteris may have allowed it to seek food on the ground or when climbing, as well as in the air. Several living bat species are similarly adapted today, their wings placing no restriction on their ability to walk or run. We do not know if Onychonycteris was nocturnal like most living bats, but adaptations in its feet imply an ability to hang upside-down in classic chiropteran fashion.
Arsinoitherium and the Evolution of Giant Mammals (Eocene)
MOST OF THE ANIMALS THAT SURVIVED THE K/PG EXTINCTION were small-bodied species that could find adequate food in resource-stripped ecosystems. As the biosphere rebuilt itself in the early Cenozoic, larger animals returned in the form of big-bodied mammalian herbivores. They initially included creatures like the globally distributed Pantodonta, which evolved in the Paleocene and developed a diverse range of omnivorous and herbivorous animals, including heavyset, 500 kg species with skulls over half a meter long. The robust, short tails in some pantadonts may have allowed them to adopt a tripodal rearing pose for browsing taller vegetation, using their tails as a prop to support their weight. Most had hoofed limbs adapted for weight bearing, although some possessed clawed digits of unknown purpose. Like some modern deer, several pantodont species bore large fangs that may—based on their lack of dental wear—have been used more for sociosexual display and combat than for foraging. Though appearing early in the Age of Mammals and lacking some anatomical innovations of later mammalian herbivores, the pantodonts held their own against competition from large herbivorous birds and mammals until the late Eocene or earliest Oligocene.
Pantodonts were not the only lineage experimenting with large body size in the early Cenozoic. The Embrithopoda, a group traditionally (though not exclusively) thought to be related to sirenians (dugongs and manatees) and elephants, were also Paleogene giants. The most famous and completely known member of this group is Arsinoitherium, a genus that occurred across northern Africa during the late Eocene and early Oligocene. It represents the last and largest of the embrithopods, with A. zitteli (shown here) reaching a rhinoceros-like shoulder height of 1.75–2 m, a body length of 3 m or more, and a body mass of at least one tonne. They inhabited swamps and mangrove-like settings, though their limb anatomy and bone chemistry suggest that they were land-based animals that ate terrestrial plants. This finding is surprising in light of their relatively poorly developed hips and shoulders for animals of their size, as well as their short limbs. These features have been historically regarded as indicating semiaquatic habits, but modern data suggests Arsinoitherium was more rhino-like than hippo-like in ecology. Arsinoitherium dentition and jaws suggest a powerful chewing mechanism adapted for shearing and grinding of bulky, malleable plant matter, such as large fruits. Fossils of such food items exist with abundance in Arsinoitherium fossil sites, perhaps representing a common component of their diet.
The most striking feature of Arsinoitherium are the twinned sets of horns atop its head: two large horns at the front, and two smaller ones at the back. These structures have hollow interiors and thin exterior bone walls, and their surface textures indicate that they were covered with a tough horn sheath. This approach to horn construction is very similar to that of modern bovids (goats, cows, and antelope) and is also seen time and time again in various mammal and reptile lineages throughout Deep Time. It blends a lightweight and bending resistant core (the horn skeleton) with a covering that is excellent at distributing impact forces evenly across its surface (the horn sheath). The result is a lightweight, damage-resistant structure supremely adapted for display and antagonistic behavior. With such horns, Arsinoitherium need not be shy about following up intimidating displays with physical aggression, perhaps locking horns with rivals or using the horns to drive away predators. Fossils show that the horns of juvenile Arsinoitherium were much smaller than those of adults, so perhaps their greatest role was settling disputes between competing Arsinoitherium individuals for food, territory and reproductive resources.
Georgiacetus, the Last Land Whale (Eocene)
THE EVOLUTION OF WHALES WAS, UNTIL COMPARATIVELY REcently, relatively poorly understood. All fossil cetaceans known before the 1980s were fully marine animals entirely divorced from terrestrial habitats, and thus species largely unhelpful in answering questions about how and when whales transitioned from land to sea, or which stock of land mammals they were descended from. Light was finally shed on this evolutionary transition when fossils representing the earliest members of the whale lineage, which had strong walking limbs and lived in freshwater habits, were discovered in India and Pakistan. In having not yet deviated markedly from the anatomy of their terrestrial ancestors, these Eocene species confirmed a long-standing hypothesis that whales had ancestry among the Artiodactyla (even-toed hoofed mammals), a finding also borne out by analysis of whale DNA. It now seems likely that, among living animals, hippos are the closest relatives of whales. Continued discoveries of Eocene whales have provided a nearly continuous sequence of whale evolution from land to sea, transforming this poorly understood portion of mammal history into one of the best-documented mammalian evolutionary transitions on record.
Cetacean evolution was rapid and was strongly directed toward the development of marine forms. It took just ten million years to evolve fully whalelike, wholly aquatic animals from the oldest known semiaquatic “proto-whales.” Cetaceans began their evolutionary life looking somewhat like heavyset dogs, and they may have run or punted through water rather than swimming, their unusually heavy bones helping them to remain submerged as they pursued aquatic prey. They eventually became more crocodile-like in overall form, becoming true swimmers with long jaws and piercing teeth. But their hind limbs remained crucial to their locomotion, their tails seemingly lacking flukes and their expanded, probably webbed feet being their main propulsors. The retention of large hips with strong connections to their spines allowed their legs to support their weight on land and permitted efficient walking and running.
From this early form, cetaceans rapidly enhanced their aquatic adaptations. They developed longer jaws for seizing prey, reconfigured their teeth for a purely carnivorous seafood diet, and modified their bodies and limbs for more efficient swimming. It was among the protocetids, a group (or maybe “grade”) of Eocene proto-whales found across the Northern Hemisphere, that cetaceans reached the end of their evolution on land. Fossils of pregnant protocetids show that some members of this group still gave birth out of water, their calves emerging headfirst rather than, as with living whales, tailfirst (an adaptation to avoid drowning during birth). But the pelvises of protocetids were joined to their spines only loosely, the connection limited to just a few vertebrae in most species, or even none in Georgiacetus vogtlensis (shown here). This would render their hind limbs less able to support their weight on land, as would the increasing length of their spinal columns. These issues would be compounded further by their growing size: Georgiacetus is estimated to been about 6 m long and might have weighed several tonnes.
We might imagine these last of the terrestrial whales as moving like large seals, undulating their bodies and using limited purchase from their limbs to clamber about beaches and exposed rocks. Protocetids retained large feet to power their swimming, but rather than paddling, they used their feet in concert with their increasingly powerful, broad tails. Animals like Georgiacetus were among the last whales to have possessed well-developed hind limbs and, once the need to visit land to give birth was lost, whales replaced their hind-limb flippers with a strong tail fluke. Flukes are entirely soft-tissue structures and thus do not fossilize readily, but they are detectable by the characteristic shape of their supporting vertebrae. We have yet to find such vertebrae in any protocetid, which suggests that they likely lacked large tail flukes. These structures quickly appeared in subsequent lineages after cetaceans committed entirely to marine life, however, just one of several changes that took place to fully adapt their bodies to life at sea.
Paraceratherium, a Giant Rhinocerotoid (Oligocene)
LIVING RHINOCEROSES ARE FAIRLY “PREHISTORIC-LOOKING” beasts thanks to their horns; their peculiarly shaped faces; and their thick, armored skin. Of course, they are as “modern” as any other animal lineage and not in any way archaic. To the contrary, they are quite anatomically distinct from many of their fossil relatives, and many of their characteristic features—like their horns and stout bodies—are relatively recent evolutionary innovations.
