Dinosaurs timeline
dinosaur, (clade Dinosauria), the common name given to a group of reptiles, often very large, that first appeared roughly 245 million years ago (near the beginning of the Middle Triassic Epoch) and thrived worldwide for nearly 180 million years. Most died out by the end of the Cretaceous Period, about 66 million years ago, but many lines of evidence now show that one lineage evolved into birds about 155 million years ago.
The name dinosaur comes from the Greek words deinos (“terrible” or “fearfully great”) and sauros (“reptile” or “lizard”). The English anatomist Richard Owen proposed the formal term Dinosauria in 1842 to include three giant extinct animals (Megalosaurus, Iguanodon, and Hylaeosaurus) represented by large fossilized bones that had been unearthed at several locations in southern England during the early part of the 19th century. Owen recognized that these reptiles were far different from other known reptiles of the present and the past for three reasons: they were large yet obviously terrestrial, unlike the aquatic ichthyosaurs and plesiosaurs that were already known; they had five vertebrae in their hips, whereas most known reptiles have only two; and, rather than holding their limbs sprawled out to the side in the manner of lizards, dinosaurs held their limbs under the body in columnar fashion, like elephants and other large mammals.
Originally applied to just a handful of incomplete specimens, the clade Dinosauria now encompasses more than 800 generic names and at least 1,000 species, with new names being added to the roster every year as the result of scientific explorations around the world. Not all of these names are valid taxa, however. A great many of them have been based on fragmentary or incomplete material that may actually have come from two or more different dinosaurs. In addition, bones have sometimes been misidentified as dinosaurian when they are not from dinosaurs at all. Nevertheless, dinosaurs are well documented by abundant fossil remains recovered from every continent on Earth, and the number of known dinosaurian taxa is estimated to be 10–25 percent of actual past diversity.
The extensive fossil record of genera and species is testimony that dinosaurs were diverse animals, with widely varying lifestyles and adaptations. Their remains are found in sedimentary rock layers (strata) dating to the Late Triassic Epoch (approximately 237 million to 201.3 million years ago). The abundance of their fossilized bones is substantive proof that dinosaurs were the dominant form of terrestrial animal life during the Mesozoic Era (about 252.2 million to 66 million years ago). It is likely that the known remains represent a very small fraction (probably less than 0.0001 percent) of all the individual dinosaurs that once lived.
The search for dinosaurs
The first finds CODE
The earliest verifiable published record of dinosaur remains that still exists is a note in the 1820 American Journal of Science and Arts by Nathan Smith. The bones described had been found in 1818 by Solomon Ellsworth, Jr., while he was digging a well at his homestead in Windsor, Connecticut. At the time, the bones were thought to be human, but much later they were identified as Anchisaurus. Even earlier (1800), large birdlike footprints had been noticed on sandstone slabs in Massachusetts. Pliny Moody, who discovered these tracks, attributed them to “Noah’s raven,” and Edward Hitchcock of Amherst College, who began collecting them in 1835, considered them to be those of some giant extinct bird. The tracks are now recognized as having been made by several different kinds of dinosaurs, and such tracks are still commonplace in the Connecticut River valley today.
Better known are the finds in southern England during the early 1820s by William Buckland (a clergyman) and Gideon Mantell (a physician), who described Megalosaurus and Iguanodon, respectively. In 1824 Buckland published a description of Megalosaurus, fossils of which consisted mainly of a lower jawbone with a few teeth. The following year Mantell published his “Notice on the Iguanodon, a Newly Discovered Fossil Reptile, from the Sandstone of Tilgate Forest, in Sussex,” on the basis of several teeth and some leg bones. Both men collected fossils as an avocation and are credited with the earliest published announcements in England of what later would be recognized as dinosaurs. In both cases their finds were too fragmentary to permit a clear image of either animal. In 1834 a partial skeleton was found near Brighton that corresponded with Mantell’s fragments from Tilgate Forest. It became known as the Maidstone Iguanodon, after the village where it was discovered. The Maidstone skeleton provided the first glimpse of what these creatures might have looked like.
