In Pleistocene northwest Europe, prehistory met modernity as recognisable species of walrus, hyena, and swans existed alongside now extinct southern mammoths, cave lions, woolly rhinoceros, and great auks. The modern natural world emerged gradually from what we class as ‘prehistory’ without a sudden transformation of flora, fauna or environment, the product of billions of years of biological evolution, extinction, and environmental change.
Building a Planet Fit for Life (Hadean)
THE STORY OF LIFE ON EARTH BEGINS SEVERAL HUNDRED MILlion years before the first organisms appeared, taking us back 4.55 billion years to the origins of our planet, sun, and solar system. While these events may seem far removed from the origin of life, they were essential in providing the right conditions for the development of life: the construction of a global habitat, the gathering of raw materials for organismal bodies, and the positioning of Earth in the right part of the solar system for plentiful, but not excessive, solar light and heat. Though it’s probable that earthly conditions are not essential for all life (life-forms on other celestial bodies, if they exist, would have to evolve with biochemistry and environmental tolerances reflecting their own environments), Earth’s properties and place in the cosmos are key factors in the presence and development of life on this planet.
Before Earth was formed, the atoms and molecules that would one day form living beings were indivisible from those that would create the sun, eight planets, and thousands of celestial bodies that make up our solar system. At this point, our constituent elements were mixed in a vast interstellar molecular cloud, the remnants of long-deceased stars. We existed as masses of hydrogen, helium, some heavier elements, and pockets of dust and gas, hanging out together in a small portion of the Milky Way galaxy. Around 4.6 billion years ago, a fragment of our molecular cloud—perhaps prompted by the energy of a local supernova—began to collapse under its own gravity and coalesced into a star: our sun. This pulled additional portions of our cloud into orbit around the new star, forming a broad ring of jostling dust and gases that collided and fused with one another. As coalescing bodies of cosmic dust created ever-larger celestial objects, the sun’s orbit became a bustling ring of planetesimals (solid objects measuring a kilometer or more across) and protoplanets (moon-sized objects), the largest growing rapidly as their increasing gravitational mass hoovered up debris from their orbits. Eventually, most of the dust and debris from this protoplanetary disk stabilized into the order of our solar system: a number of smaller stellar bodies and eight planets, one of which was Earth. But our planet was far from habitable. For at least several tens of millions of years, if not hundreds of millions, Earth remained an angry red ball floating through space: a highly radioactive planet superheated from collisions with other stellar objects and under continued bombardment from asteroids. Earth’s first 500-million-year-long eon is appropriately known as the Hadean—“hellish”—Eon.
But despite the chaotic and violent origins of Earth, its position and properties were promising for life. Earth is a relatively heavy planet and has thus been pulled close to the sun, but it is not so close as to be excessively hot, nor so far away as to receive diminished light and heat energy. Our orbit around the sun is within a narrow temperature band that allows for the existence of liquid water at the planet’s surface. Once Earth cooled, likely relatively early in the Hadean Eon, liquid water was able to cover 70 percent of the planet. This not only provided a major habitat for life, but also liberated water for use in our biochemistry. Liquid water is a critical chemical component for all organisms on Earth and life on our planet would be radically different, if it existed at all, without it. Our atmosphere was attained and lost several times in our early history thanks to influences of solar and geological events, but it was ultimately instrumental in the creation of oceans and seas. Once Earth was cool enough (about 4.4 billion years ago), the first clouds began raining the oceans into existence on our newly formed planetary crust. Tectonic activity—the creation, destruction, and motion of crustal plates—began in the Hadean and created the first small continents by the end of the eon. Subsequent tectonic activity would add continental material to these early landmasses to form the continental plates we know today. Associated volcanism transferred complex chemicals from within the Earth to its surface, and these were likely instrumental in the formation of the first nonorganic biomolecules—the raw components of life. From a vast stellar cloud of stardust, this 500-million-year process created a planet with a strong potential for originating and sustaining life.
The Origins of Life (Archaean)
DETERMINING HOW, WHEN, AND WHERE LIFE APPEARED ON Earth has preoccupied humanity for centuries. This question has been considered both philosophically, as means to understand humanity’s place in the universe, as well as religiously, with countless faiths developing creation myths to account for the advent and development of life. But our best, and realistically only, mechanism for understanding the origins of life lies with scientific approaches. While it remains true that scientists have yet to determine exactly how life began on Earth, it would be wrong to say that nothing is understood about the origins of life at all. Quite the opposite is true, and we are narrowing down how this seemingly miraculous event could happen through fundamental chemical and physical processes.
The oldest incontrovertible evidence for life on Earth is 3.5 billion years old, though other evidence indicates an even earlier origin at 3.9 billion or even 4 billion years ago. All three figures imply that life appeared rapidly after the formation of the planet, despite the early Earth being a turbulent place that would be largely inhospitable to the life of today. Earth of the early Archaean Eon was still experiencing vigorous tectonic activity and was pounded by meteors left over from the formation of the solar system. The oceans and atmosphere, although probably cooler than they had been during the Hadean, were of a composition radically different from those of more recent times.
These conditions might seem like the last place where organic beings could arise, but a planet of churning complex molecules and frequent electrical discharges was probably an ideal setting for the chemical reactions needed to catalyze life. Inorganic matter has some self-organizing properties because of inescapable laws of chemistry and physics, so we can be certain that complex chemicals and biomolecules were forming on ancient Earth without guidance or stimuli. From these came the fundamental components of organismal cells and, with them, the potential for the first basic lifeforms. There are no physical or chemical components of our bodies that are unique to us as living beings, nor is there evidence that life requires a magical “spark” or supernatural ingredient to be created and sustain itself. We are separated from inorganic forms only by specific processes of sustaining, growing, and reproducing our chemical makeup, these likely being defined and refined at a very basic level billions of years ago by chemical and biological evolution.
It is assumed that our inorganic–organic transition occurred in stages. Although true life has never been experimentally created, some of its earliest stages have been artificially generated in conditions predicted for Hadean or Archaean environments. Raw biomolecules—such as the components of cell membranes and amino acids—have been abiotically constructed from mixes of gases, liquids, and electrical charges, and the addition of certain minerals has been shown to enhance self-organization of these molecules into forms that could become “protocells” under the right stimuli. It is not at all unreasonable to assume that the full inorganic–organic transition will one day be fully understood and achieved under simulated conditions.
Inorganic biomolecules are known to occur in a number of extreme environments, such as volcanoes, mid-ocean ridges, and even interstellar space, giving scientists a number of potential sources of organic material and locations where life could have arisen. Among the most likely settings are mid-ocean ridges, where a combination of heat and abundant organic molecules could have created the first simple cells. We might not imagine life originating at the explosive, magma-spewing mid-ocean ridges we are most familiar with, however: hydrothermal fields—vast plains of limestone towers created by uprisings of geothermal water—are rich in hydrocarbons (the basic components of cell membranes) and are better candidates for life’s earliest habitat. These dark, deep-water settings form the subject of the adjacent illustration.
Stromatolites (Archaean–Proterozoic)
ALTHOUGH LIFE APPEARED ON EARTH RELATIVELY QUICKLY, IT seems to have remained relatively low-key, even microscopic, for most of Earth’s history. The proliferation of macroscopic, multicellular organisms—those we can easily see with the naked eye—is a relatively recent evolutionary event that occurred only 600 million years ago. This is not to say that early life was not conspicuous in other ways, however. Visitors to Earth as far back as 3.7–3.5 billion years ago had a good chance of seeing stromatolites, mounds of sediment bound together by films of cyanobacteria (simple, photosynthetic cells also known as blue-green algae).
Stromatolites have an excellent fossil record that reaches its zenith about 2.5 billion years ago, before declining as other, more complex life appeared at the end of the Proterozoic Eon. They survive today, but largely in places inhospitable to other organisms, such as lagoons with high salinities. Their evolutionary nemeses are grazing invertebrates, such as snails, which devour defenseless cyanobacteria with such ferocity that stromatolites can now grow only in habitats intolerable to these invertebrate herbivores.
As mounds of bacterially bound sediment, stromatolites look unremarkable. They are significant organisms for many reasons, however. Oxygen-breathing organisms such as ourselves owe much to stromatolites for their transformation of our ancient, carbon dioxide–rich ancient atmosphere into a breathable, oxygen-rich one. Without their billions of years of oxygen-deploying photosynthesis, the story of evolution on Earth would be entirely different. Stromatolites also form the majority of our record of early life and have the accolade of being the oldest known fossils. The most-ancient examples are from Australia and Greenland, and these specimens have an increasingly good chance of being the oldest fossils we’ll ever know. Early Archaean rocks are very rare on Earth’s surface today and most are so heavily distorted from ages of intense heat and pressure that their fossils and even sediments are beyond recognition. The chances of discovering new, undistorted Archaean sediments lessens as Earth’s surface geology is explored more completely and, with the loss of these sediments, our odds of finding older fossils diminishes too.
Mechanisms of stromatolite growth are also noteworthy. Their mounds—which range from narrow columns, mere centimeters across, to vast domes many meters wide and just as high—are created as layers of mud are trapped in calcium carbonate (a substance otherwise known as limestone, the skeletal material of choice for many organisms). Stromatolites produced calcium carbonate as a by-product of converting sunlight to energy and, although their mud and limestone mounds are crude structures compared to the shells and skeletons in other life-forms, they were the first examples of living organisms using rigid body frameworks to enhance their survival chances. Stacking sediment and limestone into towers and mounds lifted the bacterial cells from the sea bed, bringing them closer to sunlight; reducing their chances of burial by current-swept debris; preventing neighboring mounds from overgrowing them; and enhancing their resistance to turbulent, stormy seas. Collections of stromatolites formed the first reefs, structures that are essential foundations of certain coastal habitats even today.