The full diversity of the rhinoceros lineage is vast, with modern rhinoceroses being the proverbial tip of their evolutionary iceberg. Rhinocerotoidea split from the perissodactyls (the odd-toed hoofed mammals, a clade that also includes tapirs and horses) in the Eocene Epoch and evolved into a myriad of forms that lived across Africa, North America, and Eurasia. Some were small, fast runners; others were similar to modern horses in size and build; some were rotund, semiaquatic creatures; and another group became the heavyset herbivores that cling to survival in Asia and Africa today. It is a sad fact that poaching will likely cause the final demise of the rhinocerotoid lineage, and quite possibly within our lifetimes. Rhinoceros are killed for their horns: structures composed of the same worthless keratin proteins as our hair, skin, and fingernails, but regarded as cancer cures or luxury commodities in parts of East Asia. At time of writing, a large rhinoceros horn is worth a quarter million dollars in black market trade, a sum great enough to make rhinoceros horns of any kind a target for criminals. Museum taxidermies must now sport prosthetic horns; captive zoo rhinos are slaughtered in covert, nocturnal horn raids; and wild rhinoceroses are constantly guarded to deter poachers. It’s among the wild rhinoceros that this situation becomes most desperate, as park rangers and poachers regularly exchange gunfire, and are even killed, in their quest to obtain or protect rhinoceros keratin.
Among the most impressive fossil rhinocerotoids were the indricotherines. These enormous animals roamed central Asia during the Oligocene and achieved estimated body masses of 15–20 tonnes, sizes that were outdone only by sauropod dinosaurs and, maybe, the largest mammoths. This make indricotheres strong candidates for the largest land mammals of all time. Several different species of these giants existed, but how many, and how they are related to one another, is controversial owing to their scrappy fossil record. The most famous and largest is Paraceratherium transouralicum, shown here. The exact proportions of the largest species remain unclear because substantial fossil skeletons remain elusive. The history of indricothere research contains numerous different takes on their skeletal form, with some researchers suggesting they looked like giant versions of living rhinoceros, and others producing creatures that look like robust giraffes.
The reality may be somewhere in between. Indricotherines are related to (or may be part of) Hyracodontidae, and they shared their gracility, short torsos, and long limbs. They would thus have been svelter than modern rhinos, and were probably relatively sprightly for their size. Indricotherines also had a reasonably long neck, though exactly how long remains to be determined: modern reconstructions still differ in this regard. Their skulls are well known and, though possessing powerful rhino-like jaws and massive teeth, they lack features we associate with the presence of rhinoceros horns. A further contrast with modern rhinoceros stems from indricotherine skull anatomy indicating a tapir-style proboscis at the end of their snouts. They likely used them to browse from trees, scooping vegetation into their mouths or stripping leaves from twigs. This may seem like a bold claim, given our lack of any fossilized indricothere soft tissues, but trunks and proboscises require significant reorganization of skull anatomy to house the demands of their musculature and nervous tissues, and we can identify these adaptations in fossil animals with well-preserved skulls. Altogether, indricotheres may have looked more like gigantic, heavyset horses than rhinoceros or giraffes, although we should await more definitive assessments of their anatomy and proportions before committing fully to this reconstruction.
Bluefin Tuna and the Dominance of Teleosteans (Holocene)
OF ALL MODERN FISHES, 96 PERCENT—SOME TWENTY-EIGHT thousand species—belong to one group: the teleosteans. This remarkably adaptable lineage inhabits more aquatic environments than any other animal group, including a range of marine and freshwater settings as well as inhospitable settings such as cold arctic waters, pitch-dark caves, the deep sea, lofty mountain streams, and hypersaline lakes. The secret to this success lies in their highly adaptable bodies, which have attained a tremendous range of shapes, sizes, and feeding apparatuses to suit numerous lifestyles and environments. Teleost jaws differ from those of other fish in rapidly protruding forward during feeding, extending the reach of their mouths and also sucking food inward by creating a pressure gradient in their oral cavities. Additional characteristic features are found in their tail skeletons, which are more reinforced than those of most fish and thus are superior at generating thrust for swimming. Teleosteans have applied these anatomies to almost every conceivable vertebrate lifestyle, including predation, herbivory, filter-feeding, and parasitism.
Teleosteans have their origins in the Triassic and diversified steadily throughout the Mesozoic. Most major body plans had evolved by the end of the Cretaceous and they became the dominant fish group in the Cenozoic, radiating at a startling rate into the thousands of species known today. Teleosteans belong to the Actinopterygii, the ray-finned fish, a large fish clade that has lightweight rods of bone supporting their fins instead of robust, limb-like skeletons, as seen in the early tetrapods and coelacanths we encountered earlier. Teleost skeletons are also lightweight in other respects, their bodies being composed of thin bony scaffolds instead of heavy, robust bones. This makes them relatively light and flexible compared to other swimming vertebrates, and thus generally faster and more maneuverable.
Some of the most interesting living teleosteans are also the most familiar: tuna. We mostly know these animals from their canned flesh, steaks, or sushi, but appreciating tuna only from our dinner plates does a disservice to their amazing anatomy and optimization to the niche of a fast, large-bodied predator. Species such as the northern bluefin tuna (Thunnus thynnus), shown here, can grow over 3.5 m long and nearly 1 tonne in mass. They are effectively fishy torpedoes, equipped with a streamlined, muscular body and a narrow, thrust-optimized tail fin. Their sickle-shaped body fins can be extended for steering and stabilization or pressed into notches on their bodies to reduce drag. Unlike most fish, tuna are warm-blooded and possess an enhanced ability to circulate oxygen and absorb it into their tissues. They are among the fastest animals in the sea, and they’re dangerous predators to a variety of small fish, squid, and other invertebrates.
It is not only the remarkable physiology and anatomy of tuna that makes them famous: they also epitomize current, critical issues facing life in our seas. Demand for the flesh of the three bluefin tuna species has seen them overfished to the extent that they are all endangered, some critically so: the meat of some species is literally worth more than its weight in gold. Aquaculture—offshore vats full of captive fish—seems like an ideal countermeasure to their overfishing, but it is difficult to keep the pesticides, antibiotics, and other agents used in these farms from spreading to, and damaging, neighboring local marine habitats. Moreover, although farming predatory fish reduces pressure on wild populations of the livestock species, other fish species must be caught to supply the farm with feed. While this moves the risk of overfishing from one species to another, it does not remove it entirely. Oceanic food chains are long and complex, and there is real risk that poor fishing practices will, in the long-term, prove disastrous for many species, including ourselves. The vastness and productivity of our oceans places them as a potential solution to many of humanity’s food crises, but far stricter management is needed if we wish to enjoy healthy, fishable marine environments beyond the immediate future, and if we want to preserve the remarkable animals that have inhabited these settings for many millions of years.
Daeodon: A Formidable Piglike Creature (Miocene)
DAEODON IS THE SORT OF ANIMAL THAT PREHISTORY IS FAMOUS for: a big, intimidating species that looks like it could go seven rounds with most modern species and emerge victorious. It is part of the artiodactyl group known as Entelodontidae, a clade of omnivorous animals that lived in North America, Europe, and Asia from the mid-Eocene to the early Miocene. They bear more than a passing resemblance to pigs with their huge, gnarly skulls, large tusks, stout bodies, and hoofed feet. This similarity often sees them restored as boar- or warthog-like, and it has earned them several pig-derived nicknames among paleontological enthusiasts (“hell pigs,” “terminator pigs”). But despite appearances, they are not closely related to pigs at all. Rather, they are actually part of the terrifically named group Whippomorpha: the branch of hoofed-mammal evolution that gave us whales and hippos. Unlike either of these groups, entelodonts did not live in aquatic habitats, but frequented woodlands and plains.