Two years before the Maidstone Iguanodon came to light, a different kind of skeleton was found in the Weald of southern England. It was described and named Hylaeosaurus by Mantell in 1832 and later proved to be one of the armoured dinosaurs. Other fossil bones began turning up in Europe: fragments described and named as Thecodontosaurus and Palaeosaurus by two English students, Henry Riley and Samuel Stutchbury, and the first of many skeletons named Plateosaurus by the naturalist Hermann von Meyer in 1837. Richard Owen identified two additional dinosaurs, albeit from fragmentary evidence: Cladeiodon, which was based on a single large tooth, and Cetiosaurus, which he named from an incomplete skeleton composed of very large bones. Having carefully studied most of these fossil specimens, Owen recognized that all of these bones represented a group of large reptiles that were unlike any living varieties. In a report to the British Association for the Advancement of Science in 1841, he described these animals, and the word Dinosauria was first published in the association’s proceedings in 1842.
Reconstruction and classification
During the decades that followed Owen’s announcement, many other kinds of dinosaurs were discovered and named in England and Europe: Massospondylus in 1854, Scelidosaurus in 1859, Bothriospondylus in 1875, and Omosaurus in 1877. Popular fascination with the giant reptiles grew, reaching a peak in the 1850s with the first attempts to reconstruct the three animals on which Owen based Dinosauria—Iguanodon, Megalosaurus, and Hylaeosaurus—for the first world exposition, the Great Exhibition of 1851 in London’s Crystal Palace. A sculptor under Owen’s direction (Waterhouse Hawkins) created life-size models of these two genera, and in 1854 they were displayed together with models of other extinct and living reptiles, such as plesiosaurs, ichthyosaurs, and crocodiles.
By the 1850s it had become evident that the reptile fauna of the Mesozoic Era was far more diverse and complex than it is today. The first important attempt to establish an informative classification of the dinosaurs was made by the English biologist T.H. Huxley as early as 1868. Because he observed that these animals had legs similar to birds as well as other birdlike features, he established a new order called Ornithoscelida. He divided the order into two suborders. Dinosauria was the first and included the iguanodonts, the large carnivores (or megalosaurids), and the armoured forms (including Scelidosaurus). Compsognatha was the second order, named for the very small birdlike carnivore Compsognathus.
Huxley’s classification was replaced by a radically new scheme proposed in 1887 by his fellow Englishman H.G. Seeley, who noticed that all dinosaurs possessed one of two distinctive pelvic designs, one like that of birds and the other like that of reptiles. Accordingly, he divided the dinosaurs into the orders Ornithischia (having a birdlike pelvis) and Saurischia (having a reptilian pelvis). Ornithischia included four suborders: Ornithopoda (Iguanodon and similar herbivores), Stegosauria (plated forms), Ankylosauria (Hylaeosaurus and other armoured forms), and Ceratopsia (horned dinosaurs, just then being discovered in North America). Seeley’s second order, the Saurischia, included all the carnivorous dinosaurs, such as Megalosaurus and Compsognathus, as well as the giant herbivorous sauropods, including Cetiosaurus and several immense “brontosaur” types that were turning up in North America. In erecting Saurischia and Ornithischia, Seeley cast doubt on the idea that Dinosauria was a natural grouping of these animals. This uncertainty persisted for a century thereafter, but it is now understood that the two groups share unique features that indeed make the Dinosauria a natural group.
In 1878 a spectacular discovery was made in the town of Bernissart, Belgium, where several dozen complete articulated skeletons of Iguanodon were accidentally uncovered in a coal mine during the course of mining operations. Under the direction of the Royal Institute of Natural Science of Belgium, thousands of bones were retrieved and carefully restored over a period of many years. The first skeleton was placed on exhibit in 1883, and today the public can view an impressive herd of Iguanodon. The discovery of these multiple remains gave the first hint that at least some dinosaurs may have traveled in groups and showed clearly that some dinosaurs were bipedal (walking on two legs). The supervisor of this extraordinary project was Louis Dollo, a zoologist who was to spend most of his life studying Iguanodon, working out its structure, and speculating on its living habits.f
American hunting expeditions
England and Europe produced most of the early discoveries and students of dinosaurs, but North America soon began to contribute a large share of both. One leading student of fossils was Joseph Leidy of the Academy of Natural Sciences in Philadelphia, who named some of the earliest dinosaurs found in America, including Palaeoscincus, Trachodon, Troodon, and Deinodon. Unfortunately, some names given by Leidy are no longer used, because they were based on such fragmentary and undiagnostic material. Leidy is perhaps best known for his study and description of the first dinosaur skeleton to be recognized in North America, that of a duckbill, or hadrosaur, found at Haddonfield, New Jersey, in 1858, which he named Hadrosaurus foulkii. Leidy’s inference that this animal was probably amphibious influenced views of dinosaur life for the next century.