Cutting a stromatolite open reveals a banded structure representing periods of fast and slow growth, reflecting cycles in seasons and light availability. Variation in column thickness provides a record of the drama of a growing colony struggling against sediment being washed over the cells. In calm periods they thrived, the colony growing strong and spreading wide to create robust, thick columns. But large influxes of sediment could bring disaster, burying most of the colony and forcing the cells closest to the surface to begin the building process again, if they survived. A stromatolite fossil not only provides a record of ancient life, but also tells us much about ancient environments and climates.
MULTICELLULAR LIFE HAS ARISEN INDEPENDENTLY SEVERAL times. Plants, animals, and fungi developed the capacity to form bodies and discrete tissue types independently of one another, and in all likelihood other forms of now-extinct multicellular life existed in Deep Time, particularly in the Proterozoic. The fossilization potential of these early multicellular organisms is extremely low however, and in any case most rocks from the Proterozoic Eon are unlikely to preserve fossils on account of their deformation. Despite this, a few tantalizing fossils of early, macroscopic multicellular life, such as the 2.1-billion-year-old Gabonese Francevillian biota, have snuck through to the modern day. These recently discovered marine organisms—perhaps best interpreted as amoeba-like colonies arranged into sheetlike lobes, chains, and tubes—are known from only a single locality, but they give hope to those of us wanting to know more about the early evolution of life. It’s reassuring to know that some surprises still await us in our increasingly well-surveyed geological record.
A more detailed and well-studied suite of macroscopic, complex organisms is the Ediacaran biota. These organisms, which occur as fossils in rocks of Australia, northern Europe, Canada, Russia, and southern Africa, thrived between 600 million and 542 million years ago. Some survived into the Cambrian Period before becoming extinct 510 million years ago. Distinctive assemblages of Ediacaran species are recognized, seemingly correlating to different habitats such as coastal and deltaic settings, river systems, and deep marine environments.
What sort of organisms the Ediacarans were, and how they lived, are matters of ongoing discussion. Their anatomy is more complex than anything known on Earth until this time, but, alas, most Edi-acarans were entirely soft-bodied and fossilized only as sediment impressions, limiting our understanding of their bodies and tissues. Most species have a pleated or ribbed appearance which is often described as “quilted.” Clear morphological distinctions between different species suggest they were well differentiated ecologically. Frond-like species, such as Charnia, which could grow up to 2 m in length, seem to have been particularly diverse and abundant. Dickinsonia is generally thought to be a low-lying mobile mat, while Kimberella resembles a sluglike creature, its association with trace evidence of movement and apparent “head” anatomy adding credence to this interpretation. Some species were clearly burrowers, having left evidence of tunneling traces in ancient sediments. Microbes and different forms of algae were also abundant in Ediacaran times, and together with the Ediacaran organisms they might have formed some of the first complex ecosystems, the precise workings of which are still largely unclear.
How Ediacarans are related to living species also remains an open question. Might they be related to some of the earliest forms of animals, such as corals? Are they macroscopic, highly evolved planktonic organisms? Bizarre bodies of fungi or algae? Sophisticated bacterial mats? Or none of the above—might they be an entirely independent, now-extinct branch of multicellular life without any close living relatives? The growing consensus seems to be that Ediacarans are not a single evolutionary branch of their own, but a collection of species of varying ancestry. Some may be unique experiments in multicellular life, but others might be related to animal types still in existence today, including mollusks (the group that includes slugs, clams, and squids), cnidarians (the coral/jellyfish clade), and sponges, as well as some grades of early animal evolution that defy easy categorization. Research into Ediacarans is still in its relative infancy, the significance of their fossils being appreciated only in the 1950s and many key discoveries being made only in recent decades. Much work lies ahead to better understand the evolutionary relationships and lifestyles of these fascinating organisms.
THE FOSSIL RECORD BEGINS IN EARNEST IN ROCKS FROM THE Cambrian Period, the first interval of a 541-million-year division of time known as the Phanerozoic Eon (meaning “visible life”—a reference to the ubiquity of fossils in rocks of this time). This sudden change in fossil abundance reflects the widespread evolution of mineralized animal tissues: shells, spines, skeletons, and mouthparts composed of hard-wearing biominerals like calcium carbonate and hydroxyapatite. These hard parts are more readily fossilized than soft tissues like muscle, internal organs, or skin because they rot slowly, are less tempting to scavengers, and are more resistant to physical wear and tear. Historically, the sudden abundance of fossils in Cambrian rocks was thought to reflect an “explosion” of animal diversity—a rapid genesis and evolution of complex organisms after eons of nothing but microbes—but a more nuanced interpretation has replaced this today. Genetic data, geochemistry, and rare fossils indicate that many animal lineages were established several tens or even hundreds of millions of years before the Cambrian, but their lack of mineralized body tissues has precluded them from entering the fossil record on all but the rarest occasions. Animal life certainly diversified in the Cambrian, but it was building on a foundation of lineages established before the Phanerozoic. This was still an explosion of diversity in many respects, but perhaps not the “ground zero” of animal evolution we’ve historically assumed it was.
Many Cambrian animals defy easy interpretation, but careful research has revealed that many are related to animal lineages we know today. Arthropods—the group that includes insects, crustaceans, and arachnids—were abundant and diverse at this time, scuttling and swimming around Cambrian habitats in a great variety of forms. The famous trilobites were among them, but comprised only a fraction of Cambrian arthropod diversity—a status they would later overturn. Mollusks were also present and had already differentiated into many of their major lineages. Reefs were being constructed by sponges and mineralizing microbes. Corals were present in the Cambrian but were not yet major components of reef structures—they would not adopt this role until the next geological period, the Ordovician. Small eellike creatures existed with basic, rodlike cartilaginous skeletons. These creatures would eventually give rise to vertebrates, animals with spinal cords and internal skeletons.
A major factor in the development of mineralized body parts was the need for firm tissues for defensive and offensive use: body armor to deter predators, jaw parts to process prey, and claws and hooks to seize food or fight attackers with. Shelled animals from the close of the Ediacaran Period show that an evolutionary arms race between predators and prey was already underway before the Cambrian, and the Phanerozoic saw this develop further as animals were equipped en masse with armor to resist ever-tougher and more powerful mouthparts, pincers, and rasping equipment. Among the most formidable predators of this interval were the anomalocarids, free-swimming relatives of arthropods that grew up to a meter long. These unusual animals were equipped with a pair of gripping arms, two insect-like compound eyes, and a ring of slicing mouthparts. Two rows of segmented fins along their bodies enabled them to float around reefs in the manner of cuttlefish, seizing prey with their arms before crushing and slicing it with their jaws. Exactly what anomalocarids ate is uncertain, though various arthropods and soft-bodied organisms were probably common prey. Some species, however, have elongated arms equipped with fine combs, which they may have used to snag small prey and particles of food in the water column, rather than hunting down larger prey on the seabed.
TRUNDLING ACROSS THE SEAFLOOR FOR 270 MILLION YEARS WERE the trilobites, creatures familiar to anyone who searches for fossils among Paleozoic rocks. Trilobites lived in seas all over the planet and left an extensive fossil record of over ten thousand species, most of which stem from their heyday in the first half of the Paleozoic. Their later history was less successful: trilobite diversity dwindled in the Devonian to leave just one group, the proetids, hanging on until their final demise at the end of the Permian.
There is little doubt that trilobites are arthropods, but exactly how they are related to other members of this group is not clear. It might seem surprising that the origin of trilobites remains mysterious given their excellent fossil representation and long research history (scholarly interest in trilobites dates to 1698), but this is not uncommon when working on long-extinct animals. Evolutionary processes are complex, our fossil record is very incomplete, and even well-known groups present only limited anatomical information for piecing together their evolutionary histories. New discoveries of exceptionally preserved trilobites as well as other early arthropods promise to shed light on their affinities in future.
Well-preserved trilobite fossils have provided detailed insights into their anatomy, even their internal organs. Their calcified exoskeletons were divided into three units: the head (or cephalon), a series of body segments (thorax), and a shield-like terminal plate formed of fused body segments (pygidium). The head is often characterized by a bulbous central structure that looks like it should house a brain, but it actually held the stomach. The gut extended from this through the thorax and pygidium, and waste was expelled at the rear of the body. Their eyes were perched at the side of their heads and are the most sophisticated sense organs we know of among early animals. Like all arthropods, trilobites had eyes comprised of numerous tessellating lenses but, unlike insect or crustacean eyes, their lenses were not made from protein. Instead, each lens was a finely shaped calcite crystal: trilobite eyes were basically made of stone. This might seem primitive, but experiments actually show that trilobites had excellent visual acuity, seeing in sharp detail and with considerable depth of field. Variation in eye shape reveals the importance of vision to different trilobite species. Fast-moving swimmers had enormous eyes with wide visual ranges, some had overhanging “sun shades” to maintain their vision in strong sunlight, and low-light or burrowing species lost their eyes altogether. Further anatomical sophistication was found in trilobite legs. Each limb had two branches: a lower branch for walking or swimming, and an upper branch supporting a gill for respiration. Legs close to their mouths sometimes bore bladelike structures to assist with the processing of food, which was likely soft prey in many species, such as worms. When threatened, trilobites were able to roll into tight balls to protect their delicate undersides, and they are often found like this as fossils (though it’s best not to think about the possibility of them being buried alive to preserve in this way).
Trilobites adapted their anatomy into numerous different body shapes, sizes, and lifestyles. The most typical trilobite morph was a probably a seabed-roaming grazer or predator-scavenger, much like the large (70-cm-long), widespread species shown opposite, Ogyginus forteyi. But with only a few tweaks to their anatomy, very different lifestyles were possible. Species with small bodies but enormous, wide heads filter-fed with their legs while performing headstands. Some tiny-bodied, large-eyed species swam upside down through water. So-called effaced species—those that smoothed over their facial and body contours—were able to burrow. Taxa with numerous body segments and expanded, wide thoraxes may have relied on microbes living in their gills for sustenance. We are still learning how to interpret the different body shapes and adaptations of trilobites but, whatever they got up to, it clearly served them well for much of the Paleozoic.