Two species of the North American entelodont genus Daeodon are known. Daeodon shoshonensis, shown here, is the largest, and last, of their lineage. It was a huge animal that measured 1.8 m tall at the top of the shoulders, and it typifies entelodont anatomy in having a huge skull (some 30% of the body length); a short neck; massive shoulders that helped support the weight of the head; a deep torso; and surprisingly long, slender legs. Entelodont skulls are remarkable structures with long jaws, cavernous spaces for jaw muscles, forward-facing eyes, and a number of ornamental flanges and bosses. Versatile feeding habits are indicated by their array of tooth types: massive, peg-like incisors at the front of the jaw; long, sharp tusks behind them; triangular shearing teeth behind these; and, finally, broad, cusped teeth in the cheek region. This dentition afforded entelodonts the capacity to grip, cut, and crush their food, which was most likely plant matter (roots, tubers, fruit, and fibrous matter such as leaves and branches), as well as meat—both scavenged and predated.
Entelodonts could open their mouths exceptionally wide, permitting manipulation and strong biting action on large food items, even at the back of the toothrow. The teeth of old entelodonts are often exceptionally worn and chipped, suggesting frequent biting of hard foodstuffs. The level of wear is so extreme that it compares well to the teeth of old dogs and hyenas—animals that routinely chew into bone. That entelodonts were at least part-time carnivores is confirmed by their bite marks on fossil mammal bones as well as a remarkable fossil cache of seven Oligocene camel (Poebrotherium) individuals, all riddled with bite marks from the large entelodont Archaeotherium. Almost six hundred bones from several partly articulated camel skeletons were found in this association, and details of their arrangement implies that these carcasses were deliberately stored together, not randomly accumulated by water currents or other environmental phenomena. The camel skeletons are mostly left with their forelimbs and rib cages intact, while the pelvic regions and hind limbs are gone. Perhaps, like many modern predators, entelodonts prioritized eating the muscular, fleshy haunches over other parts of carcasses. A lack of feeding traces from other carnivores implies that the entelodonts killed the camels before storing them.
Bite marks on entelodont skulls are evidence that they grappled each other’s faces with their mouths, and this offers another potential explanation for their wide gapes. Such behavior might relate also to the function of their cranial bosses and flanges. Perhaps an impressive set of ornaments intimidated rivals and discouraged biting behaviors but, if conflict was unavoidable, they may have also deflected bites away from vulnerable areas. Entelodont limb proportions match those of running animals, and they were probably surprisingly fast despite their large size, massive heads, and deep chests. All evidence points to entelodonts being awesome animals, though probably ones we would be wise to avoid in person.
Deinotherium, the Chin-Tusker (Miocene)
THE MAMMAL LINEAGE THAT GAVE RISE TO ELEPHANTS HAS A long and relatively well-understood evolutionary history. Some parts of elephant anatomies superficially resemble those of hippopotamuses and rhinoceroses, but they are not closely related; their similarities instead reflect common adaptive responses to the challenges of supporting many tonnes of body mass on land. The true closest living relatives of elephants are much less obvious: the rodent-like hyraxes and the aquatic sirenians (dugongs, manatees, and kin). Together, these groups form the clade Afrotheria. As was discussed when we encountered the giant rhinoceratoid Paraceratherium, characteristics of animal skulls betray the presence of trunks, and we can thus conclude that virtually all elephant-line afrotherians had a trunk or a proboscis of some kind. The elephant lineage is thus aptly named Proboscidea.
Many fossil proboscideans have vaguely elephant-like proportions, but this was not always so. When proboscideans first split from other Afrotheres, sixty million years ago, members of the elephant line were squat creatures resembling hippopotamus-like pigs, and they were likely semiaquatic in behavior. They had highly mobile lips or short proboscises and small, forward-projecting tusks formed from oversize incisors—the same teeth that later proboscideans would grow to enormous proportions. These early proboscideans would eventually abandon aquatic habitats for a terrestrial existence, growing longer legs, great body size, longer trunks to reach the ground from their stately heights, and a variety of tusk shapes for differing adaptive purposes. In this guise, proboscideans colonized much of the planet, with only Antarctica and Oceania remaining beyond their reach.
Deinotherium is one of the first very large land proboscideans. A true giant even among the elephants and their relatives, the estimated shoulder height of some individuals approached 4 m and their masses were likely in the 10-tonne range. At least three Deinotherium species existed across Africa, Asia, and Europe from the Miocene to the Pleistocene. They differ only slightly from the oldest species, D. giganteum (shown opposite), suggesting that the Deinotherium body plan was a versatile one suited to changing habitats and climates.
The face of Deinotherium is remarkable for a number of reasons. It evidently had a trunk of some kind, though the regions for muscle attachment are broader and longer than those of living elephants, which have tall and narrow trunk attachment sites. Some aspects of the skull imply a shorter, maybe tapir-like proboscis instead of an elephant-like trunk, although some researchers have questioned how Deinotherium would drink with such an organ (this, of course, assumes it did drink: some mammals are capable of taking all their water from their food). Adding to this mystery are two peculiar chin tusks, structures that projected backward and downward and were evidently, because of their extensive wear, used for some practical purpose. This configuration might seem unusual because modern elephants have tusks only on their upper jaws, but proboscidean history shows a great variety of tusk configurations and many species bore them on both upper and lower jaws. Evidence of abrasion between the tusks of Deinotherium implies that food was dragged between them, so perhaps Deinotherium stripped bark or leaves as a way to better prepare their food for eating, as is done by modern elephants? In all likelihood this was not the only function of their tusks: pulling over trees, fighting, and intimidating enemies are just a few other possible uses.
Though Deinotherium was elephant-like, it is erroneous to think of this animal simply as an elephant with an unusual face. It has proportionally long limbs; a shorter torso; and a somewhat longer, more flexible neck. It was probably elephant-like in many ways, but its distinctive anatomies have implications for locomotion mechanics, digestive capability, and foraging techniques. These anatomical differences impact ecology and lifestyle, and they may explain how Deinotherium was able to live alongside other, more classically elephant-like proboscidean species.
The Killer Sperm Whale Livyatan (Miocene)
WE PREVIOUSLY MET MEMBERS OF THE WHALE LINEAGE IN THEIR final stages of becoming fully marine animals. By the end of the Eocene, whales had not only made this transition but also had split into the two major groups of cetaceans we recognize today: the toothed whales (odontocetes) and the baleen whales (mysticetes). They shared the water with another type of whale, the basilosaurids, a grade of early whales that include the famous genera Basilosaurus and Dorudon. But whereas basilosaurids perished before the end of the Eocene, odontocetes and mysticetes survived to become major predators in Earth’s seas and oceans, a role they still occupy today. Some mysticetes, such as the blue and fin whales, are the largest organisms to ever have lived. They build their immense bodies from plankton, small fish, and squid that they harvest from water by lunge-feeding on a huge scale. While swimming at their prey, they engulf tonnes of seawater and animals in a single mouthful before their powerful throat muscles force the water out through filters of baleen—a stiff, bristlelike proteinaceous structure lining the upper jaws. Anything left within in their car-sized mouths is trapped and swallowed. These amazing superpredators can eat entire schools of fish in one action.
