CHAPTER 3
THE JURASSIC PERIOD

Images

The Jurassic Period is the second of three periods of the Mesozoic Era, extending from 199.6 million to 145.5 million years ago. It immediately followed the Triassic Period and was succeeded by the Cretaceous Period. The Morrison Formation of the United States and the Solnhofen Limestone of Germany, both famous for their exceptionally well-preserved fossils, are geologic features that were formed during Jurassic times.

The Jurassic was a time of significant global change in continental configurations, oceanographic patterns, and biological systems. During this period the supercontinent Pangea split apart, allowing for the eventual development of what are now the central Atlantic Ocean and the Gulf of Mexico. Heightened plate tectonic movement led to significant volcanic activity, mountain-building events, and attachment of islands onto continents. Shallow seaways covered many continents, and marine and marginal marine sediments were deposited, preserving a diverse set of fossils. Rock strata laid down during the Jurassic Period have yielded gold, coal, petroleum, and other natural resources.

During the Early Jurassic (about 200 million to 176 million years ago), animals and plants living both on land and in the seas recovered from one of the largest mass extinctions in Earth history. Many groups of vertebrate and invertebrate organisms important in the modern world made their first appearance during the Jurassic. Life was especially diverse in the oceans—thriving reef ecosystems, shallow-water invertebrate communities, and large swimming predators, including reptiles and squidlike animals. On land, dinosaurs and flying pterosaurs dominated the ecosystems, and birds made their first appearance. Early mammals also were present, though they were still fairly insignificant. Insect populations were diverse, and plants were dominated by the gymnosperms, or “nakedseed” plants.

The Jurassic Period was named early in the 19th century, by the French geologist and mineralogist Alexandre Brongniart, for the Jura Mountains between France and Switzerland. Much of the initial work by geologists in trying to correlate rocks and develop a relative geologic time scale was conducted on Jurassic strata in western Europe.

THE JURASSIC ENVIRONMENT


The Jurassic environment was primarily characterized by the movements of various tectonic plates. At the start of the interval, the continents were grouped into two vast regions: Laurasia and Gondwana. Later each of these regions showed signs of breaking up into smaller pieces. Collisions between continents and other smaller landmasses contributed to the development of the Rocky and Andes mountain ranges. The temperature differences between the poles and the Equator remained small throughout Jurassic times, possibly due to the release of large amounts of greenhouse gases from volcanism and tectonic activity. The period was also a time of fluctuating sea levels.

PALEOGEOGRAPHY

Although the breakup of the supercontinent Pangea had already started in the Triassic Period, the continents were still very close together at the beginning of Jurassic time. The landmasses were grouped into a northern region—Laurasia—consisting of North America and Eurasia, and a southern region—Gondwana—consisting of South America, Africa, India, Antarctica, and Australia. These two regions were separated by Tethys, a tropical east-west seaway. During the Jurassic, spreading centres and oceanic rifts formed between North America and Eurasia, between North America and Gondwana, and between the various segments of Gondwana itself. In the steadily opening, though still restricted, ocean basins, there was a continuous accumulation of thick flood basalts and a subsequent deposition of sediments. Some of these deposits, such as salt deposits in the Gulf of Mexico and oil-bearing shales of the North Sea, are economically important today. In addition to ocean basin spreading, continental rifting initiated during the Jurassic, eventually separating Africa and South America from Antarctica, India, and Madagascar. The numerous microplates and blocks making up the complex Caribbean region today can be traced to this time interval.

To accommodate the production of new seafloor along the proto-Atlantic Ocean, significant subduction zones (where seafloor is destroyed) were active along virtually all the continental margins around Pangea as well as in southern Tibet, southeastern Europe, and other areas. All along the west coast of North, Central, and South America, plate tectonic activity in the subduction zones brought on the initial formation of north-south mountain ranges such as the Rocky Mountains and the Andes. Along western North America, several terranes (islands or microcontinents riding on a moving plate) were brought east on oceanic crust and collided with the continent, including parts of a microcontinent that collided into the Alaskan and Siberian regions in the northern Pacific. These collisions added to the growth of the North American continent and its mountain chains. One mountain-building event, known as the Nevadan orogeny, resulted in the emplacement of massive igneous and metamorphic rocks from Alaska to Baja California. Granites formed in the Sierra Nevadas during this time can be seen today in Yosemite National Park, California.

Images

Cross-bedded Jurassic sandstone in Zion National Park, Utah, U.S. Peter L. Kresan

In the Early Jurassic the western interior of North America was covered by a vast sand sea, or erg—one of the largest deposits of dune sands in the geologic record. These deposits (including the Navajo Sandstone) are prominent in a number of places today, including Zion National Park, Utah. In Middle and early Late Jurassic times (161.2 million to 145.5 million years ago), the western regions of North America were covered by shallow seaways that advanced and retreated repeatedly, leaving successive accumulations of marine sandstones, limestones, and shales. By Late Jurassic time the seaway had retreated, and strata bearing dinosaur fossils were deposited in river floodplains and stream channel environments, such as those recorded in the Morrison Formation, Montana.

Records of sea level changes can be found on every continent. However, because of the significant tectonic activity occurring around the world, it is not clear which of these local changes can be correlated to global sea level change. Because there is no evidence of major glaciations in the Jurassic, any global sea level change must have been due to thermal expansion of seawater or plate tectonic activity (such as major activity at seafloor ridges). Some geologists have proposed that average sea levels increased from Early to Late Jurassic time.

PALEOCLIMATE

Jurassic climates can be reconstructed from the analyses of fossil and sediment distribution and from geochemical analyses. Fossils of warm-adapted plants are found up to 60° N and 60° S paleolatitude, suggesting an expanded tropical zone. In higher paleolatitudes, ferns and other frost-sensitive plants indicate that there was a less severe temperature difference between the Equator and the poles than exists today. Despite this decreased temperature gradient, there was a marked difference in marine invertebrates from northern higher latitudes—the boreal realm—and the tropical Tethyan realm. Decreased latitudinal temperature gradients probably led to decreased zonal winds.

Large salt deposits dating from the Jurassic represent areas of high aridity, while extensive coal deposits suggest areas of high precipitation. It has been suggested that an arid belt existed on the western side of Pangea, while more-humid conditions existed in the east. These conditions may have been caused by large landmasses affecting wind and precipitation in a manner similar to that of modern continents.

Analyses of oxygen isotopes in marine fossils suggest that Jurassic global temperatures were generally quite warm. Geochemical evidence suggests that surface waters in the low latitudes were about 20 °C (68 °F), while deep waters were about 17 °C (63 °F). Coolest temperatures existed during the Middle Jurassic (175.6 million to 161.2 million years ago) and warmest temperatures in the Late Jurassic. A drop in temperatures occurred at the Jurassic-Cretaceous boundary.

It has been suggested that increased volcanic and seafloor-spreading activity during the Jurassic released large amounts of carbon dioxide—a greenhouse gas—and led to higher global temperatures. Warm temperatures and decreased latitudinal gradients also may be related to the Tethys Sea, which distributed warm, tropical waters around the world. Ocean circulation was probably fairly sluggish because of the warm temperatures, lack of ocean density gradients, and decreased winds. As stated above, there is no evidence of glaciation or polar ice caps in the Jurassic. This may have been caused by the lack of a continental landmass in a polar position or by generally warm conditions. However, because of the complex relationships between temperature, geographic configurations, and glaciations, it is difficult to state a definite cause and effect.

JURASSIC LIFE


The Triassic-Jurassic boundary is marked by one of the five largest mass extinctions on Earth. About half of the marine invertebrate genera went extinct at this time. Whether land plants or terrestrial vertebrates suffered a similar extinction during this interval is unclear. In addition, at least two other Jurassic intervals show heightened faunal turnover affecting mainly marine invertebrates—one in Early Jurassic time and another at the end of the period.

Jurassic rock strata preserve the first appearances of many important modern biological groups. In the oceans, life on the seafloor became more complex and modern, with an abundance of mollusks and coral reef builders by Middle Jurassic time. While modern fishes became common in Jurassic seas, they shared the waters with ammonites and other squidlike organisms as well as large reptiles that are all extinct today. On land a new set of plants and animals was dominant by the Early Jurassic. As previously mentioned, gymnosperms (“naked-seed” plants such as conifers) replaced the seed ferns that dominated older ecosystems. Similarly, dinosaurs and mammals, as well as amphibians and reptiles resembling those of modern times, replaced the ancestral reptiles and mammal groups common in Late Triassic times. The earliest bird fossils were found in Jurassic rocks. However, although groups now living were present in Jurassic terrestrial ecosystems, Jurassic communities would still have been very different because dinosaurs were the dominant animals.

MARINE LIFE

The earliest Jurassic marine ecosystems show signs of recovery from the major mass extinction that occurred at the Triassic-Jurassic boundary. This extinction eliminated about half of marine invertebrate genera and left some groups with very few surviving species. Diversity increased rapidly for the first four million years (the Hettangian Age) following this extinction and then slowed through the next five million years. Another extinction event occurred among benthic (bottom-dwelling) invertebrates at the Pliensbachian-Toarcian boundary in the Early Jurassic, interrupting the overall recovery and diversification. The last spiriferid brachiopod (abundant during the Paleozoic Era) went extinct at this time, and in some regions 84 percent of bivalve species went extinct. Although best documented in Europe, biodiversity during this period seems to have decreased around the globe. The extinctions may be related to an onset of low-oxygen conditions in epicontinental seas, as evidenced by the presence today of layers of organic-rich shales, which must have been formed in seas with so little oxygen that no burrowing organisms could survive and efficient breakdown of organic matter could not occur. Full recovery from this extinction did not occur until the Middle Jurassic. It has been proposed that a final interval of heightened extinction took place at the end of the Jurassic, although its magnitude and global extent are disputed. This final turnover may have been limited to Eurasian regions affected by local sea level decreases, or it may be related to a decrease in the quality of fossil preservation through the Late Jurassic.

Except for the extinction events outlined above, in general, marine invertebrates increased their diversity and even modernized through the Jurassic. Some previously abundant Paleozoic groups were extinct by the Jurassic, and other groups were present but no longer dominant. Moreover, many important modern groups first appeared in the fossil record during the Jurassic, and many important groups experienced high levels of diversification (a process known as evolutionary radiation).

A diverse group of vertebrates swam in Jurassic seas. Cartilaginous and bony fishes were abundant. Large fishes and marine reptiles were common. The largest bony fish ever to live, measuring 20 metres (66 feet) long, Leedsichthys, existed at this time, and Jurassic pliosaurs are some of the largest carnivorous reptiles ever discovered.

PROTISTS AND INVERTEBRATES

Among the plankton—floating, single-celled, microscopic organisms—two significant new groups originated and radiated rapidly: coccolithophores and foraminifera. In addition, diatoms are considered by some scholars to have originated in the Late Jurassic and radiated during the Cretaceous. The skeletons of all three groups are major contributors to deep-sea sediments. Before the explosion of skeletonized planktonic organisms, carbonates were mainly deposited in shallow-water, nearshore environments. Today the tests (shells) of coccolithophores and foraminifera account for significant volumes of carbonate sediments in the deep sea, while diatom tests create silica-rich sediments. Thus, the advent of these groups has significantly changed the geochemistry of the oceans, the nature of the deep-sea floor, and marine food webs.

Mollusks became dominant in marine ecosystems, both among swimmers in the water column (nekton) and organisms living on the seafloor (benthos). Nektic cephalopods, such as shelled ammonites and squidlike belemnites with internal skeletons, were very common. Although only one group of ammonites survived the Triassic-Jurassic mass extinction, they radiated rapidly into many different forms. Because their shells have elaborate suture lines, they are easily identifiable. This quality, along with their abundance and rapid evolution, make them useful as index fossils for correlating and sequencing rocks. Thus, ammonites are a major tool for developing relative time scales and dividing the Jurassic into finer time intervals. Other common mollusks include bivalves (pelecypods) and snails (gastropods). These forms diversified into a number of different niches. Among the bivalves, scallops (pectinids) and oysters show marked radiation. Some bivalves also are used as index fossils.