The Ordovician–Silurian Extinction
THE STORY OF LIFE ON EARTH IS NOT ONE OF EVER-INCREASING biological diversity: species also become extinct. The fossil record shows that extinction is a routine part of the evolutionary process, the natural result of organisms being unable to adapt to changing conditions. The normal rate of this process is termed “background extinction,” and it is generally offset by a comparable rate of speciation—the generation of new species. But some periods of Earth’s history show dramatic increases in extinction rates, where the fossil records of numerous species, and maybe even whole evolutionary lineages, terminate at a common level in geological sequences. These are indications of relatively sudden and widespread periods of environmental stresses which killed off lifeforms that could not adapt quickly enough to new conditions. Many such extinction events are known from Deep Time and several—colloquially known as “The Big Five”—were global-scale mass extinctions that radically altered the course of evolutionary history. The most famous of The Big Five are the Permian–Triassic and Cretaceous–Paleogene events; with the others occurring at the Devonian–Carboniferous boundary, the Triassic–Jurassic boundary, and at the end of the Ordovician Period.
Taking place 444 million years ago, the Ordovician extinction event is the oldest global mass extinction we know of. It seems to have eliminated 85 percent of species alive at that time. Many famous Paleozoic fossil groups—the trilobites, the planktonic graptolites, the hagfish-like conodonts, and the bivalved shellfish known as brachiopods—were badly affected, though few major groups were wiped out. The Ordovician extinction was a considerable squeezing of biodiversity rather than, as with other extinctions, a curtain call for major groups of plant and animal life.
The Ordovician extinction seems to have taken place in two pulses, the first being a period of global cooling. Ordovician lifeforms were adapted to very warm, greenhouse conditions but, as the period came to a close, they found themselves in an icehouse world cold enough to allow glaciers to form over the poles. The supercontinent Gondwana—a fused landmass composed of what has since become the southern continents—was situated over the south pole at this time, allowing an ice sheet more than 6,000 km wide to grow in the southern hemisphere. The expanding glaciers sequestered ever-greater stores of the planet’s water, ultimately lowering sea level by a staggering 50–100 m and emptying shallow seas around the planet. Shallow marine settings are extremely species-rich, so their loss brought heavy casualties to Ordovician life. This lowering of sea level also changed oceanic circulation and chemistry, decreasing oceanic oxygen levels while elevating concentrations of hydrogen sulfide—a substance dangerous to animals when in high concentrations. Eventually, even deep-water species that were not affected by sea level fall risked annihilation.
But just as life began to adapt to these hostile conditions, the planet warmed, the glaciers retreated, sea level rose, and oceanic chemistry and circulation reverted to pre-extinction conditions. Though seemingly a return to normal, this sudden reversal brought on a second pulse of extinction, affecting those hardy species that had been prospering in the half-million years associated with the glaciation. It took until the middle of the Silurian Period—about 430 million years ago—for life to attain its prior diversity. But unlike other large extinction events, the postextinction biosphere returned to a similar, pre-extinction configuration instead of transforming into something radically different. Ecologically speaking, the evolution of life resumed its prior course. As we’ll see later, other mass extinctions marked more dramatic changes in the development of life on Earth.
.Jawless and Jawed Fish, and “Sea Scorpions” (Silurian)
VERTEBRATES—ANIMALS WITH A STIFFENED ROD OF BONE OR cartilage supporting their bodies, surrounding a chord of nervous tissue—first appeared in the Cambrian Period. This group, to which we belong, maintained a low profile for much of the early Paleozoic Era, living in seas dominated by invertebrates. The earliest forms were probably something like modern lampreys, eellike creatures that lack a true skeleton and feed through rasping, jawless mouths. By the start of the Silurian Period, vertebrates had evolved into a number of jawless forms with distinctly fishlike body shapes, some of which bore small scales, others sporting mineralized armor. These early fish were the agnathans.
Armored agnathans include the heterostracans (middle left in opposite illustration) and osteostracans (bottom right). They are characterized by broad shields over their faces and large, overlapping scales along their trunks and tails. Thelodonts (bottom left and middle right) were not armored, but instead covered in minute, mineralized scales. Most of these early fishes were small—perhaps 10–20 cm long—but some were giants, attaining body lengths of a meter. Even so, they were far from the most formidable animals in early Paleozoic seas and likely used well-developed senses—as indicated by fossilized details of their brain and sensory anatomy—to avoid danger, along with tough dermal tissues and spines to dissuade predators. The diversity of agnathan body shapes indicates that they were adapted for a number of lifestyles, and they represent the bulk of fish diversity throughout the Silurian, declining only in the Devonian thanks to the rise of jawed fishes—the gnathostomes.
Acanthodians were among the earliest gnathostomes, first appearing in the Silurian and reaching their evolutionary acme in the Devonian. They are more typically fishlike in anatomical detail than the agnathans, including the presence of well-developed jaws and teeth. Jawed mouths had a complex evolution, with both internal and external anatomy being co-opted for biting actions. Jawbones evolved from structures that ancestrally supported a gill, while teeth share properties with structures that once covered the skin. Being able to bite, rather than merely rasp or suck, was a major development for vertebrates that was critical to their occupation of predatory roles in Devonian seas. Invertebrate predators, such as crustaceans and scorpions, rely on different structures to apprehend and process food, while vertebrate mouths permit the dismembering, immobilization, and devouring of prey with one organ. This enables a more efficient and streamlined body plan, as well as the capability to develop and optimize one organ, rather than several, for predation and ingestion. The continued success of animals like sharks—which first appeared in the Silurian, if not the Ordovician—shows the potential inherent in mounting a powerful jaw on the front of a streamlined, swimming vertebrate body.
Swimming above these early fish is a eurypterid, a giant arthropod that belongs to the same group as spiders, horseshoe crabs, and scorpions. These “sea scorpions” were major predators in early Paleozoic seas, and they have an evolutionary history from the Ordovician to the end of the Permian. They were particularly abundant and diverse in the Silurian and were able to tear into prey with large pincers. Though many eurypterids were well adapted to walking and likely patrolled the sea floor (as well as freshwater settings, and perhaps even terrestrial environments in their later evolution), some had reduced limbs save for a pair of paddle-like swimming appendages that propelled them through water. At up to 2.5 m long, the biggest eurypterids were among the largest predators of the early Paleozoic as well as the largest arthropods of all time. They must have been intimidating animals to witness in life. But the heyday of eurypterids was fleeting. Their diversity declined early in the Devonian and they lost their status as arch predators, never to regain it. The cause of this is unknown, but competition from jawed fish may have been a factor.
Plants Colonize the Land (Silurian)
FOR BILLIONS OF YEARS THE CONTINENTS WERE BARREN, LIFEless masses drifting over Earth’s mantle. These stark landscapes offered few opportunities to early life, having little in the way of food or nutrients for animals and only limited options for habitation by microbial organisms. For life to escape the seas and colonize the land, continental habitats needed to offer environments with abundant amounts of chemical energy and sufficient nutrients to build and maintain organic matter. The organisms that rose to this task, and changed the evolution of life on Earth forever, were the vascular plants.
Both animals and plants were experimenting with terrestrialization throughout the first half of the Paleozoic. Fossil footprints show small arthropods making quick trips out of the water as early as the Cambrian, genetic studies suggest that bryophytes (mosslike plants) existed on land in the same period, and fossils of the earliest land plants date to the Ordovician. But it was not until the late Ordovician and early Silurian that land plants were anything more than small mossy carpets, a change marked by newly evolved plants raising parts of their bodies from the ground via short stalks. They could do this thanks to an important innovation: vascularity. Vascular plants are distinguished from bryophytes by the presence of internal tubes that ferry water and nutrients around their bodies. The products of photosynthesis (the process of converting light to chemical energy) are carried in tubes known as phloem, while xylem, tubes made of a hard substance called lignin, transport water. Lignin is also the material that, in sufficient quantity, makes plant tissues woody and rigid.
The vascular plants colonizing mid-Silurian landscapes were small, no more than a few centimeters tall, and had a relatively simple vascular system. Their most famous genus, Cooksonia (shown here), was a globally distributed branching plant that lacked leaves but bore swollen sporangia at the tips of its stems for distributing reproductive spores. Cooksonia and other early land plants lacked roots, these having no purpose on an Earth without deep soils. As a mix of organic and rock matter, soils could not form in earnest until sufficient biomass had accumulated on land, so the soils encountered by Silurian plants were very thin, likely just millimeters or centimeters deep. It was not until the Devonian that soils of modern depths and nutrient quality were achieved.
The utility of vascularity was quickly realized by land plants. An internal transportation system allowed their bodies to grow large and differentiate their tissues, exchanging nutrients between organs specific to photosynthesis and organs adapted for absorbing water. The lignin in their tissues allowed for the development of more complex and intricate anatomies, including long branches, stabilizing grapples, and anchoring roots. By the end of the Silurian Period another plant genus, Baragwanathia, demonstrated many of these features, and compared to Cooksonia, it was a giant of many tens of centimeters in height. Plant communities from the early and mid-Devonian included multiple species further capitalizing on these benefits, forming small, shrubby forests. The first trees and seed-bearing plants appeared halfway through the Devonian, about 385 million years ago—just 50 million years after Cooksonia and kin began growing tall.
The development of land plant communities had many significant impacts on Earth’s atmosphere, geology, and biosphere. Plants cooled the planet, absorbing atmospheric carbon dioxide (a potent greenhouse gas) as they photosynthesized and locking it into biological systems, as well as slowing erosion as roots bound rocks and soils. This not only changed how landscapes were shaped, but also slowed nutrient cycling between land and sea. New, plant-dense habitats also created new environments for land animals to live in. The first to exploit these were the arthropods, creatures that followed plants onto land in the Silurian. Arthropods thrived in this world of early land plants, untroubled by other types of land animal for millions of years.