Early marine whales did not use capture-and-filter approaches to obtaining food, however: their feeding mechanics were more similar to those of modern toothed whales: the group that includes dolphins, porpoises, and sperm whales. These animals apprehend their prey by using sharp teeth or, in some species, by suction feeding, drawing prey into their mouths by means of pressure gradients within their oral cavities. Today, most odontocetes target prey species that are easily subdued, such as relatively small fish and squid. The largest of the group, the sperm whale, is famed for hunting giant and colossal squid at great depths, but these squid—which weigh hundreds of kilograms—are still much smaller than their whale predators, which routinely weigh 10–40 tonnes. Only orca (or killer whales) habitually pursue large prey, and they have a reputation for being tenacious, crafty hunters. Killer whale pods will exhaust and harass other whales through long chases before eating their tongues or their calves, and they will use a variety of tactics to disable and kill seals.
Modern orcas are an echo of a time when whales generally had an eye for larger prey. During the Miocene, several species of raptorial sperm whales roamed the globe, using their massive skulls and huge teeth to prey on other marine mammals. Among the largest was Livyatan melvillei, a 14–17-m-long Peruvian species comparable in size to the living sperm whale. Unlike the sperm whale, however, Livyatan and its relatives were equipped with huge, interlocking, tusklike teeth in both jaws, those at the front being pointed for gripping prey, and those at the rear adapted for cutting and shearing. The skulls of all sperm whales have large basins that house a fatty organ known as the junk (other odontocetes have an equivalent structure known as the melon) as well as an oil-filled spermaceti organ. These massive structures aid in the transmission of sound for echolocation, and internal reinforcement of the junk allows it to be used as a battering ram. Sperm whales are not the only toothed whales to use their heads aggressively: orcas use violent motions of their heads, as well as their tails, to stun prey before drowning it. Might ancient predatory sperm whales have weaponized their foreheads in the same way?
Livyatan was not the sole arch-predator of Miocene Peruvian seas. It shared its habitat with another giant carnivore, the enigmatic megatoothed shark Otodus megalodon—better known simply as “Megalodon.” The popularity of this famous shark is disproportionate to our understanding of it. Represented entirely by teeth and the occasional vertebra, much about this animal—its size and proportions, its relationships to other sharks, and even its appropriate scientific name—remains controversial. It may have been up to 18 m long and similar to a great white shark in behavior, and it may have competed with Livyatan for prey. But take all of this with a grain of salt: the decaying Megalodon jaws in this painting represent more anatomy of this shark than has ever been found as a single fossil.
The Aquatic Sloth, Thalassocnus (Miocene–Pliocene)
IT WOULD BE DIFFICULT FOR FOSSIL SLOTHS TO PRESENT A GREATer contrast with their extant, famously slow tree-living descendants. Modern sloths spend much of their time hanging from canopies of South America rainforests with strongly hooked claws, occasionally moving around to browse leafy vegetation. Their low-speed physiology probably reflects an energy-poor diet of hard-to-digest foliage, although some species supplement this with more nutritious food, such as insects and fruits. Sloths have many biological quirks, such as their weekly trips to the ground to defecate, their surprisingly strong swimming abilities, and their cultivation of camouflaging algae in their fur. Further peculiarities lay in their evolutionary history.
Sloths—also known as folivorans—are part of the South American mammal lineage Xenarthra, an anatomically radical group that also includes armadillos, anteaters, and the extinct, ankylosaur-like glyptodonts. Sloths appeared in the early Eocene and achieved a much broader diversity of size, geography, and ecology than is suggested by their living representatives. They became elephant-sized armored megaherbivores, bear-sized burrowers that excavated their own caves, and even aquatic species that foraged in shallow marine habitats. Sloths and other xenarthrans colonized North America when volcanic eruptions in today’s Panama created a land bridge to South America in the latest Pliocene. This connection ended sixty million years of isolation for South America, and it allowed the faunas of the different landmasses to interact for the first time. This event, which transformed the biotas of both continents, is known as the “Great American Interchange.”
The extinct giant sloths are characterized by huge claws on all limbs, necessitating many species to walk on the knuckles of their hands and the sides of their feet. Their claws likely had roles in defense, digging, and bringing foliage toward the body, whereupon powerful, muscular lips could bring leaves into the mouth. A short, stout tail may have propped up rearing sloths when they stood on two legs to feed. Trackways suggest that large sloths walked on all fours, though the bulk of their weight was carried by their hind limbs. Excellent sloth fossils exist in both Americas, showing that the last of the giant and bear-sized sloths existed just eleven thousand years ago—recently enough that mummified remains, including fur and feces, have survived to modern times in caves.
One of the most remarkable fossil sloths was Thalassocnus, a genus comprising five species adapted to aquatic life. This realm was never explored by other xenarthrans, making Thalassocnus a pioneer for its group. Over time, Thalassocnus species became increasingly adept at swimming and foraging in marine habitats. The oldest Miocene forms were semiaquatic animals that, judging from their tooth wear and bone chemistry, ate vegetation adjacent to beaches, while the more recent, Pliocene species show evidence of entering deeper water to eat seagrasses and algae. Along with increasingly long snouts and stronger lips (indicated by enlarged facial nerve openings), these more aquatically adapted Thalassocnus also had dense bones that acted as ballast in water, compensating for the buoyancy created by air in their lungs. Thalassocnus lacks strong adaptations for swimming and was likely a bottom-walker, using its large claws to grip the seafloor. A slight broadening of its shins and forearms may have enhanced their performance as paddles however, and the hind-limb reduction seen in more aquatically adapted species hints at increasing commitments to swimming behaviors in the last of the Thalassocnus lineage. We might speculate that, had they not gone extinct, another few million years of evolution might have shaped Thalassocnus into species resembling dugongs or manatees in appearance and lifestyle, sloths entirely adapted to aquatic life.
Grasses, Near-Hyenas, and Horses (Miocene)
GRASS IS SUCH A UBIQUITOUS PLANT IN THE MODERN DAY THAT it’s difficult to imagine a world without it. In fact, though grasses and sedges have their origins in the Cretaceous Period, large grassy areas such as prairies, savannas, and steppes did not appear until the Oligocene and Miocene, when woodland environments gave way to open spaces. A shift to cooler and drier global climates probably played a major role in this change, along with greater seasonality and increased wildfire frequency favoring resilient, opportunistic, and rapidly growing grasses over trees and shrubs. Grasses have long been grazed by herbivorous animals (fossilized feces show that grasses were even part of dinosaur diets), but the establishment of huge grasslands allowed mammals to become specialized graminivores—species that eat little else but grass. As tough, abrasion resistant plants, grasses are difficult to digest, forcing graminivores to evolve sophisticated digestive systems to extract their nutrients. Among the greatest challenges of eating grass are tiny silica crystals known as phytoliths. These exist inside grass leaves and rapidly wear down the teeth of animals that chew them. Grazing mammals have responded to these with stronger, deeply rooted teeth that allow them to crush and grind grasses in powerfully muscled mouths. In turn, rather than defending themselves with toxins or thorns, grasses have adapted to cope with graminivores through an elevated regenerative capability, their leaves growing from the base of the plant and thus continually regenerating after they are eaten.