Common echinoderms include crinoids (sea lilies), echinoids (sea urchins), and sea stars (starfish). Jurassic crinoids are descendants from the one group that survived the Permian-Triassic mass extinction. Their circular or star-shaped stem ossicles (plates) can be quite abundant in Jurassic sediments. Under special circumstances, articulated Jurassic crinoids are preserved. Some of these fossils suggest that certain species may have lived on floating logs and not on the seafloor. One group of regular sea urchins, radially symmetrical and living on the surface of the seafloor, radiated into a number of irregular echinoid groups (heart urchins) that could burrow into sediment.

Some lophophorates (brachiopods, or lamp shells) and bryozoa (moss animals) underwent recovery and diversification in the Jurassic but never became as dominant as they were in the Paleozoic Era. Spiriferid brachiopods went extinct during the Early Jurassic extinction event, but rhynchonellid and terebratulid brachiopods can be found throughout the period.

Among bryozoans that survived into the Jurassic, cyclostomes are found encrusting hard substrates Cheilostomes (the most common modern bryozoan) appeared in the Late Jurassic. With the extinction of trilobites, a new set of arthropods developed. The first true crabs and lobsters appeared, bearing large front claws adapted for predation. Shrimp burrows are not uncommon in Jurassic sediments, and fossil shrimp are occasionally preserved. Ostracods—small crustaceans—radiated during the Jurassic and are used today as index fossils.

Unlike today’s world, where virtually all reefs are formed by scleractinian corals, Jurassic reefs and mounds were constructed by a variety of invertebrate organisms. Buildups were constructed by siliceous sponges and serpulid tube worms as well as corals. Stromatolite mounds were formed by communities of algae, bacteria, and other microorganisms. These reefs also had a diverse set of fauna associated with them.

The ecology of the seas was changed by the diversification of marine fauna and by the adaptations of these new organisms. With the evolution and radiation of more-effective predators (crabs, snails, echinoderms, and marine vertebrates), predation pressures began to increase rapidly. For this reason, the Jurassic marks the start of the “Mesozoic Marine Revolution”—an arms race between predators and prey that led to increased diversification of marine fauna. For example, increased levels of burrowing are found in Jurassic sediments, along with an increase in the maximum depth of burrowing. These increases may have developed as a predator-avoidance adaptation, with organisms evolving that were capable of burrowing into sediment, but the activity had far-reaching effects. Burrowing changed the nature of the seafloor, the utilization of resources and space, and sedimentation style.

VERTEBRATES

Along with invertebrate fauna, a diverse group of vertebrates inhabited Jurassic seas. Some of them are related to modern groups, while others are now completely extinct. Chondrichthians (cartilaginous fishes including sharks) and bony fishes were common. Teleosts—the dominant type of fish today—began to acquire a more modern look as they developed bony (ossified) vertebrae and showed considerable change in their bone structure, fins, and tail. The largest bony fish of all time, Leedsichthys, measuring 20 metres (66 feet) long, lived during the Jurassic.

Large marine reptiles were common denizens of Jurassic seas. Ichthyosaurs had sleek profiles similar to those of modern fast-swimming fish and had large eye orbits, perhaps the largest of any vertebrate ever. Jurassic pliosaurs (short-necked plesiosaurs) could be about 15 metres (50 feet) long and are some of the largest carnivorous reptiles ever found—even rivaling Tyrannosaurus, which lived during the subsequent Cretaceous Period. Fossils of large crocodiles and elasmosaurs (long-necked plesiosaurs) are also found in Jurassic marine rocks.

TERRESTRIAL LIFE

INVERTEBRATES

Insects constitute the most abundant terrestrial invertebrates found in the Jurassic fossil record. Groups include the odonates (damselflies and dragonflies), coleopterans (beetles), dipterans (flies), and hymenopterans (bees, ants, and wasps). The discovery of Jurassic bees—which today are dependent upon flowering plants (angiosperms)—suggests either the early presence of angiosperms or that bees were originally adapted to other strategies. Snails, bivalves, and ostracods are preserved in freshwater deposits.

VERTEBRATES

Because of poor preservation of terrestrial deposits and their fossils, it is unclear whether the mass extinction at the end of the Triassic had the same impact on terrestrial ecosystems as it did in the oceans. However, there was a distinct change in vertebrate fauna by the Early Jurassic. In Triassic terrestrial ecosystems, synapsids and therapsids—ancestors of modern mammals and their relatives, often called “mammal-like reptiles”—were dominant. They occupied several ecological niches and grew to large sizes. By the start of the Jurassic, these groups became rare, a minor component of fossil assemblages. Individuals were very small—no larger than squirrel-sized—and their teeth and skeletal anatomy show that the early mammals were probably omnivorous (eating plants and animals) or insectivorous. Instead, the archosaurs (dinosaurs, crocodiles, and pterosaurs) were the dominant terrestrial vertebrates. It is not clear why this change from synapsiddominated to archosaur-dominated faunas occurred. It could be related to the Triassic-Jurassic extinctions or to adaptations that allowed the archosaurs to outcompete the mammals and mammalian ancestors (at least until the end of the Mesozoic Era). In the Late Jurassic, while some marine invertebrates were going extinct, terrestrial vertebrates may also have experienced a drop in diversity, but the evidence here, too, is inconclusive.

Pterosaurs were common throughout the Jurassic. With light skeletons and wing structure supported by a single digit on each “hand,” they were adapted to flying and gliding.

The dinosaurs are divided into two groups based on a number of skeletal characteristics: the saurischians (lizard-hipped) and the ornithischians (bird-hipped). The pubic bone of the saurischians pointed forward, while the ornithischians had an extension that pointed backward.

The saurischians, including sauropods and all carnivorous dinosaurs, were the earliest dinosaurs. Sauropods (including Apatosaurus) appeared in the Early Jurassic and reached the peak of their diversity, abundance, and body size in the Late Jurassic. Sauropods were generally long-necked and probably adapted to browsing on the leaves of tall gymnosperms. Their decline in the latest Jurassic appears to have corresponded to a decline in this type of vegetation.

Carnivorous saurischians, the theropods, include Allosaurus. The earliest allosaur is from the Middle Jurassic. Many of the theropods were globally distributed in the Jurassic. The origin of birds is still debated, but it is generally accepted that birds descended from small theropods during the Jurassic. The earliest undisputed bird fossil discovered is Archaeopteryx. Despite its feathers, this bird was saurischian in appearance: it had teeth, a tail like that of a lizard, and claws at the wing tips, and it lacked a strong breastbone keel for flight muscle attachment.

The ornithischians were all herbivorous and included Stegosaurus and Seismosaurus. By the Jurassic the earliest bipedal ornithopods had diversified into armoured dinosaurs and quadrupedal forms. The presence of heavy plates, spikes, and horns on various dinosaurs suggests that predatory pressures from the theropods may have been intense. However, some of the ornamentation also may have been used against other dinosaurs of the same species.

Images

Stegosaurus, model by Stephen Czerkas, 1986.© Stephen Czerkas; photograph, courtesy of the Natural History Museum of Los Angeles County

Other reptiles, including turtles, were present throughout the Jurassic, while modern forms of lizards made their appearance in the Late Jurassic. Amphibians present during the Triassic Period declined drastically by the Jurassic, and more modern forms developed, such as the first frog with the type of skeletal characteristics seen today.

Images

Cycas media, a treelike cycad that produces large terminal seed cones. G.R. Roberts

PLANTS

Although no new major plant groups originated during this time, Jurassic plant communities differed considerably from their predecessors. The seed-fern floras, such as Glossopteris of Gondwana, disappeared at or near the Triassic-Jurassic boundary. Their demise may be related to the mass extinction seen in marine ecosystems. True ferns were present during the Jurassic, but gymnosperms (“naked-seed” plants) dominated the terrestrial ecosystem. Gymnosperms originated in the Paleozoic Era and include three groups: cycads and cycadeoids, conifers, and ginkgos. All have exposed seeds and rely on wind dispersal for reproduction. The cycads (including the modern sago palm) and the extinct cycadeoids are palmlike gymnosperms. They proliferated to such an extent that the Jurassic has been called the “Age of Cycads.” The conifers (cone-bearing plants such as modern pine trees) also made up a large part of Jurassic forests. Almost all modern conifers had originated by the end of the Jurassic. The ginkgo, a fruit-bearing gymnosperm that is represented today by only one living species, was fairly widespread during the Jurassic.

The first undisputed fossil evidence for angiosperms (flowering plants) is not found until the Cretaceous Period. However, some pollen material similar to that of angiosperms has been reported in rocks of Jurassic age. Also present are fossils of insects whose present-day descendants depend upon angiosperms, suggesting that angiosperms may indeed have been present by Jurassic times.

SIGNIFICANT DINOSAURS OF THE JURASSIC PERIOD

During this interval, several of the more popular groups of dinosaurs emerged. Such predatory dinosaurs as Allosaurus, Ceratosaurus, and the tyranosaurs lived during Jurassic times, as well as the herbivorous Apatosaurus, formerly known as Brontosaurus, and Stegosaurus. The Jurassic also witnessed the arrival of Archaeopteryx, a genus of animals widely believed to be the first birds.

ALLOSAURUS

Allosaurus, formerly known as Antrodemus, is a genus of large carnivorous dinosaurs that lived from 150 million to 144 million years ago during the Late Jurassic Period; they are best known from fossils found in the western United States, particularly from the Cleveland-Lloyd Quarry in Utah and the Garden Park Quarry in Colorado.

Allosaurus weighed two tons and grew to 10 5 metres (35 feet) in length, although fossils indicate that some individuals could have reached 12 metres (39 feet). Half the body length consisted of a well-developed tail, and Allosaurus, like all theropod dinosaurs, was a biped. It had very strong hind limbs and a massive pelvis with strongly forward- (anteriorly) and rearward- (posteriorly) directed projections. The forelimbs were considerably smaller than the hind limbs but not as small as those of tyrannosaurs. The forelimbs had three fingers ending in sharp claws and were probably used for grasping.

The allosaur skull is distinguished by a large roughened ridge just in front of the eye. The skull was large and had sizable laterally compressed teeth, which were sharp and recurved. Allosaurus likely preyed upon ornithischian dinosaurs, small sauropod dinosaurs, and anything else that it could trap and kill. It is possible that Allosaurus was also a scavenger, feeding upon carcasses of dead or dying animals. The name Allosaurus subsumes Antrodemus, which was named earlier but was based only on an undiagnostic tail vertebra. Descendants of Allosaurus lived from 144 million to 135 million years ago, during the Early Cretaceous Period, and are known from fossils found in North America, Africa, and Australia.

APATOSAURUS

Apatosaurus, formerly known as Brontosaurus, is a genus of giant herbivorous sauropod dinosaur, one of the largest land animals of all time, that lived between 147 million and 137 million years ago during the Late Jurassic and Early Cretaceous periods. Its fossil remains are found in North America and Europe.

Apatosaurus weighed as much as 30 tons and measured up to 21 metres (70 feet) long, including its long neck and tail. It had four massive and pillarlike legs, and its tail was extremely long and whiplike. Although some scientists have suggested that the tail could have been cracked supersonically like a bullwhip, this is unlikely, as damage to the vertebrae would have been a more probable result.

The size, shape, and features of the Apatosaurus head were disputed for more than a century after its remains were first uncovered. Certainty was clouded in part by incomplete fossil finds and by a suspected mix-up of fossils during shipment from an excavation site. The head was originally and mistakenly represented in models like that of a camarasaurid, with a square, snubnosed skull and spoonlike teeth. In 1978, however, scientists rediscovered a long-lost skull in the basement of the Carnegie Museum in Pittsburgh, Pennsylvania. This was the skull that actually belonged to an Apatosaurus skeleton. It was slender and elongated and contained long peglike teeth, like those of a diplodocid. Henceforth, Apatosaurus skull models in museums around the world were changed accordingly.