MANY OF THE LARGEST CREATURES IN OUR OCEANS TODAY FEED on some of the smallest: plankton. They obtain these tiny prey items using filtration mechanisms that preclude the need for precision prey capture: they simply aim their open mouths at volumes of plankton-rich water and, after engulfing water and food alike, sieve their food using filters or combs. One mechanism for this is ram feeding, swimming with mouth agape to strain plankton through fine, comblike gill rakers. Manta rays and the largest sharks (the basking and whale sharks) use this method. Rorqual whales employ a strategy known as lunge feeding, where mouthfuls of food are sieved from water forced through sheets of brushlike baleen lining their jaws by contraction of a huge, powerful throat. The rorquals include the largest animals to have ever lived, including the fin and blue whales (up to 33 m long).
Growing large on a plankton diet is not a modern biological innovation. During the Jurassic and Cretaceous Periods the principle large ram feeders were the pachycormids, a globally distributed lineage of bony fish that matched the biggest living sharks in size. The largest was the Jurassic species Leedsichthys problematicus, which averaged between 7 m and 12 m in length but on occasion might have reached 15 m. The closely related Bonnerichthys gladius was also a large animal, perhaps achieving 5 m in length. Some members of the sand shark lineage (odontaspidids) may have also experimented with planktivory toward the end of the Cretaceous. Curiously, for all their longevity, frequent development of giant size, and anatomical diversity, the Mesozoic marine reptiles do not seem to have capitalized on this niche in any great capacity.
But the pioneer giant planktivore was the Devonian placoderm Titanichthys agassizi, pictured here. Titanichthys was among the last of a lineage known as the arthrodires, a diverse and abundant group of armored fish that occupied numerous roles in marine ecosystems throughout their fifty-million-year history. They were, for the time, the biggest animals on Earth, with both Titanichthys and the predatory arthrodire Dunkleosteus terrelli reaching around 6 m in length. Complete remains of these giants remain elusive because their skeletons were mostly composed of cartilage, which rarely fossilizes, leaving only their relatively robust skulls and armor to persist in the rock record. Fossils of Titanichthys have been found in rocks across the United States, Europe, and Africa since the late 1800s, but much remains uncertain about its paleobiology. This includes aspects as fundamental as the number of species: seven Titanichthys species have been named (five of which are from the area that is now the United States), but only three are represented by relatively good fossils. We do not yet appreciate how much Titanichthys individuals differed from one another or how their proportions changed with growth, and the provenance of many Titanichthys fossils—exactly where they were found, and in what rock layers—is poorly recorded. All these factors complicate assessments of their diversity.
It’s also only in recent years that we’ve developed a reasonable understanding of Titanichthys skull structure and found stronger evidence of a ram-feeding lifestyle. Unlike Dunkleosteus, which had famously robust jawbones shaped into bladelike biting surfaces, Titanichthys had smaller, relatively slender jawbones lacking blades or other toothlike structures. It also had small eyes and a peculiarly downturned lower jaw that likely expanded the circumference of the mouth when opened. These features are interpreted as being adaptations to ram feeding, and we should probably picture Titanichthys obtaining its food by swimming through plankton-rich seas with its mouth open, establishing a lifestyle that numerous species would emulate in the millions of years to come.
Lakes of the Early Carboniferous
THE FRESHWATER FISH OF THIS EARLY CARBONIFEROUS SCENE represent the continued success of fishes through the late Paleozoic Era. Though some early fish lineages, such as the placoderms, were now extinct, the sharks, ray-finned fishes (actinopterygians), and lobe-finned fishes (sarcopterygians) went from strength to strength. One group of sarcopterygians—the rhipidistians—had adapted to live in estuaries, rivers and lakes in the Devonian. This transition would be important for the evolution of limbed fish (including ourselves) throughout the Devonian and Carboniferous Periods. Once adapted in to life in freshwater, the rhipidistians split into two major groups: the lungfishes (Dipnoi) and the tetrapodomorphs—the fishes that gave rise to limbed vertebrates that could walk on land.
Carboniferous lakes and rivers were occupied by some of the largest freshwater fish of all time: the rhizodonts (center in opposite image). These tetrapodomorphs first appeared in the Devonian, and they became extinct in the late Carboniferous. Some were moderately sized—less than a meter long—but several species reached 5–7 m: the same length as a modern great white shark. They must have been formidable animals, having stress-resistant jaws with two rows of teeth, 20-cm-long fangs, and bodies covered in tough, platelike scales. The tissues of their teeth were arranged in convoluted rings that increased their strength, allowing the teeth to be used in forceful stabbing motions even though they were only weakly anchored to the underlying jawbones. These features cast rhizodonts as powerful predators that surely ate other large vertebrates. Details of their teeth imply that strong armor, rather than large size, was the best defense against them.
Rhizodont forefins were broad, stiff, and capable of a wide range of movement. They seem adapted to facilitating rapid changes in direction. But rhizodont bodies were otherwise long tubes, with their dorsal, anal, and pelvic fins situated far down the body, effectively being incorporated into the tail fin. This arrangement seems better suited to powerful, rapid acceleration than to high speed, and their unusually flexible vertebral columns allowed them to twist not only sideways—as we typically associate with fish—but also up and down. This body plan must have made rhizodonts agile, maneuverable swimmers despite their size, and perhaps also assisted with dismembering large prey: it’s hypothesized that rapid, forelimb-aided shaking or tail-aided corkscrewing may have been used to rend big animals into consumable pieces. The enhanced sensory system that ran across rhizodont faces and scales probably allowed these fish to detect prey and obstacles even in murky water. In short, if you ever have the chance to visit the Carboniferous, you needn’t take your swimming costume.
Rhizodonts lived alongside lungfishes (middle right) and Gyracanthus (left and right). Lungfishes have an extensive fossil record that peaks in the Triassic, the apex of their diversity and distribution. They are most famous—and from our perspective, most evolutionary-significant—for developing air-breathing lungs that are the precursors to our own. They are not the only fish capable of absorbing oxygen from air, but their lungs allow them to exist outside of water for sustained periods, a habit that some modern lungfish (such as the spotted lungfish, Protopterus dolloi) use to weather droughts in mucus-lined shallow burrows. Gyracanthus is a mysterious acanthodian known almost entirely from ornamented forelimb fin spines and shoulder bones. Many fossils of this large (perhaps 1.25 m long) fish are known from Devonian and Carboniferous rocks, and these give some indication of a small head and triangular body section. Further details of its full form remain elusive, however.
Pulmonoscorpius, Giant Scorpion of Ancient Scotland (Carboniferous)
READERS WITH A PHOBIA OF BUGS, SPIDERS, AND OTHER SPECIES with more than a sensible number of legs would not enjoy hikes in Carboniferous landscapes. Though the swamplands and the lush flora of giant-scale trees, early conifers, horsetails, and ferns would make for excellent walking country, the presence of extremely large arthropods would activate bug-based phobias in all but the steeliest individuals. Large, semiaquatic eurypterids scurried between ponds while 2-m-long millipede relatives, Arthropleura, scuttled through swampland in search of nutritious plant matter. Large dragonfly-like insects known as meganisopterans (of which the 70-cm-wingspan Meganeura is the most famous) buzzed overhead, and Pulmonoscorpius kirktonensis—a 75-cm-long Scottish scorpion (shown opposite)—stalked smaller arthropods and our own tetrapod ancestors. Scorpions have a long evolutionary history beginning in the Silurian, and Pulmonoscorpius is the largest to have ever lived.
Though the most famous of the giant arthropods are Carboniferous in age, their experiments with gigantism were a long-lived phenomenon that extended into the Permian. Species of mayfly, the extinct paleodictyopterans, and several types of wingless insects also attained elevated body sizes in these periods. Exactly what facilitated late Paleozoic arthropod gigantism remains a topic of contention. The traditional explanation is that higher levels of atmospheric oxygen (“hyperoxia”) enhanced animal respiration, permitting more powerful muscle activity and the capacity to support a larger, heavier skeleton. An alternative spin on the hyperoxia idea is that smaller arthropods found elevated oxygen levels somewhat toxic, promoting the evolution of giants that were better able to cope with high oxygen levels. Both ideas have some support from experiments on living insects.
However, there are several counterpoints to the hyperoxia hypothesis that are not easily shaken off. Among the most critical is that most Carboniferous and Permian arthropod species were not gigantic, and it has not been demonstrated that arthropods of this time were, on average, generally larger than at other times in history. This is an important point, as it means we’ve yet to determine if we’re dealing with a few exceptional species (which would likely reflect specific adaptive or ecological innovations of a few lineages) or a global trend of arthropod gigantism (which is more consistent with environmental conditions elevating arthropod size limits). It is also clear that hyperoxia is not essential for large terrestrial arthropods, thanks to the modern, strongly terrestrial coconut crab (Birgus latro). This 4-kg giant, which has a leg span of 90 cm, remains on land via a sophisticated lunglike organ adapted for breathing air, a structure somewhat analogous to the “book lungs” of spiders and scorpions. We know that Pulmonoscorpius had a similarly adapted respiratory mechanism and, if it functioned as well as its coconut crab equivalent, it may have been capable of absorbing all the oxygen Pulmonoscorpius needed even at modern-grade oxygen levels. While this does not explain gigantism in late Paleozoic insects (they rely on a less-efficient gas exchange mechanism formed from tubes invading their bodies), it demonstrates that not all arthropods need unusual environmental factors to evolve to huge size. The lack of vertebrates on land until the late Paleozoic is another factor we must consider: these animals were surely predators of large arthropods, as well as competitors for the arthropods’ food, so their absence in the early Carboniferous may have created relaxed ecological conditions for arthropods to experiment with body size. More work is needed to explain why some Carboniferous and Permian arthropods became so large, but we should note that the ideas discussed here are not mutually exclusive. Evolution and adaptation are complex, and it’s entirely plausible that hyperoxia, anatomical innovation, and relaxed ecological pressures in Carboniferous landscapes could have played complementary roles in boosting the body sizes of certain ancient arthropods.