Unlike woodlands or scrubby forests, grasslands are exposed settings that offer little opportunity to hide from predators, and good grazing pastures can be situated great distances from each another. Mammals exploiting these new open habitats were forced to adapt to face these challenges. Horses, a perissodactyl clade with origins in the early Eocene, became grassland specialists in the Oligocene and Miocene after spending most of the earlier history as woodland animals. Many of their adaptations to grassland life are typical of those of other mammals pursuing such lifestyles. Their large body size increased their travel efficiency and deterred predators, while relocating their eyes toward the top and back of their heads allowed them to scan for danger even when feeding. Elongation of limb bones below the knee and elbow, and reduction of toe and finger counts, adapted horse limbs for fast running, a useful trait for traveling long distances as well as for evading predators. Hipparion, a common Miocene–Pleistocene pony-sized horse of northern continents, embodies these features. Shown here, it likely resembled a modern horse in most regards, although if we saw one we would immediately note its three-toed feet. Hipparion walked on single hoofs on each limb, but two small toes were situated on either side of their principle digits. Accumulations of Hipparion fossils suggest it lived in groups or herds, a common antipredator behavior among modern grassland mammals. Digestive specialisms of perissodactyls mean that Hipparion was probably better equipped for eating dry, high-fiber foliage than the artiodactyls it coexisted with, allowing vast herds of these differently adapted mammals to coexist on the same grasslands.
The presence of many herbivore species in grassland settings allowed carnivorous mammals to develop their own plains specialists, adapting their anatomy to hunt big game in these new environments. Fossils show that larger members of the Carnivora (the guild that includes most mammalian carnivores) principally entered these niches. Dogs, cats, bears, and their extinct relatives either adapted to endurance running to chase down prey, or developed stealthy behavior and camouflage coloration to ambush unwary animals. One of the most formidable Miocene carnivorans was the lion-sized hyena-like predator Dinocrocuta gigantea, shown here. This large (perhaps 200 kg) predator was equipped with an enormous, powerful skull and bone-smashing teeth, and is thus very reminiscent of a hyena. Dinocrocuta was accordingly once thought to be a member of the hyena clade, but it is now considered a percrocutid, a close hyena relative. Healed injuries in the skull of a hornless rhinocerotid, Chilotherium wimani, match the teeth of Dinocrocuta and show that it pursued live animals. If percrocutids hunted like the largest living hyena (the spotted hyena, Crocuta crocuta), prey would have been worn down over long chases and weakened from crippling bites to the legs and abdomen. We might imagine that, as with most mammalian carnivores, Dinocrocuta would have begun eating their prey as soon as it was too exhausted or injured to run further, regardless of whether it was still alive.
Pelagornis, the Largest Flying Bird (Miocene)
FLYING BIRDS ARE SO FAMILIAR TO US THAT IT’S EASY TO TAKE them for granted, but they are true marvels of evolution and adaptation. Nowhere is avian adaptability more obvious than in their flight styles. Though they all share essentially the same basic body plan, birds have shaped their proportions into a myriad of forms that permit flight styles for every type of habitat. Some species, like parrots and crows, are generalist fliers that are able to flap, glide, and maneuver with equal skill. Others, including many water birds, are reliant on steady, powerful flapping to fly long distances. Birds widely regarded as poor fliers, such as turkeys, pheasants, and other gamebirds, should actually be considered launch specialists, bursting into the air almost vertically before traversing several hundred meters to avoid danger. Hummingbirds have an almost insect-like flight mechanism where highly mobile, rapidly beating wings facilitate a steady and agile aerial capability.
The birds that make flight look easiest are those adapted for soaring, a flight mechanism that exploits uplifted air currents (deflected winds and thermals) and supreme long-distance gliding abilities to fly for extended periods without flapping. Soarers are characterized by long, narrow wings and relatively large body sizes. Modern birds with the largest wingspans (about 3 m), the wandering albatross and the Andean condor, are both soaring specialists. Fossils show that, in Deep Time, soaring birds were even larger. Members of the “pseudotoothed” bird lineage known as pelagornithids had the largest wingspan of any known flying bird at 6–7 m from wingtip to wingtip. Another bird, Argentavis magnificens (a teratorn, a type of predatory birds related to New World vultures), is sometimes reported as having an even larger, 8 m wingspan. This estimate is almost certainly too generous, however, as all known Argentavis remains are smaller than those of large pelagornithids: a wingspan of 5–6 m is more likely. Pelagornithid proportions recall an exaggerated version of the albatross body plan and their wing spread may have approached the maximum size for a flying bird, any further wingspan increase being curbed by the hind-limb-dominant takeoff strategy common to all avians. Despite their magnitude, the largest pelagornithid (Pelagornis sandersi) probably weighed just 20–40 kg, thanks to its small body and short hind-limbs. It was essentially a beak with a pair of giant wings attached.
Comparative anatomy and flight models show that pelagornithids were supreme marine soarers. They might have ridden on strong winds like albatross, moving over waves using only minute motions of their wings to control their passage, or perhaps soared to great heights on over-sea thermals like frigate birds. They seem to have had restricted abilities to flap, and they probably limited this action largely to take off. The cosmopolitan distribution of pelagornithid fossils indicates an ability to cover long distances with ease. We cannot know for certain how long these birds could remain airborne, but if they were like modern oceanic soarers, they could have spent most of the year flying around the planet, returning to land only to lay eggs and feed their offspring. Their ferocious-looking jaws are equipped with toothlike spikes along each edge: these “pseudoteeth” recall the dentition of fish- or squid-eating animals, and pelagornithids surely foraged for these creatures as they toured oceans and seas. A highly modified shoulder-wing joint makes their ability to take off from water questionable, and it’s possible that they caught most or all their prey while in flight.
Pelagornithids are an ancient bird group, first appearing in the Eocene. Their relationship to other birds has been a matter of debate as, until recently, well-preserved and complete pelagornithid remains were unknown and comparisons to other bird species were limited. Traditionally, pelagornithids have been allied to other large marine bird groups, such as pelicans or albatross, but newly discovered fossils hint at affinities to nonsoaring birds including game birds (chickens, grouse, and pheasants) and waterfowl (ducks and geese), though these ideas are not without critics. While neither gamebirds nor waterfowl are known for their soaring flight, they had wide adaptive range in the early Cenozoic, including wader-like forms and giant flightless herbivores like Gastornis and mihirungs. Is it inconceivable that this lineage could develop giant soaring forms as well?B
OUR INTELLECTUAL BIAS TOWARD HUMANITY’S OWN EVOLUTIONary history means that the diversity of nonhuman fossil apes is often overlooked when we’re recounting the story of life. Humans and other modern apes—gibbons, orangutans, gorillas, and chimpanzees—arose as part of an evolutionary radiation of primates that began in the Miocene and saw apes of many kinds spread across Africa, Europe, and Asia. Ape fossils are rare but, largely through accumulations of fossil teeth and jaws in caves and other sheltered settings, we know that many parts of the world were once inhabited by multiple, coexisting ape species. The fossils of our own Homo line are among these, and scientists are still assessing how we—a lineage of technologically advanced, highly adaptable ground apes—fit into these communities, and when we began to significantly influence the history of our relatives.