Much discussion has centred on whether Apatosaurus and related forms were able to support their great bulk on the land or were forced to adopt aquatic habits. Many lines of evidence, including skeletal structure and footprints, show that Apatosaurus and all sauropods were terrestrial, like elephants. No skeletal features are indicative of an aquatic existence, and analyses suggest that the dinosaur’s bones could easily have supported its great weight Footprints show that the toes were covered in horny pads like those of elephants. Furthermore, the ribcage was heart-shaped in cross-section like those of elephants, not barrel-shaped like that of the amphibious hippopotamus. Even the massive Brachiosaurus, which weighed about 80 tons, was probably more often on land than in the water.

ARCHAEOPTERYX

This genus contains the oldest-known fossil animals generally accepted as birds. The eight known fossil specimens date to the Late Jurassic Period, and all were found in the Solnhofen Limestone Formation in Bavaria, Germany. Here a very fine-grained Jurassic limestone formed in a shallow tropical marine environment (probably a coral lagoon), where lime-rich muds slowly accumulated and permitted fossil material to be exceptionally well preserved. Several of the fossils show clear impressions of feathers. The sizes of the specimens range from that of a blue jay to that of a large chicken.

Archaeopteryx shared many anatomic characters with coelurosaurs, a group of theropods (carnivorous dinosaurs). In fact, only the identification of feathers on the first known specimens indicated that the animal was a bird. Unlike living birds, however, Archaeopteryx had well-developed teeth and a long well-developed tail similar to those of smaller dinosaurs, except that it had a row of feathers on each side. The three fingers bore claws and moved independently, unlike the fused fingers of living birds.

Archaeopteryx had well-developed wings, and the structure and arrangement of its wing feathers—similar to that of most living birds—indicate that it could fly. Skeletal structures related to flight are incompletely developed, however, which suggests that Archaeopteryx may not have been able to sustain flight for great distances. Archaeopteryx is known to have evolved from small carnivorous dinosaurs, as made evident by the retention of many features, including the teeth and long tail mentioned above. It also retained a wishbone, a breastbone, hollow, thin-walled bones, air sacs in the backbones, and feathers, which are also found in the nonavian coelurosaurian relatives of birds. These structures, therefore, cannot be said to have evolved for the purpose of flight, because they were already present in dinosaurs before either birds or flight evolved.

BRACHIOSAURS

Brachiosaurs are a group containing any member or relative of the dinosaur genus Brachiosaurus, which lived 150 million to 130 million years ago from the Late Jurassic to the Early Cretaceous Period. Brachiosaurs were the heaviest and tallest sauropod dinosaurs for which complete skeletons exist. Larger fossil bones belonging to other (and possibly related) sauropods have been found, but these specimens are incomplete. Fossilized remains of brachiosaurs are found in Africa, North America, and Europe.

Images

Model of a brachiosaur. Shutterstock.com

Brachiosaurs were built like huge giraffes. They had immensely long necks and relatively short tails. Their morphology is unusual among dinosaurs in that the forelimbs were longer than the hind limbs. These adaptations apparently enabled them to lift their heads to about 12 metres (39 feet) above the ground in order to browse the branches of tall trees. Brachiosaurs attained a maximum length approaching 25 metres (82 feet) and a weight of nearly 80 metric tons (88 tons). Their nasal bones were expanded into a broad arch that presumably allowed them to maintain some distance between the vegetation and the nasal openings so that they could breathe easily while feeding. The mouth contained a few dozen pencil-like teeth with beveled edges. Like most other dinosaurs, brachiosaurs did not chew their food but used their jaws to collect food, which the tongue presumably forced into the throat. Considering their massive size, their small heads, and the relatively poor quality of their forage, scientists have inferred that brachiosaurs must have spent nearly all their waking hours feeding.

As mentioned above, the huge size of brachiosaurs led some researchers to suggest that they spent most of their time submerged in water, which would have served to buoy up their great weight. The location of the nasal openings—on top of the head and above the eyes—lent additional support to this idea. However, water pressure at the depths needed to cover these dinosaurs would have crushed their lungs and thus made breathing difficult or impossible. Other features of their skeleton show that brachiosaurs were well adapted to a life spent on land browsing the high treetops. Their skeletons were strong but not massive, so their weight could be supported without any help from water. Their great neckbones, for example, are so deeply excavated that they function as a lightweight framework of struts and plates.

CAMARASAURUS

This genus of dinosaurs lived during the Late Jurassic Period. Its fossils, which have been uncovered in western North America, are among the most commonly found of all sauropod remains.

Camarasaurs grew to a length of about 18 metres (59 feet) and were somewhat smaller than other sauropods of the time such as diplodocids and brachiosaurs. Camarasaurs were further distinguished by their shorter necks and tails, shorter, snubnosed skulls, and large spoon-shaped teeth. The nostrils were positioned in front of the eyes—not above the eyes as in brachiosaurs, or at the tip of the snout as in the diplodocids.

When Apatosaurus (formerly Brontosaurus) was first found in the late 1800s, its skull was missing, and the skull of a camarasaur was often used in museum mounts. In 1978, however, the actual apatosaur skull was found, and it showed a distinct resemblance to diplodocids. Apatosaurus was therefore reclassified as a diplodocid rather than a camarasaur. Camarasaurs have comparatively shorter necks than brachiosaurs, and they have shorter necks and tails than diplodocids.

CAMPTOSAURUS

Specimens of Camptosaurus, a genus of large herbivorous dinosaurs, have been found as fossils in western Europe and western North America. Camptosaurus lived from the Late Jurassic to the Early Cretaceous Period.

Camptosaurus grew to a length of up to 6 metres (20 feet). Juvenile skeletons have also been found. It had very strong hind limbs and smaller forelimbs that were strong enough to support the animal if it chose to progress on all fours, as it might have done while feeding.

Camptosaurus was an ornithopod related to tenontosaurids and iguanodontids. It had the distinctive “blocky” wrist of iguanodontids that facilitated four-legged progression. Nevertheless, the hand was also prehensile and could have grasped vegetation as it was feeding. The thumb was a small spur rather than the conelike spike developed in Iguanodon. In other respects, Camptosaurus was a fairly generalized iguanodontid. The skull was low, long, and massive, with long rows of broad leaf-shaped cheek teeth. A beaklike structure (probably covered by horny pads) was effective in getting plant material into the mouth, where it was cut by the cheek teeth. Camptosaurus lacked the deep dorsal spines of many other iguanodontids, and its claws were more normally curved and less hooflike than those of other iguanodontids and hadrosaurs.

CARNOSAURS

These animals consist of any of the dinosaurs belonging to the taxonomic group Carnosauria, a subgroup of the bipedal, flesh-eating theropods that evolved into predators of large herbivorous dinosaurs.

Most were large predators with high skulls and dagger-shaped teeth that were recurved and compressed laterally with serrated keels on their front and back edges for slicing through flesh. Carnosaurs include Allosaurus and relatives that are more closely related to allosaurs than to birds. Carnosaurs are thus contrasted with coelurosaurs, which include birds and all other theropod dinosaurs more closely related to birds than to allosaurs. (The tyrannosaurs are considered to be members of Coelurosauria, not Carnosauria, despite their large size.) The carnosaurs lived during the late Jurassic Period and survived into the Cretaceous Period.

CERATOSAURUS

Fossils of Ceratosaurus, a genus of large carnivorous dinosaurs, date from the Late Jurassic Period in North America and Africa.

Ceratosaurus lived at about the same time as Allosaurus and was similar in many general respects to that dinosaur, but the two were not closely related. Ceratosaurus belongs to a more primitive theropod stock that includes the coelophysids and abelisaurids. Although it weighed up to two tons, this dinosaur was slightly smaller than Allosaurus and bore a distinctive “horn” (actually an expanded nasal crest) on its snout and a row of bony plates down the middle of its back. Ceratosaurus also differed from allosaurs in that it retained remnants of a fourth clawed finger, unlike the three typical of most theropods.

COMPSOGNATHUS

Compsognathus, a group of very small predaceous dinosaurs, lived in Europe during the Late Jurassic Period.

One of the smallest dinosaurs known, Compsognathus grew only about as large as a chicken, but with a length of about 60–90 cm (2–3 feet), including the long tail, and a weight of about 55 kg (12 pounds). A swift runner, it was lightly built and had a long neck and tail, strong hind limbs, and very small forelimbs. Of special interest is a tiny skeleton preserved within the rib cage of one Compsognathus fossil. This skeleton was once mistakenly thought to be that of an embryo, but further study has shown it to be a lizard’s and thus documents the predatory habits of Compsognathus.

Recently, a closely related theropod dinosaur was discovered in China dating from the Early Cretaceous Period This fossil, dubbed Sinosauropteryx, has filamentous structures on the skin similar to the barbs of feathers, which suggests that feathers evolved from a much simpler structure that probably functioned as an insulator. Since this discovery, several such dinosaurs related to other known theropods have also been found in China.

CONFUCIUSORNIS

This genus of extinct crow-sized birds lived during the Late Jurassic and Early Cretaceous. Confuciusornis fossils were discovered in the Chaomidianzi Formation of Liaoning province, China, in ancient lake deposits mixed with layers of volcanic ash. These fossils were first described by Hou Lianhai and colleagues in 1995. Confuciusornis was about 25 cm (roughly 10 inches) from beak to pelvis. It possessed a small triangular snout and lacked teeth.

Images

Confuciusornis, a dinosaur of the late Jurassic and Early Cretaceous, shares a number of physical characteristics with modern birds. Encyclopædia Britannica, Inc.

Confuciusornis held a number of physical characteristics in common with modern birds but possessed some striking differences. Beautifully preserved specimens show impressions of its feathers, from which it can be inferred that the wings were of comparable size to those of similar flying birds today. Unlike modern birds, however, the forearm of Confuciusornis was shorter than both its hand and upper arm bone (humerus). It also retained the feature of having three free fingers on the hand, like Archaeopteryx and other theropod dinosaurs. In contrast, the fingers of more-derived birds are fused into an immobile element. Confuciusornis had a short tail, a common feature in modern birds, and its caudal vertebrae were reduced in size and number. The terminal vertebrae were fused to form a structure known as the pygostyle. In some specimens, as in the related Changchengornis, a pair of long, thin feathers proceeded from each side of the tail and expanded distally into a teardrop-shaped surface. It has been suggested that these feathers were sexually dimorphic structures and possibly indicative of males. To date, this hypothesis has not been tested statistically or corroborated by other dimorphic features.

Whereas most living birds (including the ostrich) reach full size within a year, the internal bone structure of Confuciusornis shows that it grew more slowly. Like other small dinosaurs, Confuciusornis probably took several years to mature. This evidence suggests that birds apparently did not evolve their rapid growth rates until sometime in the Late Cretaceous.

Local farmers living in or near the Chaomidianzi Formation collected the first known remains of Confuciusornis. Although a good number of specimens have been deposited in Chinese museums, many more have been sold illegally to commercial fossil dealers.

DIMORPHODON

The remains of Dimorphodon, a genus of primitive flying reptiles, have been found as fossils in European deposits from the Early Jurassic Period. Dimorphodon is among the earliest known pterosaurs, an extinct group of reptiles related to the dinosaurs. It was about 1 metre (3.3 feet) long and had a wingspan of about 1.7 metres (5.5 m).

The head was very lightly built but large and deep; the skull had several wide openings; and the eyes were large. In the front of Dimorphodon’s jaws were several large pointed teeth, but in the back there were many smaller ones. The limbs were well developed, and, like its ancestors (which were closely related to the first dinosaurs), it probably walked on two legs. The wings consisted of thin membranes of skin stretching from the enormously elongated fourth finger of each hand rearward to the hip or hind limbs. On the ground, the animal probably folded its wings in the manner of present-day birds and bats. The first three fingers of each hand were well developed, with large claws that were probably used for grasping.