Two Americans whose work during the second half of the 19th century had worldwide impact on the science of paleontology in general, and the growing knowledge of dinosaurs in particular, were O.C. Marsh of Yale College and E.D. Cope of Haverford College, the University of Pennsylvania, and the Academy of Natural Sciences in Philadelphia. All previous dinosaur remains had been discovered by accident in well-populated regions with temperate, moist climates, but Cope and Marsh astutely focused their attention on the wide arid expanses of bare exposed rock in western North America. In their intense quest to find and name new dinosaurs, these scientific pioneers became fierce and unfriendly rivals.
Marsh’s field parties explored widely, exploiting dozens of now famous areas, among them Yale’s sites at Morrison and Canon City, Colorado, and, most important, Como Bluff in southeastern Wyoming. The discovery of Como Bluff in 1877 was a momentous event in the history of paleontology that generated a burst of exploration and study as well as widespread public enthusiasm for dinosaurs. Como Bluff brought to light one of the greatest assemblages of dinosaurs, both small and gigantic, ever found. For decades the site went on producing the first known specimens of Late Jurassic Epoch (163.5 million to 145 million years ago) dinosaurs such as Stegosaurus, Camptosaurus, Camarasaurus, Laosaurus, Coelurus, and others. From the Morrison site came the original specimens of Allosaurus, Diplodocus, Atlantosaurus, and Brontosaurus (later renamed Apatosaurus). Canon City provided bones of a host of dinosaurs, including Stegosaurus, Brachiosaurus, Allosaurus, and Camptosaurus.
Another major historic site was the Lance Creek area of northeastern Wyoming, where J.B. Hatcher discovered and collected dozens of Late Cretaceous horned dinosaur remains for Marsh and for Yale College, among them the first specimens of Triceratops and Torosaurus. Marsh was aided in his work at these and other localities by the skills and efforts of many other collaborators like Hatcher—William Reed, Benjamin Mudge, Arthur Lakes, William Phelps, and Samuel Wendell Williston, to name a few. Marsh’s specimens now form the core of the Mesozoic collections at the National Museum of Natural History of the Smithsonian Institution and the Peabody Museum of Natural History at Yale University.
Cope’s dinosaur explorations ranged as far as, or farther than, Marsh’s, and his interests encompassed a wider variety of fossils. Owing to a number of circumstances, however, Cope’s dinosaur discoveries were fewer and his collections far less complete than those of Marsh. Perhaps his most notable achievement was finding and proposing the names for Coelophysis and Monoclonius. Cope’s dinosaur explorations began in the eastern badlands of Montana, where he discovered Monoclonius in the Judith River Formation of the Late Cretaceous Epoch (100.5 million to 66 million years ago). Accompanying him there was a talented young assistant, Charles H. Sternberg. Later Sternberg and his three sons went on to recover countless dinosaur skeletons from the Oldman and Edmonton formations of the Late Cretaceous along the Red Deer River of Alberta, Canada.
Dinosaur ancestors
During the early decades of dinosaur discoveries, little thought was given to their evolutionary ancestry. Not only were the few specimens known unlike any living animal, but they were so different from any other reptiles that it was difficult to discern much about their relationships. Early on it was recognized that, as a group, dinosaurs appear to be most closely allied to crocodilians, though T.H. Huxley had proposed in the 1860s that dinosaurs and birds must have had a very close common ancestor in the distant past. Three anatomic features—socketed teeth, a skull with two large holes (diapsid), and another hole in the lower jaw—are present in both crocodiles and dinosaurs. The earliest crocodilians occurred nearly simultaneously with the first known dinosaurs, so neither could have given rise to the other. It was long thought that the most likely ancestry of dinosaurs could be found within a poorly understood group of Triassic reptiles termed thecodontians (“socket-toothed reptiles”). Today it is recognized that “thecodontian” is simply a name for the basal, or most primitive, members of the archosaurs (“ruling reptiles”), a group that is distinguished by the three anatomic features mentioned above and that includes dinosaurs, pterosaurs (flying reptiles), crocodiles, and their extinct relatives.