Among the most mysterious of all fossil apes is Gigantopithecus blacki, a large species known from Miocene–Pleistocene fossils of southeast Asia. Thousands of teeth of Gigantopithecus have been recovered, but the rest of its skeleton, save for a handful of broken lower jawbones, is entirely unknown. This leaves much about this famous primate shrouded in mystery, and any reconstruction of it—including the one opposite—is extremely speculative. Even its size is not well constrained. Its teeth and jaws are slightly larger to those of the biggest living ape, the gorilla, and it is generally assumed that G. blacki was among the largest apes of all time. However, with only a few broken jaws hinting at the size of the skull, and no idea how large the head was in relation to the body, our size estimates are wide-ranging and of questionable reliability. Conservative estimates suggest that G. blacki may have stood just a little taller than a large gorilla (around 2 m), while others predict a gigantic standing height of 4 m. The latter seems overoptimistic given the size of the jaw fossils, and the depiction opposite accordingly shows an animal somewhat, though not unduly, bigger than a large silverback gorilla. It towers over the early human (Homo erectus) in this scene, but bear in mind that H. erectus is somewhat smaller than H. sapiens, with an average height of about 1.65 m.
Gigantopithecus is thought to be a member of the orangutan line (Ponginae), but it is unlikely to have been a giant version of these living apes. At times Gigantopithecus has been restored as a fully upright, somewhat humanlike ape because features of its lower jaw have been linked to a humanlike neck posture. This idea has caught on in some circles, particularly among cryptozoologists hoping that Sasquatch or yetis might be surviving Gigantopithecus, but in reality this interpretation is very speculative: there is nothing about Gigantopithecus fossils that convincingly indicates an upright, humanlike posture. If it was anything like large living apes, Gigantopithecus was likely a quadruped, and, in being the size of a large gorilla, it probably did not spend much time in trees. Its jawbones imply a relatively short, deep skull, and extensive wear on its teeth suggest the presence of very large, powerful jaw muscles, perhaps more akin to those of gorillas than orangutans or other apes. The same dental wear patterns imply a diet of very tough, coarse vegetation. Bamboo was abundant in the tropical regions inhabited by Gigantopithecus and was likely a common food source, along with other types of foliage. Such a diet implies a large gut to digest fibrous plant matter, another feature that adds to our mental image of Gigantopithecus as a heavyset, ground-based herbivorous ape. These lifestyle inferences contrast with the ecology of orangutans, which mainly forage for fruit and insects in trees. Perhaps, despite its pongine affinities, Gigantopithecus was much more gorilla-like than orangutan-like in habit and form. These ideas are mainly conjecture, of course, and will remain so until we develop a better understanding of Gigantopithecus anatomy.
Insect Societies and the Giant Asian Pangolin (Pleistocene)
MANY ANIMAL SPECIES EXPLOIT THE ADVANTAGES OF LIVING IN groups, but few have made sociality and cooperation as essential to their existence as the eusocial insects. These are insect species with highly organized societies, characterized by a cooperative approach to rearing young, the year-round presence of adults, and segregation of individuals into castes (most often into roles for reproduction [queens], foragers and builders, guards, and flying individuals that disperse and create colonies elsewhere). With all members of the colony sharing the same genes, eusocial species are considered “super organisms”: species for which life as individuals is impossible, and only collectives can survive.
The hymenopterans (bees, ants, and wasps) and termites are the most dedicated eusocial insects. Ants and termites are major contributors to the planet’s biomass thanks to some species forming colonies with millions of individuals. The nests constructed by these animals rank among the most sophisticated natural structures on the planet, drawing parallels to human settlements in their ability to provide their inhabitants with comfort and security. Nests can be made of many materials in varied settings, including the inside of rotting wood, within soil, as mud mounds of varying size (some gigantic, being many tens of cubic meters in volume), and in elevated positions (such as within tree branches or under overhanging rock) using paper or wax. Each nest provides its owners with safety and shelter, as well as dedicated spaces to store or farm food and to raise offspring. Particularly sophisticated nests, such as termite mounds, have mechanisms to keep the inhabitants cool against elevated external air temperatures.
The patchy insect fossil record means some uncertainty exists regarding when various insect lineages committed to communal living, and fossils of ancient nests are very rare. Many alleged fossils of Mesozoic nests have been identified, but most of these lack characteristic features of true insect colonies and their identification as ancient ant or termite structures is highly controversial. Recently discovered fossils of Early Cretaceous termites preserved in amber show that they had evolved eusocial behavior by one hundred million years ago. Ants of this time probably also lived in groups, though perhaps not yet in huge colonies. The capability to farm fungus for food is predicted to have appeared at some point in the Cenozoic for both groups, an idea consistent with the identification of fossil “fungus gardens” among Tanzanian Oligocene termite nests. Evolutionary models predict that Late Cretaceous bees had also developed various grades of social behavior.
Eusocial insect colonies represent enormous quantities of protein to any animal that can breach their nest defenses to harvest their inhabitants. To ants and termites, such creatures are the stuff of nightmares: animals with huge claws and powerful limbs that can excavate their underground shelters or smash through walls; thick, sometimes armored skin that resists attack from guard castes; and extremely long, sticky tongues that extend through nest corridors and chambers to grab panicked citizens. These are creatures like anteaters, armadillos, aardvarks, and—as shown here—pangolins. Though sharing similar adaptations, these animals are not closely related, their common features being the result of convergent evolution. Pangolins seem to have evolved in the Eocene from the same branch of mammals that gave rise to Carnivora, though their slow, trundling habits and massive scales (made from the same material as fingernails) clearly distinguish them from their carnivorous cousins. Today confined to Africa and Asia, pangolins also once roamed Europe and North America. A giant Asian species—the 2–2.5-m-long Manis paleojavanica—lived in parts of Indonesia during the Pleistocene. Much of southeast Asia was covered in savanna-like habitat at this time and was populated by mound-building Macrotermes termites: ideal prey for this large pangolin. The extinction of the giant Asian pangolin coincides with the arrival of humans into its range, and human predation may have had a role in their demise. Similar fates await the eight pangolin species alive today, all of which are hunted in huge numbers because of pseudoscientific beliefs about the medicinal properties of their scales, as well as for their meat. The long-lived pangolin lineage may be extinct before the year 2050 because of these practices.
FEW SPECIES ARE AS EMBLEMATIC OF OUR RECENT ICE AGE AS Mammuthus primigenius, the woolly mammoth. The recovery of numerous frozen mammoths from permafrost in Russia and Alaska, as well as abundant skeletal remains from other countries, has provided an extremely detailed insight into their biology. Our mammoth specimen inventories span the full spectrum of age and gender and preserve their tissues to the cellular level, allowing for exceptional insights into their genetics, stomach contents, life appearance, and growth regimes. These large proboscideans are closely related to Asian elephants and existed in large numbers across the Northern Hemisphere throughout the Pleistocene. They were among the last of the mammoths, but they were not, as sometimes depicted, the largest—they were actually similar in size to our modern African elephants. They were well adapted to life in cold climates by having small ears, a short tail, sophisticated body integument comprising dense underfur and longer outer hair, and a layer of insulating fat around the body. Stomach contents show that they did not live among deep snow and glaciers but spent much of their time in open habitat known as “mammoth steppe”—a cold-adapted grassy ecosystem that still survives in a handful of locations in Siberia. Mammoths and other large herbivores were essential to maintaining these environments, their removal of shrubs and trees preventing forests from overwhelming the grasslands.