Dimorphodon, like other early pterosaurs, had a long tail that probably helped stabilize it during flight. It also had a large breastbone and a large crest on the humerus to which the powerful flight muscles were attached. Like all but the largest pterosaurs, Dimorphodon was well suited for flapping flight.

DIPLODOCUS

Fossil remains of Diplodocus, a genus of dinosaurs known for their gigantic size, have been found in North America in rocks dating from the Late Jurassic Period. Diplodocus is perhaps the most commonly displayed dinosaur. It, along with sauropods such as Apatosaurus (formerly Brontosaurus), belong to a related subgroup of dinosaurs called diplodocids, members of which were some of the longest land animals that ever lived.

The skull of Diplodocus was unusually small and rather light. Elongate like that of a horse, it sat atop a very long neck. The brain was extremely small. The body was comparatively light and was well supported by limb girdles and pillarlike legs. While most of these dinosaurs weighed slightly more than 30 tons, some members of the genus may have weighed as much as 80 tons.

The tail was very long and probably extremely flexible. It most likely provided an anchoring site for the powerful hind leg muscles. The tail may also have functioned as a defensive weapon that could lash out at predators with great force. At some distance down the tail, certain arched structures beneath the tail vertebrae change in shape from spoonlike to canoelike. This flattening of the arches occurs at approximately the same height as where the base of the tail is located above the ground, which suggests that the tail could have been used as a prop for the hindlimbs. This arrangement may have enabled the animal to rear up on its back legs to feed on high vegetation.

DOCODON

This extinct genus of mammals was originally known only from fossilized teeth. The dentition patterns of the cusps and other molar structures are complex and distinct, resembling those of modern mammals. However, Docodon and its close relatives, the docodonts, are only distantly related to living mammal groups. Whether or not these animals are considered mammals is a controversial matter—Docodon fits only the broadest definition of mammal, having the typical mammalian jaw joint between the dentary and squamosal bones.

Docodonts are found in European and North American deposits of the Middle and Late Jurassic Period. The best-known docodont is Haldanodon from the Late Jurassic of Portugal. Haldanodon is recognized from a virtually complete skeleton that suggests that it was fossorial, or burrowing.

IGUANODON

Fossil remains belonging to the genus Iguanodon, a group of large herbivorous dinosaurs, dating from the Late Jurassic and Early Cretaceous periods (roughly 161 million to 100 million years ago) have been found in a wide area of Europe, North Africa, North America, Australia, and Asia; a few have been found from Late Cretaceous deposits of Europe and southern Africa.

Iguanodon was the largest, best known, and most widespread of all the iguanodontids (family Iguanodontidae), which are closely related to the hadrosaurs, or duck-billed dinosaurs. Iguanodon was 9 metres (30 feet) long, stood nearly 2 metres (6.5 feet) tall at the hip, and weighed four to five tons. The animal probably spent its time grazing while moving about on four legs, although it was able to walk on two. Iguanodontid forelimbs had an unusual five-fingered hand. The wrist bones were fused into a block; the joints of the thumb were fused into a conelike spike; the three middle fingers ended in blunt, hooflike claws; and the fifth finger diverged laterally from the others. Furthermore, the smallest finger had two small additional phalanges, a throwback to more primitive dinosaurian configuration. The teeth were ridged and formed sloping surfaces whose grinding action could pulverize its diet of low-growing ferns and horsetails that grew near streams and rivers. Most bones of the skull and jaws were not tightly fused but instead had movable joints that allowed flexibility when chewing tough plant material.

In 1825 Iguanodon became the second species to be described scientifically as a dinosaur, the first having been Megalosaurus. Iguanodon was named for its teeth, whose similarity to those of modern iguanas also provided the dinosaur’s discoverer, the English physician Gideon Mantell, with the first clue that dinosaurs had been reptiles. In his first reconstruction of the incomplete remains of Iguanodon, Mantell restored the skeleton in a quadrupedal pose with the spikelike thumb perched on its nose. This reconstruction persisted in London’s famous Crystal Palace dinosaur sculptures by Waterhouse Hawkins (1854) until many complete skeletons were found in Bernissart, Belgium, during the 1880s. Reconstructions of the Belgian skeletons mistakenly placed the animal in an upright, kangaroo-like stance with its tail on the ground—a misconception not corrected until the late 20th century, when a posture based upon a nearly horizontal backbone was adopted.

The fossil remains of many individuals have been found, some in groups, which suggests that iguanodontids traveled in herds. Fossilized tracks of iguanodontids are also relatively common and are widespread in Late Jurassic and Early Cretaceous deposits.

ORNITHOLESTES

Ornitholestes is a genus of small, lightly built carnivorous dinosaurs found as fossils from the Late Jurassic Period in North America. Ornitholestes is known from a nearly complete skeleton found in Wyoming, U.S. It was about 2 metres (6.6 feet) long, with a long skull, neck, and tail. The neck was apparently very flexible. The forelimbs were well developed and ended in three long clawed fingers, which indicates that Ornitholestes could catch quick and elusive prey. Its name means “bird robber,” but it probably ate small, speedy lizards and even early mammals. The hind limbs were well developed, with strong running muscles. Some authorities have equated Ornitholestes and Coelurus, but they appear to be separate genera.

PTERODACTYLS

“Pterodactyl” is an informal term for a subgroup of flying reptiles (Pterosauria) known from the Late Jurassic through Late Cretaceous periods.

Pterodactyls, or, more correctly, pterodactyloids, are distinguished from basal pterosaurs by their reduced teeth, tail, and fifth toe. Pterodactyloid metacarpals (palm bones) were more elongated than those of earlier pterosaurs, which instead had elongated phalanges (finger bones). There are also proportional differences in the skull, neck, pelvis, and wing bones. Pterodactyloid genera include Pterodactylus, a Late Jurassic form from Germany with a wingspan ranging from 50 cm (20 inches) to well over 1 metre (3.3 feet). It is likely that all fossils of Pterodactylus represent different stages of growth within a single species Pteranodon, a Late Cretaceous form found in North America, had a long cranial crest and a wingspan exceeding 7 metres (23 feet). Other crested genera are found in Late Cretaceous deposits of Brazil and include Tupuxuara, Anhanguera, and Santanadactylus. Dsungaripterus and several other crested forms have been discovered in China. A group of Late Cretaceous pterodactyloids called azhdarchids includes Montanazhdarcho and Quetzalcoatlus from North America, Europe, and Africa. The wingspan of these reptiles ranged from 2 to 11 metres (6.5–36 feet), which makes them the largest-known flying animals.

RHAMPHORHYNCHUS

Specimens of Rhamphorhynchus, a genus of flying reptiles (pterosaurs), were uncovered as fossils from the Late Jurassic Period in Europe. The finds suggest that the animal had a diamond-shaped rudder at the tip of its tail. Rhamphorhynchus was about 50 cm (20 inches) long, with a long skull and large eyes; the nostrils were set back on the beak. The teeth slanted forward and interlocked and were probably used to eat fish. The body was short, and each thin wing membrane stretched from a long fourth finger. The wing membrane probably attached near the hind limbs.

SCUTELLOSAURUS

This genus of small ornithischian dinosaurs from the Early Jurassic Period is characterized by the presence of small scutes along the back and sides of the body. Scutellosaurus had small forelimbs and robust hind limbs indicative of a bipedal stance. However, some authorities maintain that its forearms were strong enough to support quadrupedal movement. Scutellosaurus reached lengths of 1.5 to 2 metres (about 5 to 6.5 feet). Its skull grew to about 9 cm (about 3.5 inches) in length, and it contained several broad incisors and a row of fluted leaf-shaped cheek teeth that appear to be adapted for feeding on plants.

The first remains of Scutellosaurus, which made up a nearly complete skeleton, were found in the Kayenta Formation of Arizona by Douglas Lawler in 1971. Lawler, then a graduate student at the University of California, Berkeley, took the remains to American paleontologist E.H. Colbert at the Museum of Northern Arizona in Flagstaff. In 1981 Colbert described the remains (collected by a Harvard University field party in 1977), along with a second specimen, as Scutellosaurus lawleri. The remains of six additional specimens were recovered from other Kayenta localities in Arizona in 1983 by American paleontologist James M. Clark.

Colbert identified the new find and inferred that it was closely related to Lesothosaurus diagnosticus, a basal ornithischian, and so he placed it in the family Fabrosauridae. However, Scutellosaurus possessed scutes, whereas the fabrosaurs did not. The presence of scutes and other features of the skeleton, such as the curve and shape of the lower jaw, demonstrated that Scutellosaurus is more closely related to the stegosaurs and the ankylosaurs in suborder Thyreophora.

Most authorities now recognize Scutellosaurus as the most primitive known member of the Thyreophora. In fact, it is so basal that it does not belong to either subgroup. Ankylosaurs improved upon the body armour seen in Scutellosaurus by making it more robust and massive, which resulted in a sculpted, tanklike appearance. Stegosaurs, on the other hand, lost all the armour except a single row of parasagittal scutes alternating along the spinal column. These scutes were successively modified into various combinations of broad plates and narrow spikes. Although many authorities have long noted the defensive and thermoregulatory functions of these structures may have principally served as indicators that allowed different species of stegosaurs to recognize each other. In Scutellosaurus, however, the scutes were far too small to serve these functions. Embedded in the skin like those of crocodiles, the scutes were probably barely visible.

STEGOSAURUS

Stegosaurus, one genus of various plated dinosaurs (Stegosauria), lived during the Late Jurassic Period. Fossil remains of this animal are recognizable by the presence of a spiked tail and series of large triangular bony plates along the back. Stegosaurus usually grew to a length of about 6.5 metres (21 feet), but some reached 9 metres (30 feet). The skull and brain were very small for such a large animal. The forelimbs were much shorter than the hind limbs, which gave the back a characteristically arched appearance. The feet were short and broad.

Various hypotheses have attempted to explain the arrangement and use of the plates. Paleontologists had long thought that Stegosaurus had two parallel rows of plates, either staggered or paired, and that these afforded protection to the animal’s backbone and spinal cord. However, new discoveries and reexamination of existing Stegosaurus specimens since the 1970s suggest that the plates alternated along the backbone, as no two plates from the same animal have exactly the same shape or size. Because the plates contained many blood vessels, the alternating placement appears consistent with a hypothesis of thermoregulation. This hypothesis proposes that the plates acted as radiators, releasing body heat to a cooler ambient environment. Conversely, the plates could also have collected heat by being faced toward the sun like living solar panels

Two pairs of pointed bony spikes were present on the end of the tail. These are presumed to have served as defensive weapons, but they may have been ornamental. The spinal cord in the region of the sacrum was enlarged and was actually larger than the brain, a fact that gave rise to the misconception that Stegosaurus possessed two brains. It is more likely, however, that much of the sacral cavity was used for storing glycogen, as is the case in many present-day animals.

Stegosaurus and its relatives are closely related to the ankylosaurs, with which they share not only dermal armour but several other features, including a simple curved row of small teeth. Both groups evolved from a lineage of smaller armoured dinosaurs such as Scutellosaurus and Scelidosaurus of the Early Jurassic Period. Stegosaurs lost the armour from the flanks of the body that these early relatives had. Plating among different stegosaurs varied: some forms apparently had parallel rather than alternating plates, and some, such as Kentrurosaurus, had plates along the front half of the back and spikes along the back half and tail. These variations cast doubt on the hypothesis of a strong thermoregulatory function for the plates of Stegosaurus, because such structures were not optimized in all stegosaurs for collecting or releasing heat. Furthermore, it is puzzling why other stegosaurs and other dinosaurs lacked elaborate thermoregulatory structures. Display and species recognition remain likely functions for the plates, although such hypotheses are difficult to investigate.

STENEOSAURUS

Steneosaurus is a genus of extinct crocodiles that inhabited shallow seas and whose fossils are found in sediments of the Jurassic Period in South America, Europe, and North Africa. The skull of Steneosaurus was very light and narrow, with large openings and a long and narrow snout. The nostrils were at the tip of the snout and connected to the throat by a long bony tube. Many sharp teeth were present, which were probably used to eat fish.