The Tetrapods Invade the Land (Carboniferous)
LIFE’S INVASION OF LAND TOOK OVER ONE HUNDRED MILLION years to complete. We have already met some of the pioneering land plants and arthropods, but we’ve yet to cover the final stage in this process: when fish hauled, crawled, and slid from the water onto land, eventually evolving to walk and breathe in terrestrial settings. The evolution of land vertebrates, or tetrapods, is widely regarded as one of the most significant steps in vertebrate evolution, and it was a far-reaching event for the Paleozoic biosphere.
The broad strokes of tetrapod evolution have been known for some time, but only in recent decades have we begun to uncover the details and specifics of this major evolutionary event. The creatures shown in this early Carboniferous scene represent some of the first tetrapod-like animals, species that had limb-like fins but were not yet capable of carrying themselves over land in a true walking gait. They likely propelled themselves along on their bellies, using their appendages to push and pull their bodies across lake margins and riverbanks. The landscapes they entered were not the more familiar richly forested swamps of the latter Carboniferous Period, but the less densely vegetated precursors to these environments.
Different grades of tetrapod evolution are shown in this illustration. Animals such as the unnamed species on the far left of the scene and the fishlike whatcheeriid on the right represent the epitome of vertebrate land evolution at this time. They probably still spent much of their time in water, feeding on fish or aquatic invertebrates, but their deep bodies and relatively powerful limbs enabled them to enter terrestrial settings when they desired. Seeking safety, exploiting new foraging opportunities, and travelling between aquatic habitats could have been catalysts driving this behavior. It’s likely that their forays onto land did not require particularly unusual gaits, because many near-tetrapod fish—both living and extinct—can crawl or walk with their fins even when they’re submerged in water. Thus, the major challenge for early tetrapods was not walking or crawling per se, but simply carrying their bodies out of the supportive medium of water. Fossils show that tetrapods quickly developed longer and stronger legs once they were experimenting with land-based locomotion, allowing for longer bouts of terrestrial behavior and more efficient means of moving around out of the water.
The large, eellike species in the center of the scene is a colosteid, a long-bodied animal with diminutive limbs and a flattened body profile. Although they’re somewhat more closely related to the true tetrapods than to the other animals shown here, colosteids were likely very sluggish land creatures and may have reverted to an entirely aquatic way of life from a semiterrestrial ancestor. The fact that colosteids split from the main tetrapod line and reversed the general trend of their evolution is a great example of the complexity of natural selection. When discussing grand evolutionary events such as vertebrate terrestrialization, we can give the false impression of species moving toward goals, striving for a generationally distant biological optimum. But lineages like the colosteids show that evolution is far more opportunistic: organisms make the best of whatever works for them at the time, not what might be ideal in one hundred generations. It seems that, for colosteids, abandoning experiments with terrestriality in favor of returning to a more aquatic way of life was more successful than following the adaptive trends of their relatives. As we learn more about the early history of Tetrapoda, we see that it is a story not just of fish leaving the water, but of vertebrate groups adapting to life at the water’s edge. Only some of these groups would seek further evolutionary novelty by pushing further inland.
Platyhystrix, a Sail-Backed “Amphibian” (Carboniferous–Permian)
BY THE END OF THE CARBONIFEROUS, TRUE TETRAPODS WERE roaming the landscapes and swampy waterways of the world. One group, the amniotes, had already split from the rest of the tetrapod lineage and would, aside from the need to drink, sever all essential ties with water. Their final hurdle to an entirely land-based existence concerned reproduction: while the first tetrapods were competent land animals, they still had to lay their eggs in aquatic settings. This was overcome with the evolution of the amniote egg: a tough-shelled, desiccation-resistant capsule containing adequate energy and fluids needed to support and nourish a developing embryo, as well as an efficient gas exchange system to supply oxygen and expel carbon dioxide. Now fully able to exploit terrestrial settings, the amniotes gave rise to the two great lineages of land vertebrates: the diapsids (reptile-line animals) and synapsids (mammal-line animals).
Another major tetrapod lineage arose in the Carboniferous: the temnospondyls. These diverse animals are what we think of as “prehistoric amphibians,” although their relationship to living amphibians is debated. Temnospondyls were mostly somewhat salamander-like species that lived in or around water, but some species were more terrestrially adapted. They attained a range of body shapes and sizes that permitted occupation of a number of ecological niches. Many were crocodile-like, snatching fish or ambushing shorebound prey with broad, flattened heads filled with teeth. Some spent much of their time on land and may have competed with carnivorous amniotes for prey, while others became giant marine predators. Like living amphibians, they had larval stages where they bore long gills emerging from the back of their heads, and we sometimes find these in their fossils. Temnospondyls were a long-lived lineage that was particularly speciose in the Carboniferous, Permian, and Triassic Periods and, although their diversity and abundance lessened throughout the Mesozoic Era, they persisted in some parts of the world until the Early Cretaceous (and maybe longer, if living amphibians are their descendants).
Among the most remarkable of the temnospondyls was Platyhystrix rugosus, a large (about 1 m long) species from latest Carboniferous and earliest Permian rocks of the southern United States. The sail of this animal is most striking and was formed of elongate extensions of its vertebrae fused to bones in its skin. Platyhystrix is a dissorophid, a temnospondyl lineage characterized by strong terrestrial capabilities and extensive body armor. It is characterized by long, robust limbs; well-developed limb girdles; a strong spinal column reinforced by bones in the skin; and a high level of skeletal ossification: attributes which equipped Platyhystrix for sustained periods of walking and running. Dissorophid skulls were powerfully built and bore numerous conical teeth around the jaw margins and within the mouth. These leave little doubt of their predatory habits, and their well-developed eye and ear anatomy likely outfitted them with suitable senses to locate prey as well as detect danger. With these adaptations and their widespread distribution, dissorophids may have been important land predators during the late Carboniferous and the early Permian. Their carnivorous diet is confirmed by bite marks and shed teeth associated with fossils of Permian synapsids: this temnospondyl not only competed with amniotes for food, but actually made meals of the competition.
Several dissorophids have sails, but they are not ubiquitous among the group. The function of the sails is not clear, but several ideas have been suggested: to regulate body temperature, to reinforce the spinal column, as defense structures, or as sociosexual display devices. We’ll return to the issue of sailed animals soon.
Harlequin Mantella Frog and Other Amphibians (Holocene)
MODERN AMPHIBIANS—THE CLADE LISSAMPHIBIA—CAN BE DIvided into three major lineages: frogs, salamanders, and caecilians. Frogs are overwhelmingly the most speciose group, representing something like 90 percent of all living lissamphibians. In contrast to the large, robust tetrapods we met on previous pages, lissamphibians are generally small-bodied animals with delicate skeletons. This means they do not fossilize easily and have an accordingly patchy fossil record. Long-standing disagreements over their relationships to other tetrapods complicate our understanding of their early history. Some models suggest Lissamphibia are a subgroup of the temnospondyls (in which case temnospondyls did not go extinct in the Cretaceous), while others posit that the lepospondyls, another group of early amphibious tetrapods, are their true evolutionary home. A third model splits these ideas, with some modern lissamphibians arising from temnospondyls, and others from lepospondyls. A wholly temnospondyl origin is currently preferred, but it does not answer all the questions we have over lissamphibian evolution, such as which fossil temnospondyls are their closest relatives, or whether they evolved in the Permian or the Carboniferous.
Lissamphibians are an overlooked tetrapod group in modern culture. Being mostly small creatures that inhabit dark, damp environments, and that struggle to live in close association with humans, especially in polluted ecosystems, they fly under our collective radar far more than birds, reptiles, or mammals. Our general unfamiliarity with amphibians is our loss. These remarkable animals offer some truly spectacular takes on tetrapod anatomy: peculiar skeletons, a suite of remarkable adaptations related to breathing (including being able to breathe through their skin; using a throat-pump to initiate respiration; and, in some species, having no lungs at all), life cycles that involve transitioning from gilled swimming larvae to air-breathing adults, and the ability to completely regenerate lost body parts. They are also amazingly diverse in anatomy and lifestyle. Frog hind-limb and pelvic anatomy is strongly specialized to leaping, but some species also excel at climbing, digging, and swimming. Salamanders are not only newt-like in form; they also exist in eellike varieties that are strongly specialized for aquatic lifestyles. Caecilians are perhaps the strangest of all, being limbless creatures that at home underground or in stream substrates, and resembling worms or snakes depending on their size. Fossils show that lissamphibians occupied similar body forms in the deep past: dinosaurs would not be startled by modern-looking frogs, salamanders, or caecilians.
Today, amphibians face real adversity. Drastic falls in amphibian populations have been recognized in thousands of species across the world since the 1950s, and their demise continues today as severely as ever. Hundreds of species are now classed as critically endangered following decades of continued pressure on amphibian communities, and many species now only survive in captivity. The necessity for both land and water in amphibian life cycles means they are especially vulnerable to environmental modification, placing climate change (and the associated rise in ultraviolet light radiation) and habitat degradation as major drivers in their population crisis. Diseases (possibly spread by travelling humans and exacerbated by habitat changes) and the introduction of foreign predators into amphibian habitats also play a role. Wild collecting for the pet trade has reduced some wild amphibian populations to critical levels and has even—as with the famous gilled salamander, the axolotl—rendered them extinct in the wild. In many cases, such as with the harlequin mantella frog (Mantella cowani, shown here), amphibian species have become endangered before we even have time to understand their biology. The unavoidable fact of amphibian conservation is that this great branch of tetrapod evolution will be decimated within our own lifetimes unless significant and immediate efforts are made to conserve them.