Woolly mammoths have a long and complex history with several types of humans, including early members of our own species and Homo neanderthalensis—the Neanderthals (opposite). We both relied on mammoth bones and tusks for a variety of uses, including the fashioning of tools as well as the creation of shelters. Human hunters would have wanted to remain on mammoth steppe because of the wealth of game but, without forests, wood would have been scarce, and mammoth bones and tusks were often the only objects available to create large structures. Multiple examples of humans having processed mammoth carcasses are known, but how frequently we or other humans hunted mammoths remains controversial. Healed spear wounds in mammoth fossils record failed predation efforts, but they are not particularly common. Mammoths were surely dangerous game for our Pleistocene ancestors, so perhaps even powerful, robust humans like Neanderthals generally avoided them in favor of smaller, less challenging prey. However frequently we hunted them, mammoths clearly made an impression on early humans, their forms being depicted frequently in Pleistocene cave art.
Our relationship with and reverence for the woolly mammoths have not halted despite these animals’ extinction fourteen thousand to ten thousand years ago. We continue to use their tusks to create art and objects, because the decline of living elephants and the heightening of antipoaching laws make it easier to trade ivory from mammoths than from modern proboscideans. The resurrection of mammoths by cloning is also a continued point of discussion among scientists and journalists, a question with more at stake than simple scientific curiosity: if a long-extinct animal such as a mammoth can be re-created, extinction need not be forever. Opinion is split about the feasibility of cloning mammoths, though even optimistic scientists would concede that numerous hurdles currently stand in the way of this goal. Even the best mammoth genetic material is degraded to an extent that it is not a viable blueprint for a living individual, and—even if we had a perfect sample—the complexities associated with turning a genome into a flawless set of chromosomes, and ultimately mammoth cells, are huge. If we get to the stage of having a lab-grown mammoth egg cell, we then face the challenges of implanting them into surrogate mothers—2.5 tonne Asian elephants. These animals are rare, are protected because of their endangered status, are an inevitable practical nightmare as laboratory subjects (consider that we’d need many, many elephants to have a shot at cloning success), and they’re also prone to developing tumors when their reproductive cycles are interrupted. Science aside, this latter point is the tip of an ethical iceberg for this most ambitious “de-extinction” project. In all likelihood, mammoths will remain extinct, at least for the foreseeable future.
A Dwarfed Giant Horned Turtle (Pleistocene)
WE HAVE MADE IT NEARLY TO THE MODERN DAY WITHOUT MEETing one of the most familiar, and also most bizarre and mysterious, of all reptiles: the testudinatans—better known as turtles. The turtle branch of evolution is a long one, stretching back to at least the Triassic, with a Permian origin predicted in some evolutionary models. Although occupying a range of habitats, body sizes, and lifestyles, throughout that time turtles have remained dedicated to their Bauplan of a shelled body; a beaked face; and short, stout limbs. The number of living turtle species is estimated to exceed 350, but some data indicates that this count may be too conservative, and that as many as 470 species may exist today. At least half of these are at risk of extinction, and a full third are classed as “endangered” or “critically endangered,” mostly because of habitat degradation.
The relationships of turtles with other reptiles are hotly debated. Turtles lack the twin jaw muscle openings that characterize most reptile skulls, and this has linked them with the Parareptilia, a group that contains the pareiasaurs and procolophonids we met earlier in the Permian and Triassic Periods. Parareptiles branched off the reptilian tree close to its roots, and if turtles belong to this group, they would be the sole survivors of a very ancient reptile lineage. Turtle DNA, however, suggests that this is incorrect. Genetically speaking, turtles seem more closely related to modern reptiles, either as close relatives of lizards and their kin, or as relatives of the archosaurs. Uncertainty on this issue is compounded by a historic lack of fossils representing the early phases of turtle evolution. For many decades, the oldest known member of the turtle line was the Triassic Proganochelys quenstedti, a relatively “primitive” turtle compared to later species, but still a fully formed turtle with all the unique anatomical features of the group. Thankfully, newfound Chinese fossils have begun to shed light on the morphology of turtle ancestors and, although their evolutionary significance is contested at present, their discovery gives hope that we will eventually pin down how turtles are related to other reptiles.
Turtles are anatomically alarming even to experienced biologists. Though outwardly unassuming, they are some of the most extremely modified of all tetrapods. Their shells are made up of two elements: an underside (plastron) and an upper portion (carapace). These are joined by bony bridging elements running along the side of the animal. The carapace is made from bone formed within the skin, as well as incorporating ribs, vertebrae, shoulder blades, and pelvic elements. It requires some serious rejigging of typical tetrapod anatomy to enclose the limb girdles within the rib cage! Virtually all turtle-line reptiles have toothless, beaked mouths, but only some species can withdraw their heads and necks into the shell. Most turtles, including a large portion of living species, can pull their head and neck in only sideways, partially covering those body parts with the front of the shell. Turtles likely had a terrestrial, burrowing origin, but they have adapted to aquatic life numerous times in their evolutionary history. It’s for this reason that terms like tortoise and terrapin lack strict definitions: these are categories for lifestyle and appearance than true evolutionary groupings.
Oceania was once home to a remarkable turtle group: the meiolaniids, or horned turtles. Meiolaniid origins stretch back into the Cretaceous, and these animals must have been remarkable to see alive, being large (up to 2.5 m long) and bearing spiked, armored tails in addition to impressive-looking cranial horns. They were fully terrestrial in habit and probably subsisted on low-lying vegetation, primarily grasses. They became extinct only three thousand years ago. Among the last of their kind was the 1-m-long dwarf island species, Meiolania platyceps (shown here), which existed on Australia’s Lord Howe Island. Fossil sites with hundreds of human-butchered Meiolania bones suggest that human hunting had some role in their extinction.
FOR THE LAST EIGHTY MILLION YEARS, NEW ZEALAND HAS BEEN isolated from the other southern continents, allowing for the development of unique ecosystems containing many types of flightless bird. Flightless parrots, rails, and kiwis survive today, albeit in such low numbers that many species only survive through intensive conservation efforts. The largest and most spectacular of New Zealand’s flightless birds, the moas, are no longer with us, following zealous predation by human settlers arriving in New Zealand just a few hundred years ago. Moas are ratites, and thus part of the same avian lineage as ostriches, emus, and kiwis, though they are not closely related to kiwis despite their shared New Zealand home. DNA analysis has revealed the surprisingly complicated evolutionary history of ratites, wherein moas have closer affinity to the South American tinamous than they have to kiwis or even Australian ratites, such as cassowaries and emus. This is far from the only shakeup of bird relationships brought on by genetic data. Many branches of the avian family tree are now differently arranged compared to evolutionary relationships deduced from physical anatomy alone.
Except for a few highly stylized paintings by historic Maoris, no records detailing moa appearance or habits are known. Thus, while moas died out around 1400 CE—a nanosecond ago in the context of geological time—we must reconstruct their lives as if they had been extinct for millions of years. This is a sharp reminder of the finality of extinction, and a sobering thought given the critically low populations of so many species across the planet today. The moa fossil record begins 16-19 million years ago, but it took just 150 years from the arrival of humans in New Zealand to reduce their populations to unsustainable levels. Archaeological sites preserve the rafts and cooking tools which transported and processed the carcasses of these often-enormous birds, animals that—until our arrival—had never been concerned about large ground predators. It seems the only natural predator of adult moa was the Haast eagle (Harpagornis moorei), a predatory bird comparable in size to our largest living raptors. As moa populations dwindled, the Haast eagle also became extinct. Both are part of one of the largest extinction events of the Holocene Epoch: the loss of thousands of bird populations throughout Polynesia, brought on by humans spreading south.