TYRANNOSAURS

This group of predatory dinosaurs lived from the Late Jurassic Period to the latest Cretaceous Period, at which time they reached their greatest dominance. Most tyrannosaurs were large predators, with very large, high skulls approaching or well exceeding a full metre (more than three feet) in length. The best-known and largest member of the group is Tyrannosaurus rex, or T. rex. The “king of the tyrant lizards,” as its Latin name is usually translated, walked on powerfully developed hind limbs. If the animal had stood upright, it would have been more than 6.5 metres (21 feet) tall, but the usual posture was horizontal, with the body carried parallel to the ground and the tail held off the ground as a counterbalance. In this position a large adult, weighing 4,000 to 7,000 kg (about 9,000 to 15,000 pounds), could measure 14 metres (46 feet) long.

The longest known tyrannosaur skull is 1.3 metres (more than 4 feet) long. The skull bones of large tyrannosaurs are often several centimetres thick and are strongly braced to each other, which suggests a resistance to the forces of biting, both inflicted upon and received from other tyrannosaurs. Engineering models, in fact, show that the bite force of T. rex would easily have been capable of ripping through a car roof, as portrayed in the 1993 motion picture Jurassic Park, directed by Steven Spielberg. The huge mouth contained some 60 teeth, which could protrude as far as 15 cm (6 inches). The crowns of the teeth were shed and regrown frequently (every 250 days or so, on the basis of microscopic lines visible within the teeth). Serrations of the teeth bear deep pocketlike recesses in which bacteria may have flourished to provide an infectious bite.

Images

Robotic adult and baby Tyrannosaurus rex models used in a live show titled “Walking with Dinosaurs,” performed in 2009. Oli Scarff/Getty Images

Tyrannosaur teeth are distinctive. The front teeth are small and U-shaped. The side teeth are large, and in adults they become even larger, fewer in number, and D-shaped in cross section rather than daggerlike as in most theropods, or flesh-eating dinosaurs. In juveniles the teeth are laterally compressed and serrated front and back, like those of other theropods. In mature individuals, however, the teeth fall neatly into three general classes: upper front teeth, upper side teeth, and lower jaw teeth. Gut contents and coprolites (fossilized feces) of tyrannosaurs, as well as remains of other dinosaurs preserved with tyrannosaurid bite marks, show that tyrannosaurs were voracious predators that could easily bite through skulls, pelvises, and limbs of other dinosaurs.

In contrast to the powerful jaws and legs, the forelimbs of tyrannosaurs were very small (less than the length of the shoulder blade), and in some forms the hands were reduced to only two digits. Although a mechanical reconstruction suggests that the musculature of the arms of T. rex and some other large tyrannosaurs could have lifted about 180 kg (400 pounds), the hands would not have been able to reach the mouth or grasp prey. The hind limb bones appear massive but are lightly constructed: the thickness of the bone wall is only about 20 percent of the bones’ diameter—a figure approaching that of many birds.

The age of individual dinosaurs and other vertebrates can be determined by counting the annual growth rings that are laid down in the long bones, in a manner somewhat analogous to counting tree rings. By using a series of bones from early growth stages to adulthood, the life history of an animal species can be reconstructed. Such studies have shown that T. rex effectively reached full size in less than 20 years—approximately the same period as for human beings. Of course, at a length of 6.5 metres (21 feet) and a mass of six tons, T. rex reached a much larger size than humans in 20 years. But its growth rate was not as high as that of some herbivorous dinosaurs such as the hadrosaurs (duck-billed dinosaurs), which reached full size in seven or eight years, or the sauropods (the largest plant-eating dinosaurs), which attained most of their gigantic size in 14 years or so. On the other hand, the growth rate of T. rex was higher than that of the African elephant, which has a similar mass yet a longer time to maturity. Some of the known specimens of T. rex did not quite reach full size. Others do not seem to have survived long after achieving it. This may testify to the hard life of Mesozoic dinosaurs.

Although it was once thought that male and female tyrannosaurs could be distinguished by the shape of the tail vertebrae near the pelvis, this feature turns out not to be diagnostic. However, one subsequently discovered feature does establish sex. During the reproductive cycles of female birds, a layer of bone (medullary bone) is often deposited on the inner wall of the long bones. This process has been recognized in some fossils of tyrannosaurs (and of a few other dinosaurs), indicating that these specimens are female.

Fossils of T. rex are found only in the Hell Creek Formation of Garfield county, Montana, and adjacent areas of the United States, in deposits dating from the Maastrichtian Age, the last time unit of the Cretaceous Period—although slightly earlier relatives such as Tarbosaurus are known from Asia. Found in the same deposits as T. rex are fossils of the ceratopsians (giant horned dinosaurs) on which they likely preyed. There is some question about whether tyrannosaurs killed their food or simply scavenged it. However, neither predatory nor scavenging behaviour need be excluded, since T. rex, like many large carnivores today, probably fed opportunistically, scavenging when it could and hunting when it had to. One argument for predation emphasizes T. rex’s vision. The eye sockets tend to be keyhole-shaped and directed forward, which has been taken as evidence for accurate depth perception, because the fields of view of the eyes would overlap. Other evidence supporting predation is the well-protected skull and formidable jaws. Wounds in the bones of its prey indicate that T. rex ate by using a “puncture and tear” stroke, planting its feet and using the powerful muscles of the neck and legs to anchor itself and pull flesh off bones.

Before 1980 all knowledge of T. rex was based on only four skeletons, none very complete. The Latin name was given to the first specimen by American paleontologist Henry Fairfield Osborn in 1905 and was based on partial specimens collected from the Hell Creek Formation by renowned fossil hunter Barnum Brown. Remains found by Brown are on display at the Carnegie Museum of Natural History in Pittsburgh, Pa., the American Museum of Natural History in New York City, and the Natural History Museum in London. Since 1980 more than two dozen other specimens of T. rex have been discovered in western North America, some very complete. However, some are in private collections and are therefore lost to science and education. Two of the best specimens, consisting of almost complete adult skeletons, were unearthed in 1990. One, the 85-percent-complete “Wankel” T. rex, is on display at the Museum of the Rockies in Bozeman, Mont., and the other, the 90-percent-complete “Sue,” is displayed at the Field Museum in Chicago. Other T. rex specimens are mounted at other natural history museums in North America, such as the Denver Natural History Museum, the University of California Museum of Paleontology in Berkeley, the Natural History Museum of Los Angeles County, and the Royal Tyrrell Museum in Drumheller, Alta., near Dinosaur Provincial Park.

Images

Sue, a 76-million year-old Tyrannosaurus rex skeleton, is displayed in Washington, D.C.’s Union Station in 2000. Mark Wilson/Getty Images

In 2000 five T. rex specimens were discovered in the Hell Creek Formation. Several are now on display at the Museum of the Rockies. One of them, the “B-rex,” preserves soft tissues and also medullary bone that indicates the specimen was female. The soft tissues preserve transparent, flexible, hollow blood vessels that contain small round microstructures highly reminiscent in structure of red blood cells. The preservation of these structures is one of the most amazing features of the entire known fossil record.

Tyrannosaurs are generally divided into the large but more lightly built and slightly earlier albertosaurines and the still larger, more robust, and later tyrannosaurines. Most tyrannosaurs are known from the latest Cretaceous, but some basal forms are now known from the Early Cretaceous and even the Late Jurassic, though these earlier forms share few features with their later, well-known relatives Guanlong, an animal about 3 metres (10 feet) long from the Late Jurassic of Xinjiang province, western China, is the earliest well-known member of the group. It has some primitive and unique features—the most notable being a complex skull crest consisting of a hollow bone running atop the midline of its skull. Dilong, an early tyrannosaur 1.5 metres (5 feet) long from the spectacular Liaoning deposits of northeastern China, is preserved with a covering of simple, filamentous “protofeathers” like those seen on many other Early Cretaceous theropod dinosaurs. Eotyrannus, from Early Cretaceous deposits on Britain’s Isle of Wight, is also lightly built and relatively small (some 4.5 metres, or 15 feet, long). These three tyrannosaurs are so primitive that they retain three fingers on their hands.

Several small tyrannosaur fossils from the latest Cretaceous of western North America were once assigned to separate taxa, but most scholars now consider them to be merely young tyrannosaurs. For example, specimens once given the names Nanotyrannus and Stygivenator are now considered to be juvenile tyrannosaurs, and the former Dinotyrannus is now seen as a subadult T. rex. T. rex is the only tyrannosaur known from the late Maastrichtian Age (i.e., the latest Cretaceous Period) in North America. As is mentioned above, Tarbosaurus is a slightly earlier and very similar form from the latest Cretaceous of Mongolia.

Tyrannosaurs were long thought to be one of the carnosaurs (“flesh-eating lizards”), related to other large theropods such as the allosaurs. These resemblances have proved to be superficial, related to large size alone. Today tyrannosaurs are considered to be gigantic members of the coelurosaurs (“hollow-tailed lizards”), a group largely composed of smaller, more-gracile forms. Frequently they have been related to the largely toothless, ostrichlike ornithomimids, mainly because tyrannosaurs and ornithomimids share a peculiar foot with “pinched” foot bones. Tyrannosaurs may turn out to be closely related to the dromaeosaurs, the “raptors” of Jurassic Park, though evidence for this hypothesis is as elusive as any other.

YINLONG

This ceratopsian dinosaur genus is known from a single nearly complete skeleton taken from the Junggar Basin of western China. Yinlong was discovered in rock deposits dating from 159 million to 154 million years ago, during the Oxfordian and Kimmeridgian stages of the Late Jurassic Epoch. The genus is recognized as the most primitive ceratopsian dinosaur known, and it is also the earliest indisputable ceratopsian described from the Jurassic Period. The genus name is derived from Chinese words meaning “hiding dragon,” because the fossil was found near a filming location of the movie Crouching Tiger, Hidden Dragon, directed by Ang Lee (2000). The genus contains only one species, Yinlong downsi, named for the American vertebrate paleontologist William R. Downs III.

In addition to being the earliest ceratopsian, Yinlong is distinctive because its skeleton shares many features with Heterodontosaurus, a genus of ornithopod dinosaurs, and the pachycephalosaurians (such as Pachycephalosaurus). These features are important for determining the evolutionary relationships between all ornithischian dinosaurs.

Yinlong was herbivorous and measured 1.2 metres (about 4 feet) long. Like the pachycephalosaurians, Yinlong walked bipedally, whereas most ceratopsians relied on quadrupedal locomotion. It also shared a number of pachycephalosaurian skull characteristics not seen in other, more advanced ceratopsians. The combination of ceratopsian and pachycephalosaurian skeletal features in Yinlong strengthens the argument that the pachycephalosaurians were the closest relatives of the ceratopsian dinosaurs. It also suggests that later pachycephalosaurians retained more of the primitive characteristics initially shared between the two groups, while the skeletons of ceratopsians became much more derived with time.

OTHER SIGNIFICANT LIFE-FORMS OF THE JURASSIC PERIOD

Although the Jurassic was a time of great dinosaur speciation, other forms of life (mammals, fishes, mollusks, etc.) also continued to evolve. Several interesting examples of mammalian dentition also emerged during the Jurassic. Diarthrognathus retained both mammal-like and reptilelike features in its jaw. Other mammals, such as the multituberculates, Spalacotherium and Triconodon, developed specialized molar shapes and configurations. The Jurassic also marked the beginning of the halcyon times of pliosaurs, enormous reptilian carnivores that stalked Jurassic seas.

AUCELLA

This genus of clams is characteristically found as fossils in marine rocks of the Jurassic Period (between about 176 million and 145.5 million years old). The shell has a distinctive teardrop shape and is ornamented with a concentric pattern of ribs. The apex of one valve (shell half) is often curved over the other. A distinctive and commonly found Jurassic species is Aucella piochii.