Caseids: The Land Vertebrates Declare War on Plants(Permian)
COMPARED TO CARNIVORY, WHERE MEALS TEND TO BE FLIGHTY, armed, or angry (or all three), herbivory may seem like an adaptive walk down easy street. After all, what is difficult about eating plants? They’re abundant, accessible, and mostly inoffensive to the body parts of the individuals devouring them. But herbivory is actually a much tougher lifestyle than it first seems. The flesh of our fellow animals is relatively easy to digest and rich in nutrients, on account of it being in the same chemical format as our own tissues. Plants, in contrast, are composed of a different cellular makeup that makes them tough to digest and nutrient-poor, to the extent that many living herbivores have to supplement their diet with animal tissues on occasion. It actually takes a number of significant adaptations to eat a plant-based diet: abrasion-resistant mouthparts that can reach and crop plant tissues, a digestive mechanism that can break into tough cells, a gut that can hold huge quantities of bulky vegetative matter and absorb its nutrients, and a body capable of supporting and moving a relatively bulky torso. Herbivory may look easy, but it’s actually more complex and physiologically challenging than carnivory.
It took tetrapods several million years to develop lineages capable of eating plants. Among the first major successes in this niche were the caseids, a group of large-bodied Carboniferous and Permian amniotes that look like reptiles but are in fact more closely related to mammals. Caseids are synapsids, the major branch of tetrapod evolution that contains mammals and our ancestors. Our early synapsid relatives were the dominant terrestrial vertebrates in the late Paleozoic, transforming from superficially reptile- or amphibian-like creatures in the Carboniferous to a diverse range of increasingly mammal-like animals through the Permian. Caseids were some of the first synapsids to become widespread and abundant, living across North America, Europe, and Russia for much of the Permian and outlasting several other early synapsid attempts and herbivory. They were among the largest animals on land in the Permian. Some species, such as the completely known Cotylorhynchus romeri, regularly attained lengths exceeding 3.5 m and masses of 300 kg, but other species—such as C. hancocki, shown here—reached 5–6 m long and weighed half a tonne.
The first caseids were likely carnivorous, but the group quickly switched to dedicated herbivory, becoming barrel-chested, small-headed animals with enormous, sprawling limbs. Their small heads look ridiculous at first glance, but they are not so different in proportion to those of herbivorous tortoises or sauropod dinosaurs. If herbivores have an efficient enough gut mechanism, their mouths simply have to harvest plant matter, negating the need for a large head and robust teeth for chewing. Caseid jaws were equipped with a dentition similar to that of iguanas, along with smaller teeth across the roof of their mouths and powerful tongues (the latter not being directly preserved, but evidenced by robust bones that anchor throat and tongue tissues). Collectively, these anatomies allowed caseids to shear and rip away plant material before sending it to their expansive guts for a long digestive process. Their gut capacity and dentition seem suited to high-fiber vegetation, one of many paleobiological details indicating a fully terrestrial lifestyle for these creatures.
Caseid limbs are extremely stout, and their enormous hands and feet were equipped with huge, pointed claws. Much of their limb anatomy has been regarded as an adaptation for weight-bearing at large size, but the fact that even small caseids have robust limbs suggests a function beyond supporting huge bodies. They were likely excellent diggers, their robust forelimbs and powerful hands perhaps being used to uproot plants for access their nutritious roots, or perhaps to dig burrows. The idea of such large animals excavating burrows may seem peculiar, but we know that similarly sized ground sloths and bears are capable of such feats, and it would be remiss to preclude this behavior for these evidently mighty extinct animals.
THE MOST FAMOUS AND CHARISMATIC OF ALL EARLY SYNAPSIDS is surely Dimetrodon, an animal best described as an alligator–bull terrier cross with a penchant for sails—the ultimate retro prehistoric accessory. Dimetrodon is a sphenacodontid, a group of European and North American predators that evolved in the Carboniferous and went extinct in the mid-Permian. They represent a grade of synapsid evolution that was somewhat more mammal-like than a caseid, but was still superficially reptile-like in many respects. Many sphenacodontids were small (just over half a meter long) but some, including several Dimetrodon species, attained total lengths of 4.5 m and body masses of 250 kg. Large sphenacodontids were probably top predators in many terrestrial Permian food webs.
Dimetrodon occurred in Permian landscapes of the United States and Germany, and up to twenty species have been recognized from its abundant fossils. Modern assessments of its diversity suggest that only thirteen or so of these proposed species are valid, however. The taxon illustrated here is the large Texan species D. grandis, one of the best-represented and studied taxa. Fossil bone chemistry, which reflects some details of what animals were eating and where they obtained their food, suggests that Dimetrodon was primarily a terrestrial predator but probably not a fussy eater. Small individuals likely ate insects and small vertebrates, while larger animals preyed on fish, amphibians, and other synapsids. These interpretations are supported by possible Dimetrodon stomach contents and bite marks on prey species, including the “boomerang-headed” lepospondyl Diplocaulus and freshwater sharks. In the adjacent scene, a Diplocaulus has been pulled from its burrow before being eaten by a Dimetrodon family.
The function of the Dimetrodon sail is a subject of much intrigue. Sails were common to several sphenacodontid species and several other early synapsids, but they are not so universal that they were physiologically essential. Indeed, large sail-less sphenacodontids, such as Sphenacodon, lived in the same time and place as Dimetrodon, which rules out environmental conditions as promoting sail development. Sails might have imparted practical benefits, such as helping their owners warm up in direct sunlight or cool down in a breeze. But they also have drawbacks: significant resource investment is required for their growth and maintenance, they restrict mobility when the animal is moving through cluttered habitats, and they make their owners conspicuous to predators. Studies of Dimetrodon sail growth suggest that they grew much faster and larger than expected for a device required to regulate body temperature, instead matching the growth rates of sociosexual display structures in living species. This implies a social selection pressure—the sail as a display device to attract mates or deter rivals—over a mechanical one, a finding that also matches the somewhat random distribution of sails among Permian animals. The display structures of living animals often have the same traits—a complex, seemingly erratic occurrence across evolutionary lineages, and detachment from fundamental physiological functions—and we can assume some elaborate anatomies of past species served the same purpose.
Investigations into Dimetrodon sails have revealed unexpected insights into its soft-tissue composition. Each bony spar supporting the sail has variable bone surface texture, implying that three types of tissue covered the sail in life. The base was anchored in muscle (as expected—the struts composing the Dimetrodon sail are the same structures that support back muscles in all vertebrates), and much of their length was encased in skin webbing. The exact nature of this tissue is unknown, but healed fractures on sail spars show it was strong enough to hold spines in place when they snapped. A third texture type is found at the tips, implying that the spines projected above the tissues of the sail webbing. Some Dimetrodon spines twist into unusual directions at their ends, perhaps further evidence of their liberation from the webbing beneath them.
THE WORD ENIGMATIC IS OVERAPPLIED TO FOSSIL ANIMALS, BUT there’s little doubt that Helicoprion, a cartilaginous fish that swam Early Permian seas across the world, warrants such description. Approximately one hundred specimens of Helicoprion are known, and virtually all of them represent the same body part: a spiraling whorl lined with numerous (sometimes over 130) triangular, laterally compressed teeth along the outer margin. The cartilage jaw apparatus that housed this bizarre structure, as well as the rest of the Helicoprion skeleton, has long proved elusive, leaving us puzzling over its function and anatomical configuration since the first Helicoprion fossils were discovered in the late 1800s. Artists have done their best to make some implausible interpretations look convincing. Was it a projecting structure from the upper jaw, somewhat like a coiled swordfish bill? Did it droop from the lower jaw like a ragged, spiraling beard of teeth? Was it located in the mouth somehow? Was it even a jaw—might it have been a bizarre fin?
The recent discovery of a Helicoprion specimen with preserved jaw tissues has brought this century-long mystery to an end. It seems the whorl was, in fact, the majority of the lower jaw, with the largest teeth being the “active” teeth in the mouth. Older, smaller teeth and whorl elements spiraled into the jaw as the animal grew. The whorl is thus somewhat comparable to a shark jaw where new teeth rotate into place as old ones are pushed out, with the distinction that Helicoprion maintained its entire dental history within its jaw rather than shedding teeth as they left active use. The same Helicoprion specimen that revealed this surprising anatomy also suggests that the upper jaw lacked dentition entirely, and that the lower jaw only possessed a single row of teeth: a configuration recalling a circular saw blade and its guard more than a conventional oral cavity. It seems that only the front and top of the tooth whorl were exposed, and that as the mouth opened and closed the lower jaw moved forward and backward. This mechanism, in concert with the recurved teeth, may have acted to snag and cut prey while also drawing it into the mouth. A lack of wear on Helicoprion teeth suggests that they ate largely soft-bodied animals like cephalopods (octopus, squid, and their shelled relatives, such as nautiloids, shown here). The tooth whorl was prohibited from biting the upper jaw by a strap of cartilage that stabilized the side of the lower jaw and limited how far the mouth could close.
Unfortunately, while the mystery of Helicoprion jaws is on the way to being solved, many questions about the rest of the animal remain unanswered. It seems that Helicoprion belongs to a fish clade known as the Eugeneodontida, a lineage related to sharks and rays, and that its closest living relatives are the chimeras, a large group of small-to-medium-sized, sharklike fish. But we have no concrete ideas about the anatomy of Helicoprion beyond its jaws. Even basic information like body size and general proportions is unknown. This limits our understanding of its lifestyle as well as its appearance, and any full-body restoration you see of this fish is—at best—an educated guess. We can, however, be sure that Helicoprion was a very successful and widespread animal. Fossils of Helicoprion are found in rocks representing Permian seas all over the globe, and sometimes in relative abundance. It seems to have endured for at least fifteen million years and diversified into several species in that time. A solitary record from the Triassic Period has been argued as evidence that Helicoprion survived into the Mesozoic Era, but modern researchers are skeptical of this interpretation because the source of this fossil was poorly documented by its discoverers. Associated geological details of the alleged Triassic Helicoprion specimen match those of other Permian examples and, in all likelihood, its interpretation as a Triassic specimen was erroneous.