Moa have left behind an extensive fossil record including skeletons, mummified remains, and footprints, providing a detailed picture of their biology. Their modernity also means we have access to their genetic information, and this has helped resolve a long-standing controversy over the number of moa species. Moas vary in many aspects of proportion and body size, and we once had Pleistocene and Holocene moa divided into over twenty species. Only nine are now recognized, however, thanks to DNA showing that pronounced size differences between moa individuals were a result of gender dimorphism, not taxonomic separation. Female moas were typically the larger gender, with female members of the Dinornis genus being particularly huge: 280 percent larger than males. This is the most significant dimorphism of any bird species or land mammal.
Though superficially ostrich-like in appearance, moas were generally heavyset birds better adapted to walking than running. Some, like the giant moa (Dinornis robustus) shown here, were huge animals that stood taller than a human and weighed almost a quarter of a tonne, but others—like the bush moa (Anomalopteryx didiformis)—were much smaller, just over 1 m tall and only 20–50 kg in mass. All had lost their wings entirely, the only hint of a forelimb being a finger-sized remnant of their shoulder girdle. At least some were covered head to toe in feathers, though whether this was true of all species remains unknown. Their diets and preferred habitats have been revealed by preserved stomach content and careful assessments of their fossil sites. All moas were herbivores, but they differed in their habitat and food choices, allowing many species to live across the varied landscapes of New Zealand without stepping on each other’s ecological toes.
APPROXIMATELY THREE HUNDRED THOUSAND YEARS AGO MODern humans, Homo sapiens, speciated from other members of the Homo group in Africa. Genetic data shows that we are a mongrel species, having interbred with other Homo species encountered in our travels away from our African home. The DNA of Neanderthals and another, largely mysterious human lineage, the Denisovans, still lingers in our genes.
Humans are part of the ape lineage Homininae, a group of especially intelligent primates that, on the evolutionary path to you and me, became especially adept at technological innovation, problem-solving, and communication. Though seemingly anatomically odd compared to other apes—most obviously because of our long legs and short arms (these seeming to be adaptations to an obligate upright stance and long-distance travel), our anatomy is entirely normal for primates. Our limb proportions, for example, are not so different from those of monkeys. Our bodies look largely devoid of fur, but only because our hair is mostly a short, fine pelt that has the appearance of naked skin to all but the closest inspection. We are, in fact, as hairy as any other primate. Our adaptations for standing and walking on two legs are better developed than those of our relatives, but are not unique to us: many primates have adaptations for bipedality. Our faces and bodies are also ornamented with hair and fatty tissues to advertise our vigor and fitness, just like other primates. And beyond our anatomy, even our social structure, though culturally varied, is primate-typical, revolving around prolonged care of offspring and the formation of clans comprising related and unrelated individuals.
What really sets us apart from other apes, and perhaps all other animal species, is our technological prowess. Tool use is common to animals of all kinds, even in species that lack grasping hands, such as birds or dolphins. But no other species have developed their technological capability to a point where they can bypass many major challenges of natural selection. Our technology has removed barriers against our spread and proliferation, allowing us to quickly overcome natural challenges that would take typical evolutionary mechanisms multiple generations to respond to, and our unprecedented ability to store collective knowledge allows successive generations to improve on the innovations of our ancestors. These abilities have allowed us to transform much of the planet into settlements entirely conducive to our own needs and safety, and in the process we have created a new, now-widespread form of habitat: the urban environment. Humans are an exceptional and remarkable lineage, the likes of which has never existed on Earth before.
Our success has not been without cost, however. The results of our continued population expansion and resource use have come into sharp focus in recent years, it being apparent that human activities are having environmental impacts on a global scale. The emissions from our chosen energy sources are causing rapid, worrying shifts in global climate. Our waste and pollutants are found in all habitats and environments around the world, even drifting into the deep ocean and to landmasses uninhabited by humans. Widespread transformation of land into farms and homes has left little wilderness and has reduced animal and plant populations to dangerously low levels. Our actions are now even detectable at the geological level, leading some to propose an “Anthropocene” Epoch—a period of geological time defined by the presence of humanity.
These actions are causing a biodiversity crisis comparable to the “Big Five” great mass extinctions. We are living through a biological cataclysm, and our data is clear: we are the cause. Long-term degradation of the biosphere means that uncountable, maybe most, species are at risk, whereby even relatively common “low concern” species are struggling. This crisis will ultimately affect us too. Widespread waste and pollution, collapsing marine and terrestrial ecosystems, and shifting climates are already impacting food availability and quality, our health, and the habitability of our towns and cities. The impact we are having on Earth is not just a problem for wildlife: it is a problem for all life. How we choose to respond to these facts in the next few years and decades will determine as much about our own future as it will the fate of the natural world.
IN 1778 CE, THE BRITISH EXPLORER CAPTAIN JAMES COOK LANDED the HMS Resolution on the Hawaiian Islands. The arrival of Europeans on Hawaii instigated major changes to the natural history of the islands, bringing a suite of human hunters and habitat changes as well as introducing cats, mongooses, and pigs into ecosystems ill-equipped to cope with them. Hawaii’s wildlife had suffered a suite of extinctions since the arrival of Polynesian settlers in the ninth to tenth centuries, but the influx of Europeans increased the extinction rate considerably. Among the affected species was the Hawaiian goose, or nene (Branta sandvicensis). This mid-sized, soft-voiced goose is endemic to Hawaii, having evolved from Canada geese blown to the islands in storms. It is characterized by half-webbed feet, long legs, distinctively textured plumage, and relatively terrestrial habits compared to other geese. From an estimated population of about twenty-five thousand in 1778, populations fell to just twenty to thirty wild birds in 1951, with thirteen birds in captivity. It seemed near certain that the nene would join the hundreds of endemic Hawaiian animals to have become extinct in the last thousand years.
Nene, however, are still with us, and their wild population is slowly growing. They are alive today because the same species that almost drove them to extinction decided to save them. From the 1950s onward, intensive nene conservation efforts were introduced that included captive breeding and release programs, control of introduced predators on Hawaii, and designation of sanctuary areas for wild goose populations. From those twenty to thirty birds, over one thousand wild Hawaiian geese now exist, with another thousand in captivity in zoos and wildlife parks around the world. The species is still at risk because of its small population and considerably reduced genetic diversity, but it now has a fighting chance of survival.
The story of the nene has been replicated numerous times by biologists and conservationists across the world. Restorative efforts have seen iconic species like pandas become abundant enough in the wild to be removed from endangered species lists. Eradication of introduced species on islands and careful control of fishing quotas are restoring ecosystems to historic balances and are allowing native species to re-establish themselves. Entire populations of endangered amphibians have been captured to exist in captivity to preserve them against habitat loss. The amount of effort that goes into such schemes is incredible: even small conservation projects have enormous practical and organizational demands, as well tremendous scientific and economic requirements. The challenges of rearing captive organisms or keeping tabs on wild individuals are vast, especially against the continued background of habitat degradation, poaching, and shifting climates. In reality, the resources needed to preserve species and environments often fall well short of what is needed, and not all efforts are successful. The sad fact of modern conservation is that we have to choose our battles, weighing available resources against conservation needs and their likelihood of success.
But at the core of any conservation effort is a decision that a species or an environment is worth saving, and with enough groundswell action can happen. Our biodiversity and environment are at crisis point, and it is only through changing our views on the importance of the natural world, and taking responsibility for the impact we’ve already had, that we will avert further loss among our remaining species and habitats. Unlike the meteorite strikes or volcanism that catalyzed previous extinction events, we are a mass extinction with a conscience. For the first time in our planet’s history, the decisions of conscious, sentient beings will determine the shape of life through future ages.