CARDIOCERAS

An extinct genus of ammonite cephalopods, Cardioceras is related to the modern pearly nautilus. Cardioceras appears as fossils in rocks of the Late Jurassic Period. The several species known are excellent index, or guide, fossils for Jurassic rocks, enabling them to be correlated over widely separated areas. The shell of Cardioceras is circular in outline and ribbed, with a prominent crest along the outer margin.

DIARTHROGNATHUS

Diarthrognathus is a genus of extinct, advanced mammallike reptiles found as fossils in Early Jurassic terrestrial deposits about 200 million years old in southern Africa. Diarthrognathus was contemporaneous with a host of other mammal relatives but is nearer than many of them to the line leading to the true mammals because of its unspecialized features of skeletal anatomy and dentition. In true mammals, one jaw joint is formed by the squared bone of the skull and the dentary bone of the lower jaw. In other tetrapods, the location of this joint is determined by the intersection of the quadrate bone above and the articular bone below. In Diarthrognathus, both configurations are preserved, and both the quadrate and articular bones are reduced. These bones evolved to become two of the middle-ear bones in mammals.

GRYPHAEA

An extinct molluskan genus, Gryphaea fossils occur in rocks from the Jurassic period to the Eocene epoch (that is, between about 200 million and 34 million years ago). Related to the oysters, Gryphaea is characterized by its distinctively convoluted shape. The left valve, or shell, was much larger and more convoluted than the flattish right valve. Fine markings extended across the shell at right angles to the direction of growth. In some mature specimens, the coiling of the shell became so pronounced that it is unlikely that the shell could be opened at all, at which point the animal must have died.

HOLECTYPUS

Holectypus is a genus of extinct echinoids, animals much like the modern sea urchins and sand dollars. Holectypus fossils appear exclusively in marine rocks of Jurassic to Cretaceous age (that is, between roughly 200 million and 66 million years ago). Holectypus was bun shaped with a flat bottom and arched back.

INOCERAMUS

Fossils of the extinct pelecypod (clam) genus Inoceramus appear as fossils in Jurassic to Cretaceous rocks. Especially important and widespread in Cretaceous rocks, Inoceramus had a distinctive shell. It is large, thick, and wrinkled in a concentric fashion, making identification relatively simple. The many pits at the dorsal region were the anchoring points for the ligaments that closed the shell.

MULTITUBERCULATE

A multituberculate is any member of an extinct group of small, superficially rodentlike mammals that existed from about 178 million to 50 million years ago (that is, from the middle of the Jurassic Period until the early Eocene Epoch). During most of this span, they were the most common mammals. Adult multituberculates were usually the size of mice, though the largest species approached the size of beavers. They were dominantly herbivorous and granivorous. The distinguishing characteristic of multituberculates is the construction of their molars, with two or three longitudinal rows of cusps. In fossils of more primitive forms, there are five or six cusps, whereas up to 30 cusps are present in advanced genera. Multituberculates had a single pair of long lower incisors and possibly one to three pairs of upper incisors. In most genera, the anterior lower premolars were large shearing teeth.

The relationship of multituberculates to living mammals is controversial. Some authorities argue that they branched off before the emergence of the last common ancestor of monotremes, marsupials, and placentals. Other authorities argue that multituberculates are more closely related to the latter two groups.

PLIOSAURS

These large carnivorous marine reptiles are characterized by their massive heads, short necks, and streamlined, tearshaped bodies. Pliosaurs have been found as fossils from the Jurassic and Cretaceous periods. They are classified in Order Plesiosauria, along with their long-necked relatives, the plesiosaurs. Pliosaurs possessed powerful jaws and large teeth, and they used four large fins to swim through Mesozoic seas.

One notable pliosaur is Liopleurodon, a genus found in Middle Jurassic deposits in England and northern France. Liopleurodon is significant in that several fossils of variable quality that range in length from 5 to 25 metres (16 to 85 feet) have been placed in this genus, leading many authorities to question whether such specimens should be reclassified into other genera.

On the other hand, some groups did indeed grow quite large. For example, Kronosaurus, an Early Cretaceous pliosaur from Australia, grew to about 12 metres (about 40 feet) long. The skull alone measured about 3.7 metres (12.1 feet) long. An even larger pliosaur from the Jurassic, dubbed “Predator X,” was unearthed in Svalbard in 2009. Although it remains unclassified at present, some details are known. Its length and weight are estimated at 15 metres (about 50 feet) long and 45 tonnes (almost 100,000 pounds), respectively. The jaws of this creature are thought to have produced a bite force of 33,000 pounds per square inch, perhaps the highest bite force of any known animal.

Images

Illustrated Liopleurodon catching its prey. DEA Picture Library/Getty Images.

PYCNODONTIFORMES

Pycnodontiformes is an order of extinct fishes of the class Actinopterygii, containing the genus Pycnodus, common in Jurassic seas. Pycnodus is typical of pycnodonts, which were characterized by deep, narrow bodies that were very circular in outline in side view. The pycnodonts had a downturned beak and small mouth with an abundance of bulblike, rounded teeth with thick enamel surfaces. These structures enabled pycnodonts to crush their prey, such as the shelled invertebrates of coral reefs.

SPALACOTHERIUM

An extinct genus of primitive, probably predaceous, mammals, Spalacotherium is known from fossils found in European deposits dating from the Late Jurassic and Early Cretaceous periods (some 160 million to 100 million years ago). The genus Spalacotherium has a symmetrodont dentition, characterized by molar teeth with three cusps arranged in a triangle. The symmetrodonts are among the oldest known mammals and also among the most common European faunas of the time.

TRICONODON

Triconodon is a genus of extinct mammals found in European deposits of the Late Jurassic Period. The genus is representative of the triconodonts, known from fossils throughout North America, Europe, Africa, and China. Triconodon was about the size of a domestic cat. Triconodon was relatively large for its time, since most early mammals were very small. However, its brain was smaller than that of most living mammals. The canine teeth were large and strongly developed, so it is probable that Triconodon was an active predator. The premolars are simple, but the molars—for which the genus is named—have three distinctive cone-shaped cusps.

TRIGONIA

A genus of mollusks that first appeared during the Jurassic period, Trigonia still exists today. It has a triangular shell with distinctive concentric ridges on its surface as well as nodular outgrowths. A different ornamental pattern is present in the posterior parts of the shell.

JURASSIC GEOLOGY


The Jurassic Period was characterized by its high level of tectonic activity. The supercontinent Pangea broke apart into several pieces as a result of the processes of continental rifting and seafloor spreading. Igneous rocks laid down during the Jurassic were largely the result of such processes, whereas the production of sedimentary rock was influenced by rising sea levels. In contrast, the production of metamorphic rocks resulted from the subduction of tectonic plates and mountain-building processes.

THE ECONOMIC SIGNIFICANCE OF JURASSIC DEPOSITS

Jurassic igneous rocks have yielded uranium and gold in the Sierra Nevada range of North America, including placer deposits that were mined during the California Gold Rush of the mid-1800s. Some of the diamonds in Siberia were emplaced during Jurassic times. The shallow seas inundating Jurassic continents allowed for extensive deposition of sedimentary rocks that have provided important resources in many regions. For example, clay and limestone have been used for brick, cement, and other building materials in various areas of Europe; iron ore is prevalent in western Europe and England; and Jurassic salt is mined in both the United States and Germany.

Energy resources have also been derived from Jurassic deposits. Jurassic coals are found throughout Eurasia. One significant example is from the Late and Middle Jurassic Yan’an Formation in the Ordos Desert of China. A significant amount of American petroleum production comes from deposits trapped against salt domes of Jurassic age in the Gulf Coast of the United States. The North Sea and Arabian oil fields can also be traced back to organic-rich deposition in restricted Jurassic marine basins. Oil also is found in northern Germany and Russia.

THE OCCURRENCE AND DISTRIBUTION OF JURASSIC ROCKS

Jurassic rocks are widely distributed and include sedimentary, igneous, and metamorphic rocks. Because of continuous subduction and destruction of ocean crust in trenches, Middle Jurassic oceanic crust and sediments are generally the oldest sediments remaining in the deep sea. The Jurassic was a time marked by a high level of plate tectonic activity, and igneous rocks of Jurassic age are concentrated in the areas of activity, such as spreading centres (rifts and oceanic ridges) and mountain-building areas near subduction zones. In the areas where the Atlantic Ocean was opening and other continents were splitting apart, basalts that make up oceanic crust today accumulated in the basins. Notably, basalts are found along the east coast of North America and in southern Africa where it was connected to Antarctica. Volcanic ash can also be found near active margins. For example, many ash beds occur in the Late Jurassic Morrison Formation in western North America. Granite batholiths (igneous rocks that were emplaced at depth) can be found along the western margin of North and South America where subduction was occurring during the Jurassic.

Jurassic sedimentary rocks can be found on all modern continents and include marine, marginal marine, and terrestrial deposits. Jurassic marine sediments are also found on the modern seafloor. Because sea levels were high enough to cover large portions of continents, seaways formed on landmasses throughout the Jurassic. Thus, marine sandstones, mudstones, and shales often alternate with terrestrial conglomerates, sandstones, and mudstones. Marine carbonate limestones are mostly found in the tropics and the midlatitudes, where waters were warm and faunal productivity high. In Europe, black shales are common where restricted circulation in shallow marine basins caused bottom waters to become oxygen-deficient. Red beds, windblown sands, lake deposits, and coals can be found in terrestrial systems. Deltaic sands and salt deposits are found in what were once marginal marine environments.

NORTH AMERICA

The geologic profile of Jurassic North America is best separated into three different zones: the east coast, where rifting opened the Atlantic Ocean; the western interior, where continental sediments and epicontinental seaway sediments accumulated; and the west coast, where deformation occurred because of the presence of offshore subduction trenches.

In eastern North America, Late Triassic–Early Jurassic extensional basins were filled with red beds and other continental sediments, and pillow lavas were extruded into lake basins. The basaltic Watchung Flows of the Newark Basin are Early Jurassic in age, based on potassium-argon dating techniques that show them to be 185 to 194 million years old. More than 150 metres (500 feet) of Lower Jurassic lake beds were deposited in various basins on the east coast. Some of these bedded sediments may reflect orbital cycles. Middle Jurassic volcanoclastic rocks have been found beneath sediments on the continental shelf of New England. Upper Jurassic marine sediments include clastics interfingering with carbonates in the Atlantic and Gulf Coast basins. Middle Jurassic strata include evaporites, red beds, carbonates, and shelf-margin reefs. The Smackover Formation of the Gulf Coast sequences is a sedimentary unit typical of the Middle Jurassic.

In the western interior of North America, the Middle Jurassic is characterized by a series of six marine incursions. These epicontinental seaways are referred to collectively as the Carmel and Sundance seas. The Carmel Sea is older and not as deep as the Sundance. In these epicontinental seaways, marine sandstones, mudstones, limestones, and shales were deposited—some with marine fossils. Fully marine sequences interfinger with terrestrial sediments deposited during times of low sea levels and with marginal marine sediments that accumulated in environments bordering the seaways.

In the Late Jurassic, sea levels dropped in North America, and terrestrial sedimentation occurred across much of the continent. The Morrison Formation, a clastic deposit of lacustrine and fluvial mudstone, siltstone, sandstone, and conglomerate, is famous for fossil-rich beds that contain abundant plant and dinosaur remains. Uplift of the continental interior occurred between central Arizona and southern California from the Late Triassic until the Middle Jurassic.

Throughout the Jurassic the western margin of North America was bounded by an active subduction zone. This led to very complex geology and much plate tectonic activity, including collisions between terranes and North America, creation of volcanoes, and mountain-building episodes. Accretion of microcontinents and volcanic island arcs to the continent occurred along the entire coast of North America. More than 50 Jurassic terranes have been incorporated onto the continent. Some of the terranes may have originated from tropical areas and traveled far before colliding into North America. During the Nevadan orogeny, volcanic island arcs, including the Sierra Nevada, collided with the continent from northern California to British Columbia, and this resulted in the development of faults and emplacement of igneous intrusions. Deformation of the Foothills Terrane in the Sierra Nevada occurred 160 to 150 million years ago. Jurassic deep-sea rocks now uplifted and exposed in California are between 150 million and 200 million years old, as are intrusive igneous bodies such as the granite batholiths of Yosemite and the High Sierra. During the Jurassic, sediments accumulating off the continental margin were accreted along with the terranes. Many formations in the region are composed of ophiolites (oceanic crust), basalts, and deepwater marine sediments such as cherts, slates, and carbonates. Such a variety of rock types, deposited in a number of different environments, makes this region a geologic patchwork.