THE ADAPTIVE POTENTIAL OF SYNAPSIDS IS WELL DEMONSTRATed by the dicynodonts, a widespread, abundant, and diverse group of herbivores that existed across much of the planet during the Permian Period and Triassic Period. Dicynodonts developed a more complex approach to herbivory than the caseids we met earlier, being able to slice plant matter in their jaws before digesting it in their large guts. Their efficient herbivory may explain how they became the dominant herbivorous species for much of the Permian and Triassic, before meeting their end at the close of the latter period. A fragmentary jawbone from Cretaceous rocks in Australia has hinted at dicynodonts surviving over one hundred million years beyond their assumed extinction, but this exciting claim has now been compelling challenged, and is likely erroneous. Instead, the specimen probably represents a badly preserved jawbone from a Pliocene or Pleistocene marsupial mammal.
Dicynodonts evolved in the Middle Permian from the therapsid branch of Synapsida. Therapsids showed greater mammal-like qualities than synapsids like Dimetrodon in having more upright limbs; hands and feet that were losing their lizard-like asymmetry; mammal-like skulls, teeth and sensory capabilities; and they seemingly possessed elevated growth rates and body temperatures. But while these were important steps toward defining mammalian traits, we should be careful not to overstate their “mammaliness.” Early therapsids like dicynodonts still had some way to go before attaining truly mammal-like anatomy, and we have yet to find evidence of characteristic mammalian soft tissues, like fur, in these species.
The rapid spread of dicynodonts made them the most abundant large-bodied terrestrial animals on Earth by the end of the Permian. Some genera, like Lystrosaurus, were so widespread that they occurred on multiple continents, and their fossils were instrumental in validating early ideas about continental drift. In form, dicynodonts ranged from small, long-bodied species that were adapted for burrowing (including Cistecephalus microrhinus, the small animal in the right of this image), to elephant-sized, 4.5 meter long and 5-7 tonne megaherbivores. All were robustly built with proportionally large heads that generally lacked teeth except for, in most species, prominent tusks (or tusklike structures, in some unusual Triassic forms). They used narrow, deep beaks to crop plant matter, an action aided by powerful jaw muscles. Their beaked lower jaws slid backward when closed to create a shearing motion that chopped plant matter in a scissorlike fashion. Chewing or slicing food before swallowing is a great advantage for an herbivore in that it helps break down tough plant tissues before digestion, leading to readier absorption of their nutrients. Chewing mechanisms have evolved numerous times among animals for this reason, and we often see plant chewers, rather than gulpers, as dominating herbivore ecological space throughout geological time. Many dicynodonts have bosses and other ornamental structures on their faces (including the South African Permian species, Aulacephalodon peavoti, shown here), as well as reinforced skull bones that—along with their tusks—may indicate antagonistic or defensive behaviors. They were likely common prey for predatory synapsids that existed in the Permian (such as the famous saber-toothed gorgonopsians) and for the various types of carnivorous reptiles that lived in the Triassic.
Dicynodonts were among the few major components of Permian synapsid ecosystems to survive into the reptile-dominated world of the Mesozoic, where they continued to be the major land herbivores until dinosaurian plant eaters took over their roles. Many other Paleozoic lineages were not so fortunate, however: for most species alive at the end the Permian, events were about to take a very, very bad turn.
The Great Dying: The End-Permian Extinction
AS THE PERMIAN PERIOD DREW TO A CLOSE, LIFE ON EARTH WAS subjected to its most grueling test: an extinction event that wiped out around 95 percent of marine species and over 70 percent of tetrapods. Evolutionary history was essentially reset as major lineages became extinct and biological communities crumbled, collapsing much of the ecological complexity attained in the Paleozoic and wreaking havoc in terrestrial and aquatic ecosystems alike. No other extinction event of the last few hundred million years had such an impact on life as the end-Permian catastrophe, or, as it is often called, “the Great Dying.”
Some controversy exists over the exact cause of the Permian crisis. The Permian was already a somewhat turbulent period for life because the ever-wandering continental plates had drifted into one another, forming the supercontinent Pangaea. Supercontinents have formed and split apart multiple times in our planet’s 4.5-billion-year history, but Pangaea is the only supercontinent to have existed during the Phanerozoic Eon. The formation of a supercontinent has a number of effects on climate and environment, including the reduction of coastline and shallow marine habitats. These are among the most speciose environments on the planet, so their removal leads to a rapid reduction in diversity, and we see this reflected in latest Permian fossil beds. This in itself does not explain the Great Dying, however, because the dramatic loss of life at the end of the Permian is very sharply observed, not the result of a long decline as continents slowly coalesced into one landmass. Moreover, while the construction of Pangaea led to loss of life in some communities, other environments responded positively to its formation, achieving greater diversity. It seems more likely that the cause of the end-Permian extinction was a relatively sudden event 252 million years ago, at the very end of the Paleozoic Era.
A major geological episode precisely dating to this interval was probably the major catalyst for the Permian extinction event. At the close of the Permian, a tremendous eruptive phase began in what is now northern Siberia, launching enormous amounts of ash and greenhouse gases into the atmosphere. The scale of this event is unparalleled in the Phanerozoic. Geological data suggests that the eruption spewed lava and other pyroclastic material over two million square kilometers of what is now northern Russia. This output stemmed not from stereotyped prehistoric volcanoes—the conical sort you see in the background of unimaginative paleoart—but from massive fissure eruptions, splits in Earth’s crust that can run for many kilometers and release massive amounts of volcanic material. The Permian fissure eruptions persisted for hundreds of thousands of years, dimming the planet as they pumped ash and dust into the atmosphere, causing acidic rain, and raising global temperatures through greenhouse gases. The fact the eruptions occurred through coal beds worsened this effect considerably, as it liberated huge volumes of carbon dioxide—a potent greenhouse gas—into the atmosphere.
Marine life was hit especially hard during the Permian event. Skyrocketing carbon dioxide levels raised oceanic acidity, making it difficult to create calcareous shells and skeletons, and temperature shifts altered oceanic circulation. Marine oxygen levels dropped and surface water temperatures rose to intolerable levels—perhaps as high as 40°C. Life on land also endured extremely high temperatures, turning lush, diverse forested habitats into barren vistas. For several hundred thousand years, life was baked and choked to death, and came the closest it ever has to total annihilation.
The Foundation of the Modern Age (Triassic)
MOST OF OUR INTEREST IN MASS EXTINCTIONS IS TUNED TO THE exciting parts: the cataclysmic events, the extreme environmental conditions, the huge taxonomic death tolls. But the aftermaths of these events are just as important in our consideration of life’s evolution. The effects of mass extinctions are not easily shrugged off, often leading to cascading, runaway impacts on environmental conditions and global habitability lasting millions of years after the initial extinction episode. Our biosphere is a robust and adaptable one, but if basic foundations of climate, seawater chemistry, and oceanic circulation are knocked too far from their typical operation, the return to habitable conditions can happen only at a geological pace. These findings are arguably some of the most important to stem from paleontological research and are especially significant in light of growing concerns about the rapidly changing climates, biodiversity crisis, and shifting oceanic conditions of our modern age.
The aftermath of the end-Permian extinction was a difficult period for life in both terrestrial and marine settings. The absence of extensive vegetation on land reduced the diversity of habitats and the availability of food, as well as exposed soils and bedrock to erosive forces, thus allowing large volumes of terrestrial debris to be swept into bodies of water. This may not seem too significant—what harm can a little windblown sand do?—but it actually created chaos in the oceans. High sediment influxes compromised the ability of marine species to feed, reproduce, regulate their internal salt balances, and develop strong skeletons. They promoted blooms of microbes that, upon death, had a stagnating effect on oceanic oxygen levels and further stressed marine life. Some aquatic habitats were so inundated with sediment that species adapted to living in or on soft substrates thrived, but those adapted for other habitats went into decline. With global temperatures and oceanic acidity also remaining generally high, it’s unsurprising that many marine species—even those that survived the initial end-Permian event—did not endure these conditions for long. About five million years passed before biodiversity started to recover, and it took another few tens of millions of years to achieve pre-extinction diversity levels.
The cast of animals that repopulated Earth in the Mesozoic Era was not the same as that of the Permian. Trilobites, eurypterids, acanthodians, several major coral and insect groups, numerous synapsids, and some early reptile lineages were gone. Animals that anchored to the seafloor—such as the once-abundant brachiopods—were especially hard hit, and they never recovered their former diversity. What replaced them were animal types and ecosystems that we would find more recognizable: the basis of our “modern” natural world was born out of the ashes of the Permian. New types of corals, bivalves, snails, and sea urchins became abundant in marine environments and, on land, ecosystems were populated by the mammal-like cynodonts and a hitherto fairly innocuous evolutionary lineage: the reptiles. With reptiles having largely been sidelined by synapsids since their evolution in the Carboniferous, the Triassic marked the interval where they made a play for world domination. Starting their time in the Triassic as relatively unassuming, somewhat crocodile-like species—such as the Brazilian Teyujagua paradoxa (shown here, middle right) or its squat contemporary Procolophon trigoniceps (bottom)—by the end of the period reptiles had come to dominate land ecosystems, had achieved a tremendous diversity of seagoing forms, and had even adapted to powered flight—a first for vertebrates. Dicynodonts showed some resilience against the rise of reptiles but relinquished control of herbivorous niches to saurian lineages at the end of the Triassic. This surge in reptile evolution saw several major lineages competing for roles as dominant terrestrial predators and herbivores as the Triassic drew to a close: a highly adaptable and successful clade, the dinosaurs, were the principle victors.