An early candidate for the ancestor of dinosaurs was a small basal archosaur from the Early Triassic Epoch (252.2 million to 247.2 million years ago) of South Africa called Euparkeria. New discoveries suggest creatures that are even more dinosaur-like from the Middle Triassic (247.2 million to 237 million years ago) and from an early portion of the Late Triassic (237 million to 201.3 million years ago) of South America; these include Lagerpeton, Lagosuchus, Pseudolagosuchus, and Lewisuchus. Other forms, such as Nyasasaurus and Asilisaurus, date from the Middle Triassic of East Africa; Nyasasaurus is considered by some to be the oldest known member of Dinosauria. Other South American forms such as Eoraptor and Herrerasaurus are particularly dinosaurian in appearance and are sometimes considered dinosaurs.
The earliest appearance of “true dinosaurs” is almost impossible to pinpoint, since it can never be known with certainty whether the very first (or last) specimen of any kind of organism has been found. The succession of deposits containing fossils is discontinuous and contains many gaps; even within these deposits, the fossil record of dinosaurs and other creatures contained within is far from complete. Further complicating matters is that evolution from ancestral to descendant form is usually a stepwise process. Consequently, as more and more gaps are filled between the first dinosaurs and other archosaurs, the number of features distinguishing them becomes smaller and smaller. Currently, paleontologists define dinosaurs as Triceratops (representing Ornithischia), birds (the most recent representatives of the Saurischia), and all the descendants of their most recent common ancestor. That common ancestor apparently had a suite of features not present in other dinosaur relatives, including the loss of the prefrontal bone above the eye, a long deltopectoral crest on the humerus, three or fewer joints on the fourth finger of the hand, three or more hip vertebrae, a fully open hip socket, and a cnemial crest on the shin bone (tibia). These features were passed on and modified in the descendants of the first dinosaurs. Compared with most of their contemporaries, dinosaurs had an improved stance and posture with a resulting improved gait and, in several independent lineages, an overall increase in size. They also were more efficient at gathering food and processing it and apparently had higher metabolic rates and cardiovascular nourishment. All these trends, individually or in concert, probably contributed to the collective success of dinosaurs, which resulted in their dominance among the terrestrial animals of the Mesozoic.
Modern studies
During the first century or more of dinosaur awareness, workers in the field more or less concentrated on the search for new specimens and new types. Their discoveries then required detailed description and analysis, followed by comparisons with other known dinosaurs in order to classify the new finds and develop hypotheses about evolutionary relationships. These pursuits continue, but newer methods of exploration and analysis have been adopted. Emphasis has shifted from purely descriptive procedures to analyses of relationships by using the methods of cladistics, which dispenses with the traditional taxonomic hierarchy in favour of “phylogenetic trees” that are more explicit about evolutionary relationships. Phylogenetic analyses also help us to understand how certain features evolved in groups of dinosaurs and give us insight into their possible functions. For example, in the evolution of horned dinosaurs (ceratopsians), it can be seen that the beak evolved first, followed by the frill, and finally the nose and eye horns, which were differently developed in different groups. The hypothesis that the frill was widely used in defense by ceratopsians such as Protoceratops can thus be tested phylogenetically. On this basis, the idea is now generally rejected because the frill was basically just an open rim of bone in nearly all ceratopsians except Triceratops, which is often pictured charging like a rhinoceros.
Functional anatomic studies extensively use analogous traits of present-day animals that, along with both mechanical and theoretical models, make it possible to visualize certain aspects of extinct animals. For example, estimates of normal walking and maximum running speeds can be calculated on the basis of the analysis of trackways, which can then be combined with biomechanical examination of the legs and joints and reconstruction of limb musculature. Similar methods have been applied to jaw mechanisms and tooth wear patterns to obtain a better understanding of feeding habits and capabilities.