EURASIA AND GONDWANA

Similar to those in North America, Jurassic rocks in the rest of the world can be divided into three types: igneous rocks associated with continental rifting and seafloor spreading, sedimentary rocks associated with epicontinental seaways and terrestrial systems, and deformed deposits associated with subduction and mountain-building (orogenic) zones. Continental rifting between the regions of the Gondwana continent resulted in vast outpourings of basalts similar to those in the Newark Basin (although not as large in extent). These flood basalts are most notable in southern Africa, though thick volcanic sequences are also found on other landmasses that were breaking up at the time—Australia, South America, and India. Other rift-related sedimentary rocks also accumulated in these spreading centres.

The warm, shallow trough of the Tethys Sea between Eurasia and Gondwana accumulated thick sequences of Jurassic sediments. Carbonates are predominant and include fossiliferous shallow-water marls, limestones, and reefs. Siliceous limestones are fairly common, suggesting that an abundance of sponges were available to provide the silica. Evaporites formed along marginal environments around the seaway, while fine sandstones and mudstones are present mainly in nearshore environments near highlands. Deformation of these sediments began in the Late Jurassic, but most of the folding and faulting occurred after the Jurassic. The deformed sediments are exposed today in the Alps.

As seafloor spreading continued, Tethys widened further. Deeper-water sediments present within the Tethyan realm suggest that deepening basins developed during this time. The interiors of continents experienced different levels of marine inundation. As sea levels rose, Tethys expanded and at times covered large parts of the continental interior of Eurasia, allowing for the deposition of the sediments discussed above. Jurassic carbonates can be found in the Jura Mountains and southern France and in England. Fossiliferous, fine-grained lithographic limestones of Germany were deposited in lagoonal and marginal marine environments adjacent to the seaway. Clastic facies include the Early Jurassic shales of western Europe, the Late Jurassic clays of England and Germany, and the clays of the Russian Platform. The Arctic region was primarily a clastic province dominated by clay-rich rocks, shale, siltstone, sandstone, and conglomerate.

There are many examples of Jurassic black shales in Europe that represent intervals of low oxygen conditions at the seafloor. These conditions may have been developed because of restricted circulation and high levels of productivity. Some of the black shales contain exceptionally preserved vertebrate and invertebrate fossils that provide much of the paleontological information about the Jurassic.

On most of the southern landmasses (India, Antarctica, Africa, Australia, and New Zealand), marine deposits are generally restricted to the edges of continents because the continents were mainly above sea level for much of the Jurassic. Continental deposits consist mainly of red beds, sandstones, and mudstones that were deposited under fluvial, lacustrine, and eolian (wind-dominated) environments. Many parts of Eurasia also were dominated by terrestrial environments, accumulating coal beds and other continental sediments.

The Pacific margin of Asia, which was surrounded by subduction zones such as those along the west coast of North America, developed volcanic island arcs and associated basins from Japan to Indonesia. As the Pacific plate subducted under New Zealand during the Late Jurassic, terrane accretion, volcanic activity, and deformation occurred. Subduction zones off the west margin of South America resulted in igneous activity, deformation, and mountain building similar to that occurring in North America.

OCEAN BASINS

The oldest oceanic evidence for seafloor spreading (and magnetic anomalies) dates from about 147 million years ago, and the oldest oceanic sediments date from the Middle Jurassic. The Indian Ocean began to open at this time as India separated from Australia and Antarctica. The oldest crust of the Pacific basin dates from the Late Jurassic.

By the Early Jurassic, much of the flood basalts associated with the opening of the Atlantic Ocean had already been formed, and some significant basalts are found in the Newark Basin. In the early stages of formation of the Atlantic, nonmarine deposits such as fluvial (river), deltaic, and lacustrine (lake) sediments accumulated within the basin. In other cases, as on the Gulf Coast, marginal marine deposits such as evaporites (salt deposits) accumulated. The Jurassic Gulf Coast salt domes are huge (200 metres, or 660 feet, tall and 2 km, or 1.2 miles, in diameter), suggesting prolonged intervals of seawater evaporation. As the basins grew larger, connections were made with the open ocean, and the basins filled with marine waters and normal marine sediments. However, because these new oceans were still restricted and did not have vigorous circulation, oxygen content was low, allowing for the deposition of organic-rich shales. The source of North Sea oil comes from organic material buried during the Jurassic.

THE MAJOR SUBDIVISIONS OF THE JURASSIC SYSTEM

The Jurassic Period is divided into three epochs: Early Jurassic (about 200 million to 176 million years ago), Middle Jurassic (175.6 million to 161.2 million years ago), and Late Jurassic (about 161 million to 145.5 million years ago). (These intervals are sometimes referred to as the Lias, Dogger, and Malm, respectively.) Rocks that originated during the Jurassic period compose the Jurassic System. This system in turn is subdivided into stages, which are often established by using ammonites, bivalves, and protozoans (single-celled organisms) as index fossils. Some controversy exists among researchers as to where the boundaries between the stages should be drawn and what the dates of the boundaries should be. Difficulties arise because many Jurassic ammonites have only a limited geographic distribution. Regional ammonite zones have been established for many areas, but their exact placement in relationship to global correlations is unclear.

THE STAGES OF THE JURASSIC PERIOD

The Jurassic Period is divided into 11 stages. The Early Jurassic rock system has four stages—the Hettangian, Sinemurian, Pliensbachian, and Toarcian. The Middle Jurassic also has four stages—the Aalenian, Bajocian, Bathonian, and Callovian, whereas the Late Jurassic has three stages—the Oxfordian, Kimmeridgian, and Tithonian. The stages of the Jurassic Period are described in detail below.

HETTANGIAN STAGE

This stage is the lowest of the four divisions of the Lower Jurassic Series. It is the interval that represents all rocks formed worldwide during the Hettangian Age, which occurred between 199.6 million and 196.5 million years ago. The Hettangian Stage underlies the Jurassic Sinemurian Stage, and it overlies the Rhaetian Stage of the Triassic Period.

The name of this stage refers to its type district, located at the village of Hettange-Grande, near Thionville in the Lorraine region of France. The type district consists of a thick succession (57 to 70 metres, or 187 to 230 feet) of basal sandstones overlain by limestones and marls. The limestones bear the bivalve Gryphaea arcuata and other fossils that correlate to the biozone of the ammonite Psiloceras planorbis. Other species of this genus occur throughout eastern Siberia, North America, and South America, but the definitions of and relationships between Hettangian ammonites are not well established, making correlations difficult. In northwestern Europe the Lower Hettangian is referred to as the Planorbis Zone, the Middle Hettangian as the Liasicus Zone, and the Upper Hettangian as the Angulata Zone.

SINEMURIAN STAGE

The Sinemurian Stage is the second of the four divisions of the Lower Jurassic Series. It is the interval that contains all rocks formed worldwide during the Sinemurian Age, which occurred between 196.5 million and 189.6 million years ago. The Sinemurian Stage overlies the Hettangian Stage and underlies the Pliensbachian Stage.

The Sinemurian Stage was named for exposures at Semur (the ancient Roman town of Sinemurum) in northeastern France, where a condensed sequence of limestones contains fossils of ammonites that lived during this time interval. In northwestern Europe, six major ammonite biozones have been recognized for the Lower and Upper Sinemurian. Because Sinemurian ammonites are less geographically differentiated than earlier Jurassic forms, theoretically there should be more possibilities for large-scale regional stratigraphic correlations. However, many of the ammonite species are rare outside of northwestern Europe, and detailed fine-scale correlations and temporal divisions have not yet been developed for most regions around the world.

PLIENSBACHIAN STAGE

The third of the four divisions of the Lower Jurassic Series, the Pliensbachian Stage represents all rocks formed worldwide during the Pliensbachian Age, which occurred between 189.6 million and 183 million years ago. The Pliensbachian Stage overlies the Sinemurian Stage and underlies the Toarcian Stage.

The stage’s name is derived from the village of Pliensbach, Germany, which is near Boll in the Swabian Alps. The Pliensbachian Stage is represented by up to 195 metres (640 feet) of deposits, mostly marls, in Germany, Belgium, and Luxembourg. Five ammonite biozones, beginning with Uptonia jamesoni and ending with Pleuroceras spinatum, are recognized for the Lower and Upper Pliensbachian of northwestern Europe. The ammonites of this age worldwide exhibit a high level of regional differentiation, making global correlation difficult.

TOARCIAN STAGE

The fourth and uppermost division of the Lower Jurassic Series, the Toarcian Stage represents all rocks formed worldwide during the Toarcian Age, which occurred between 183 million and 175.6 million years ago. The Toarcian Stage overlies the Lower Jurassic Pliensbachian Stage and underlies the Aalenian Stage of the Middle Jurassic Series.

The stage’s name is derived from the village of Thouars (known as Toarcium in ancient Roman times) in western France. The standard succession is better known from the Lorraine region of northeastern France, where about 100 metres (330 feet) of marls and shales with nodular limestones are represented. In northwestern Europe there are two ammonite zones each in the Lower, Middle, and Upper Toarcian, ranging from the Tenuicostatum Zone to the Levesquei Zone. Many Toarcian ammonites are distributed widely around the world, which allows for better global correlations of Toarcian rocks than for those of some other Jurassic stages. However, some differences in species’ longevities and their definitions in various regions complicate correlation efforts.

AALENIAN STAGE

The first and lowest division of the Middle Jurassic Series, this interval corresponds to all rocks formed worldwide during the Aalenian Age, which occurred between 175.6 million and 171.6 million years ago. The Aalenian Stage underlies the Bajocian Stage and overlies the Toarcian Stage of the Lower Jurassic Series.

The name for this stage is derived from the town of Aalen, located 80 km (50 miles) east of Stuttgart in the Swabian Alps of Germany. The Aalenian Stage is divided into the Lower Aalenian and the Upper Aalenian, each of which in Europe is subdivided into two standard ammonite biozones: the Opalinum and Scissum zones for the Lower Aalenian and the Murchisonae and Concavum zones for the Upper Aalenian. In other parts of the world there is an almost complete absence of the ammonite group upon which the standard European zonation is based, so that several different zonation sequences have been recognized in different parts of Asia and North America. Some of these zones are approximately coeval and equivalent to the standard European zonations.

BAJOCIAN STAGE

The Bajocian Stage is the second of the four divisions of the Middle Jurassic Series, representing all rocks formed worldwide during the Bajocian Age, which occurred between 171.6 million and 167.7 million years ago. (Some researchers have proposed a longer time span for this stage that extends into more recent time.) The Bajocian Stage overlies the Aalenian Stage and underlies the Bathonian Stage.

The name for this stage is derived from the town of Bayeux in northwestern France. Bajocian rocks exhibit great variation and include coral reef limestones, oolitic deposits, and crinoidal limestones. Eight standard ammonite biozones have been recognized in the majority of European strata—five zones in the Lower Bajocian and three in the Upper Bajocian. However, because of significant differentiation of ammonites in various parts of the world, it is impossible to use these ammonite chronologies outside of Europe. Other regions employ zonation schemes for Bajocian strata that recognize different numbers of zones based on alternative species.

BATHONIAN STAGE

The Bathonian Stage is the third division of the Middle Jurassic Series. It encompasses all rocks formed worldwide during the Bathonian Age, which occurred between 167.7 million and 164.7 million years ago. (Some researchers have proposed a longer time span for this stage that extends into more recent time intervals.) The Bathonian Stage overlies the Bajocian Stage and underlies the Callovian Stage.