The Basilisk Lizard, a “Lower Vertebrate” (Holocene)
THE REPTILES WE KNOW TODAY GIVE ONLY A NARROW, AND PERhaps somewhat misleading, characterization of this group compared to their anatomical and ecological diversity throughout evolutionary history. Modern reptiles are the turtles, the crocodylians, and the lepidosaurs (the group that includes lizards, snakes, and tuatara). They are easy to classify from their scaly bodies, sprawling limbs, and environment-controlled “cold-blooded” physiologies (all features seen in the common basilisk, Basiliscus basiliscus, right). If we consider the broader picture of reptile evolution, however, we find that it’s much harder to stereotype what a reptile is. In addition to species like those alive today, the reptile fossil record contains “warm-blooded” animals that had elevated metabolic rates, upright postures, and fuzzy skin. Ancient reptiles include dinosaurs, pterosaurs, several great marine lineages, and a number of bizarre forms that defy easy categorization—all animals with lifestyles and anatomies very different from those of their cousins living now. We also have to classify birds as a type of reptile, their ancestry being among the predatory dinosaurs. From this perspective, reptiles are not just the scaly sprawlers we know today, but an evolutionary tour de force, a lineage that has been prominent in a range of habitats across the world since the Triassic.
Reptile phylogeny has been studied intensively and the broad picture of their evolution is now well understood, though the origin of turtles remains challenging to pin down (we’ll return to this later). Lepidosaurs represent one major branch of the reptile line, the other belonging to the archosaurs and their relatives. Archosaurs are the evolutionary home of crocodylians and their kin, as well as the lineage that gave us pterosaurs, dinosaurs, and birds. The origins of these groups are ancient, with lepidosaurs and archosaurs first appearing in the Permian, and the archosaurs splitting into the bird and crocodylian branches in the Triassic. With such deeply rooted divisions, it’s not surprising that living reptiles are only superficially similar. Once we look closely, we find they are quite distinct from one another in detailed anatomy.
Humans have often regarded reptiles as second-class animals despite their evolutionary success. Scientists have historically considered reptiles, along with amphibians and fish, as “lower” vertebrates: dim-witted, anatomically primitive animals that are physiological and behavioral shadows of “higher” species, the birds and mammals. In broader culture, reptilian features—scales, “cold-bloodedness,” and crawling gaits—are often applied to subjects of revile or suspicion. But ongoing studies of the “lower” vertebrates demonstrate that our traditional view of their physical abilities and intelligence has been biased by the idea that humans, and species most biologically similar to us, are an evolutionary peak that all organisms aspire to. For instance, while upright limbs may confer energetic advantages over sprawling configurations, sprawlers enjoy greater stability and advantages in climbing, sprinting, and frequent acceleration. These properties strongly suit the lifestyles of many small tetrapods, and their retention of this so-called “primitive” stance likely reflects the fact that a mammal-like upright limb posture would compromise their evolutionary fitness. Similarly, a reptilian physiology requires only about 10 percent of the food needed by an equivalently sized warm-blooded creature, making reptiles better suited to life in areas of low productivity than mammals or birds. The idea that a slow metabolism is associated with lack of behavioral sophistication or low intelligence has also not been borne out under study. Reptiles seem to have good memory, flexible (instead of purely instinctive) behavior, and problem-solving abilities. They even engage in play behavior, just like mammals and birds. Crocodylians will play with tethered balls, and turtles engage in tug-of-war with zookeepers or toss objects gleefully around their ponds. Komodo dragons, the largest living lizards, take part in especially sophisticated recreational behavior where they play with their handlers and make toys of balls, buckets, and old shoes. These so-called lower vertebrates are far from second-fiddle to mammals and birds: they are our adaptive peers, and their prominence and persistence in Earth’s evolutionary history is far from a fluke.
Amphibious Ichthyosaurs (Triassic)
IT’S A SOMEWHAT PERVERSE FACT OF EVOLUTIONARY HISTORY that, within a few million years of amniotes evolving to live independently of water, several lineages performed an adaptive U-turn to reinvade the aquatic realm. Of course, by this point amniotes had long abandoned key adaptations to aquatic existence: they had limbs instead of fins, they breathed air, and they laid eggs that would drown if submerged. If we ever need an example of how random, opportunistic, and counterintuitive natural selection can be, the evolution of secondarily aquatic Permian and Triassic amniotes is a great example.
The first of these U-turns was launched by the mesosaurids, a group of lizard-like Permian animals with uncertain relationships to reptile-line amniotes. Mesosaurids seem to have been a relatively shortlived experiment with secondary aquatic lifestyles however, and it was not until the Triassic that amniotes fully committed to life at sea. From the Triassic through to the end of the Cretaceous, Earth’s seas were full of reptiles: ichthyosaurs, plesiosaurs, mosasaurs, turtles, and many other unusual forms of uncertain evolutionary affinities. Many of these reptiles were as comfortable in the water as a whale or a dolphin, being efficient swimmers; accomplished fish predators; and capable of giving birth to live offspring, negating the need to leave the water to lay eggs. Making this more remarkable is the fact that “marine reptiles” are not a single group but several different lineages that entered the seas independently, converging on the same adaptations for wholly marine lifestyles. Quite how many times reptiles developed aquatic adaptations is a matter of contention, as the evolutionary origins of some groups—including the famous ichthyosaurs and plesiosaurs—remain difficult to pin down.
The Triassic was a special time in marine reptile evolutionary history, thanks to the presence of many unusual lineages that would not survive into the later Mesozoic. Among them was Cartorhynchus lenticarpus, a small (about 40 cm long) Chinese marine reptile that represents an early stage of ichthyosaur evolution. The ichthyosaurs modified their bodies for swimming more than any other aquatic reptile, eventually assuming a fishlike form entirely unlike that of their terrestrial ancestors (hence the name “ichthyosaur”, which translates to “fish lizard). But they did not attain this anatomy overnight. The earliest ichthyosaurs retained several relatively “terrestrial” characteristics compared to later forms, including a long, slender tail instead of a tail fin, relatively long hind limbs, short bodies, and generalized skulls and teeth. Many of these features were present in Cartorhynchus, as were unusually long, flexible and curving forelimbs that may have allowed this species to move about on land. Cartorhynchus was clearly a primarily aquatic animal however, possessing dense, heavy bones to reduce its buoyancy, true flippers instead of walking limbs, and a jaw apparatus adapted to suction feeding (an aquatic foraging mechanism where prey is sucked into the mouth though quick, powerful expansion of the mouth cavity). If Cartorhynchus did amble about on land, it probably was not especially speedy or agile. Our mental image might be better informed by sea turtles and mudskippers than by seals or otters. Perhaps it left water only to escape predators, to rest, or to access water bodies that were cut off from the sea.
Curiously, though Cartorhynchus seems like a “transitional” species, moving from life on land to life in the oceans, its placement in current schemes of ichthyosaur evolution suggests that it arose from species that were already strongly adapted to marine life. Thus, if Cartorhynchus was an amphibious ichthyosaur, it represents an aquatic species adapting to life on land, not the other way around. Perhaps Cartorhynchus represents an ichthyosaur lineage that never committed to land or sea, but instead ebbed between the two as adaptive opportunities arose.
THE TERM LIVING FOSSIL HAS A NUMBER OF DEFINITIONS. ONE applies to modern animals that closely resemble their fossil relatives, where slow evolutionary rates preserve anatomy developed by ancient, now-extinct relatives until modern times. A second definition applies to species known from fossils before living examples of the same lineage are found alive in the modern world. Such species are sometimes called “Lazarus taxa”—organisms that have, like the biblical Lazarus, come back from the dead.
Both definitions are met by the coelacanth (Latimeria), a large (2 m long) and robust lobe-finned fish most closely related to lungfishes and tetrapods. Coelacanths have a good fossil record from the Devonian to the Cretaceous as well as a long research history, their fossils first being studied in the early 1800s. 19th century paleontologists found that the coelacanth fossil record ran dry in rocks less than seventy million years old, so they sensibly assumed that coelacanths were long extinct, perhaps victims of the same mass extinction that wiped out many marine species at the end of the Cretaceous Period. We can only imagine the surprise paleontologists felt when a living coelacanth was caught in the Indian Ocean in 1938, and especially because this living example was superficially little different to those known from the geological record—a true “living fossil” in every sense. Two living species of coelacanth are now recognized: the critically endangered West Indian coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis).
Modern fish experts are now pushing back against the concept of coelacanths being “living fossils”, however. First, coelacanth DNA shows a typical rate of mutation and change, without any evidence of slow or static mutation rates. This is not unexpected, because although the deep-water habitats occupied by coelacanths are often regarded as archaic “lost worlds,” they are actually just as dynamic and changing as any other setting. There is no reason why coelacanths should have, or would benefit from, reduced evolutionary rates. Second, fossil coelacanths were very diverse, with over 130 known species in a myriad of sizes and forms. Variations in body proportions represent vastly different approaches to locomotion, and differences in jaw anatomy hint at different dietary preferences. Some coelacanths were huge, 5-m-long ocean cruisers, while others were small, compact fish better adapted to skirting over the seafloor than living in the open sea. We know that many ancient coelacanths lived in freshwater habitats instead of marine settings, and that swim bladder (organs that help fish regulate buoyancy) anatomy differs fundamentally between living and extinct species, suggesting different habitual water depths. Just one example of a fossil coelacanth unlike Latimeria is the 1.3-m-long Triassic species shown here, Rebellatrix divaricerca. The most obvious feature of this Canadian coelacanth is its large forked tail, a fin morphology unknown in other coelacanths and thought to be linked with a powerful, sustained swimming ability. The skull of Rebellatrix is unknown so its exact diet is not clear, but its streamlined body shape and powerful tail compare well to certain sharks and predatory fish, suggesting that it may have been a fast pursuit predator. This lifestyle is very different from the relatively sedate bottom-feeding habits of living coelacanths, which exploit deep-sea currents to drift over foraging areas, swimming only when necessary.
It is thus difficult to consider modern coelacanths “living fossils.” There is no stereotyped coelacanth anatomy or lifestyle for them to conform to, just as there is nothing sluggish about their rate of evolutionary change. Indeed, the whole concept of “living fossils” is a problematic one: any study of life that delves beneath superficial details invariably shows that evolution and adaptation are rarely static.