The stage’s name is derived from the city of Bath in the historic county of Somerset, England. The rock units that make up this stage include about 130 metres (430 feet) of strata, including parts of the Great Oolite and the Cornbrash Beds.

The Bathonian Age is divided into Early, Middle, and Late Bathonian, which are subdivided into various ammonite biozones. Many Bathonian ammonites have only a regional occurrence, so that different zonation schemes came to be established for various parts of the world. For example, unlike the Aalenian and Bajocian stages, two different ammonite sequences are used for the sub-Mediterranean region and northwestern Europe. Several distinct zonations have been developed for separate regions of the circum-Pacific belt. Some of these can be well correlated to the standard European ammonite biozones. However, many stratigraphic sequences in Asia and North America are difficult to correlate globally.

CALLOVIAN STAGE

This interval is the uppermost of the four divisions of the Middle Jurassic Series, corresponding to all rocks formed worldwide during the Callovian Age, which occurred between 164.7 million and 161.2 million years ago. (Some researchers have proposed a longer time span, from 160 million to 154 million years ago, with concomitant changes in the dates of other Jurassic stages.) The Callovian Stage overlies the Bathonian Stage and underlies the Oxfordian, the lowest stage of the Upper Jurassic Series.

The name for this stage is derived from the Kellaways area in Wiltshire, England, which was known as Callovium during Roman times. In England the Callovian includes strata from the Cornbrash Beds, Kellaways Beds, and Oxford Clay. The Callovian is subdivided into the Lower, Middle, and Upper Callovian, and throughout Europe each of these intervals is further subdivided into two standard ammonite biozones. Outside of Europe the Callovian sequences are not well developed, because of gaps in marine strata, small geographic ranges among the ammonites, and the presence of long-lived species that are unsuitable for correlation. In some regions ammonite associations are present but cannot easily be correlated to European forms. However, in certain other regions and time intervals in the circum-Pacific belt (such as the Lower Callovian of Mexico), a large number of European species can be found, permitting global correlations.

OXFORDIAN STAGE

The Oxfordian Stage is the first and lowest of the three divisions of the Upper Jurassic Series. The interval encompasses all rocks formed worldwide during the Oxfordian Age, which occurred between 161.2 million and 155.6 million years ago. (Some researchers have proposed a longer span for this stage that extends into more recent time.) The Oxfordian Stage underlies the Kimmeridgian Stage and overlies the Callovian Stage of the Middle Jurassic Series.

The name for this stage is derived from Oxford, Oxfordshire, England. The stage includes up to 90 metres (295 feet) of strata, including portions of the Oxford Clay and the Corallian Beds. The Oxfordian is divided into the Lower, Middle, and Upper Oxfordian, each of which is further subdivided into zones. In Europe there are seven standard ammonite biozones, with two (the Mariae and Cordatum) in the Lower Oxfordian, two (the Plicatilis and Transversarium) in the Middle Oxfordian, and three (the Bifurcatum, Bimammatum, and Planula) in the Upper Oxfordian. Outside of Europe the distribution of ammonites and other fossils used in correlations is often patchy because of unsuitable habitats during the Oxfordian and deformation of the strata after deposition. In addition, because of the limited geographic range of many species, it is difficult to correlate strata between regions of the world. In North America a number of different zones have been established for different areas, but gaps within the sequences prevent the zones from spanning the entire Oxfordian. In Asia and the southern Pacific there are fewer established zones, and their exact placement in relationship to global correlations is unclear.

KIMMERIDGIAN STAGE

The Kimmeridgian Stage is the second of the three divisions of the Upper Jurassic Series, encompassing all rocks formed worldwide during the Kimmeridgian Age, which occurred between 155.6 million and 150.8 million years ago. (Some researchers have proposed a more recent endpoint for this stage.) The Kimmeridgian Stage overlies the Oxfordian Stage and underlies the Tithonian Stage.

The name for this stage is derived from the Kimmeridge area in Dorset, southern England. In England the Kimmeridgian includes the Kimmeridge Clay. The Kimmeridgian Stage is divided into the Lower Kimmeridgian and the Upper Kimmeridgian, each of which contains three standard ammonite biozones—the Platynota, Hypselocyclum, and Divisum in the Lower Kimmeridgian and the Acanthicum, Eudoxus, and Beckeri in the Upper Kimmeridgian. In North America only Mexico has a detailed ammonite stratigraphic zonation developed for much of the Kimmeridgian. In other regions the presence of ammonites that are useful for correlation is sparse, and often these species cover a wide range of time or are not easily correlated to other areas. There are large regions where no ammonites have been found, making the development of stratigraphic zones and global correlations very difficult. Bivalves such as Buchia and Retroceramus have been used to correlate strata from the circum-Pacific region.

TITHONIAN STAGE

This interval is the uppermost of the three divisions of the Upper Jurassic Series. It corresponds to all rocks formed worldwide during the Tithonian Age, which occurred between 150.8 million and 145.5 million years ago. The Tithonian Stage overlies the Kimmeridgian Stage and underlies the Berriasian, the lowest stage of the Cretaceous Period.

The name of this stage is derived not from a geographic source but from the Greek mythological figure Tithonus, who was the consort of Eos (Aurora), goddess of the dawn. The Tithonian Stage has replaced the Volgian and Purbeckian Stages, which were previously locally recognized in Russia and England, respectively.

In Europe the Tithonian is divided into the Lower, Middle, and Upper Tithonian. Each of these intervals is further divided into numerous standard European ammonite biozones: the Lower Tithonian includes the Hybonotum and Darwini zones; the Middle Tithonian includes the Semiforme, Fallauxi, and Ponti zones; and the Upper Tithonian includes the Micracanthum and Durangites zones.

In other parts of the world, Mexico is one of the few regions where an extensive, detailed ammonite stratigraphic zonation has been developed. Elsewhere only a few zones have been recognized, and in some areas the exact timing and correlations of these zones have not been finalized. As with the other Upper Jurassic stages, the lack of well-developed global correlations is due to patchy distribution of ammonites and tightly constrained geographic distributions for individual species.

SIGNIFICANT JURASSIC FORMATIONS AND DISCOVERIES


Certain geologic structures have greatly increased the scientific understanding of Jurassic time. Three of the four items described below are units of rocks that contain examples of some of the fossil life-forms described above. Although the rock formations are useful for preserving the harder and more durable parts of dinosaurs and other forms of Jurassic life, the fourth structure, the coprolite, possessed the ability to protect the less-durable remains of other organisms. Without coprolites, the remains of several species would have been completely destroyed and thus unknown to science.

COPROLITES

A coprolite is defined as the fossilized excrement of animals. The English geologist William Buckland coined the term in 1835 after he and fossilist Mary Anning recognized that certain convoluted masses occurring in the Lias rock strata of Gloucestershire and dating from the Early Jurassic Period had a form that would have been produced by their passage in the soft state through the intestines of reptiles or fishes. These bodies had long been known as fossil fir cones and bezoar stones (hardened undigestible contents of the intestines). Buckland’s conjecture that they were of fecal origin and similar to the excrement of hyenas was confirmed upon analysis. They were found to consist essentially of calcium phosphate and carbonate, and they not infrequently contained fragments of unaltered bone. The name coprolites, from the Greek kopros (“dung”) and lithos (“stone”), was accordingly given them by Buckland. Coprolites often preserve the remains of plants and small animals that would otherwise be destroyed or lost. They are therefore important sources of concentrated information about ancient biota and environments.

THE MORRISON FORMATION

The Morrison Formation is a series of sedimentary rocks deposited during the Jurassic Period in western North America, from Montana to New Mexico. The Morrison Formation is famous for its dinosaur fossils, which have been collected for more than a century, beginning with a find near the town of Morrison, Colorado, in 1877. Radiometric dating indicates that the Morrison Formation is between 148 million and 155 million years old. Correlation of fossils indicates that it was deposited during the Kimmeridgian and early Tithonian ages and possibly during the latest Oxfordian Age.

The sediments in the Morrison Formation include multicoloured mudstones, sandstones, and conglomerates, as well as minor amounts of marls, limestone, and clay-stones. The sediments were derived from western mountains, such as the Sierra Nevada range, that were uplifted during Late Jurassic time. There are also numerous volcanic ash beds within the formation that have been used to date the deposits through radiometric techniques. Some sediments in the lowest portion of the Morrison Formation are marine in origin, but the majority of the sediments were deposited along rivers, streams, lakes, mudflats, swamps, and alluvial plains that covered the western interior of North America during the Late Jurassic.

The nonmarine sediments contain abundant fossils—plants as well as the famous invertebrate and vertebrate animals. Dinosaur National Monument in eastern Utah was established to preserve and exhibit fossils from the Morrison Formation. Many of the dinosaur fossils are found as jumbled accumulations consisting of dozens of partially disarticulated skeletons. These probably resulted from the transportation of dinosaur carcasses along streams and their subsequent burial on sandbars. The dinosaurs are quite diverse and represent a number of different habitats. Mollusks, fishes, insects, crocodiles, turtles, and other fossils suggest that some lakes in the area were freshwater but that saline, alkaline lakes were also present.

THE PURBECK BEDS

This unit of exposed sedimentary rocks occurs in southern England and spans the boundary between the Jurassic and Cretaceous periods, approximately 145.5 million years ago. The highly varied Purbeck Beds, which overlie rocks of the Portland Beds, record a marked change in sedimentary facies, indicating major alterations in environmental conditions. Limestones, marls, clays, and old soil horizons are present in thicknesses of up to 170 metres (560 feet).

The type section is at Durlston Bay near Swanage, Dorset. Each of the Lower, Middle, and Upper Purbeck beds contains distinctive units. The Lower Purbeck is completely Jurassic in age, having been deposited during the Tithonian Age, and the Upper Purbeck is entirely Cretaceous in age, having been deposited during the Berriasian Age. The boundary between the two geologic time periods appears to occur in the Cinder Bed unit of the Middle Purbeck.

The varied rock types of the Purbeck Beds were deposited in marine, marginal marine (such as brackish lagoons), and freshwater settings. Ancient land soils in the Lower Purbeck include the fossilized stumps of coniferous trees and primitive palmlike cycads. In addition, shales and clays occasionally contain fossil insects. The Middle and Upper Purbeck consist of freshwater limestones that are quarried for use as building stone. Marls and shales are interbedded with the limestones.

The lowest unit of the Middle Purbeck, the Marly Freshwater Beds, has a Mammal Bed containing about 20 mammalian species. The Cinder Bed, located within the Middle Purbeck, is a marine unit containing varied fauna, including large quantities of oysters, trigonids (a type of Mesozoic clam), and fragments of echinoids (sea urchins). The Upper Building Stones unit of the Middle Purbeck contains fossils of turtles and fish that probably lived in brackish water. The Upper Purbeck contains freshwater fossils and is the source of “Purbeck Marble” building stones.

THE SOLNHOFEN LIMESTONE

This famous Jurassic Period limestone unit located near the town of Solnhofen, southern Germany, contains exceptionally preserved fossils from the Tithonian Age. The Solnhofen Limestone is composed of thin beds of fine-grained limestones interbedded with thin shaley layers. They were originally deposited in small, stagnant marine basins (possibly with a very high salt content and low oxygen content) surrounded by reefs. The limestones have been quarried for hundreds of years for buildings and for lithographic printmaking. The Solnhofen Limestone is also known as Solnhofen Plattenkalk.

More than 750 plant and animal species have been described from the Solnhofen Limestone. The most common fossils are crinoids, ammonites, fishes, and crustaceans. The most famous fossil from Solnhofen is Archaeopteryx, an ancient bird that left impressions of its feathers preserved in the rock. It is the oldest bird fossil to have been found by paleontologists.

The Solnhofen is well known for the exceptional preservation of soft-bodied organisms such as jellyfish, squid, and insects that are not usually incorporated into the fossil record. The burial of such organisms in the fine-grained sediments of stagnant marine basins allowed even the impressions of internal organs to be preserved.