The soft parts of dinosaurs are only imperfectly known. Original colours and patterns cannot be known, but skin textures have occasionally been preserved. Most show a knobby or pebbly surface rather than a scaly texture as in most living reptiles. Impressions of internal organs are rarely preserved, but, increasingly, records of filaments and feathers have been found on some dinosaurs. The discovery of Kulindadromeus zabaikalicus, an early ornithischian dinosaur whose remains show evidence of featherlike structures on its limbs, suggests that feathers may even have been widespread among the dinosaurs. Gastroliths (“stomach stones”) used for processing food in the gizzard have been recovered from a variety of dinosaurs.
Extinction
A misconception commonly portrayed in popular books and media is that all the dinosaurs died out at the same time—and apparently quite suddenly—at the end of the Cretaceous Period, 66 million years ago. This is not entirely correct, and not only because birds are a living branch of dinosaurian lineage. The best records, which are almost exclusively from North America, show that dinosaurs were already in decline during the latest portion of the Cretaceous. The causes of this decline, as well as the fortunes of other groups at the time, are complex and difficult to attribute to a single source. In order to understand extinction, it is necessary to understand the basic fossil record of dinosaurs.
Faunal changes
During the 160 million years or so of the Mesozoic Era (252.2 million to 66 million years ago) from which dinosaurs are known, there were constant changes in dinosaur communities. Different species evolved rapidly and were quickly replaced by others throughout the Mesozoic; it is rare that any particular type of dinosaur survived from one geologic formation into the next. The fossil evidence shows a moderately rich fauna of plateosaurs and other prosauropods, primitive ornithopods, and theropods during the Late Triassic Epoch (237 million to 201.3 million years ago). Most of these kinds of dinosaurs are also represented in strata of the Early Jurassic Epoch (201.3 million to 174.1 million years ago), but following a poorly known Middle Jurassic, the fauna of the Late Jurassic (163.5 million to 145 million years ago) was very different. By this time sauropods, more advanced ornithopods, stegosaurs, and a variety of theropods predominated. The Early Cretaceous (145 million to 100.5 million years ago) then contained a few sauropods (albeit they were all new forms), a few stegosaurian holdovers, new kinds of theropods and ornithopods, and some of the first well-known ankylosaurs. By the Late Cretaceous (100.5 million to 66 million years ago), sauropods, which had disappeared from the northern continents through most of the early and mid-Cretaceous, had reinvaded the northern continents from the south, and advanced ornithopods (duckbills) had become the dominant browsers. A variety of new theropods of all sizes were widespread; stegosaurs no longer existed; and the ankylosaurs were represented by a collection of new forms that were prominent in the North America and Asia. New groups of dinosaurs, the pachycephalosaurs and ceratopsians, had appeared in Asia and had successfully colonized North America. The overall picture is thus quite clear: throughout Mesozoic time there was a continuous dying out and renewal of dinosaurian life.
It is important to note that extinction is a normal, universal occurrence. Mass extinctions often come to mind when the term extinction is mentioned, but the normal background extinctions that occur throughout geologic time probably account for most losses of biodiversity. Just as new species constantly split from existing ones, existing species are constantly becoming extinct. The speciation rate of a group must, on balance, exceed the extinction rate in the long run, or that group will become extinct. The history of animal and plant life is replete with successions as early forms are replaced by new and often more advanced forms. In most instances the layered (stratigraphic) nature of the fossil record gives too little information to show whether the old forms were actually displaced by the new successors (from the effects of competition, predation, or other ecological processes) or if the new kinds simply expanded into the declining population’s ecological niches.
Because the fossil record is episodic rather than continuous, it is very useful for asking many kinds of questions, but it is not possible to say precisely how long most dinosaur species or genera actually existed. Moreover, because the knowledge of the various dinosaur groups is somewhat incomplete, the duration of any particular dinosaur can be gauged only approximately—usually by stratigraphic boundaries and presumed “first” and “last” occurrences. The latter often coincide with geologic age boundaries; in fact, the absence of particular life-forms has historically defined most geologic boundaries ever since the geologic record was first compiled and analyzed in the late 18th century. The “moments” of apparently high extinction levels among dinosaurs occurred at two points in the Triassic (about 221 million and 210 million years ago), perhaps at the end of the Jurassic (145 million years ago), and, of course, at the end of the Cretaceous (66 million years ago). Undoubtedly, there were lesser extinction peaks at other times in between, but there are poor terrestrial records for most of the world in the Middle Triassic, Middle Jurassic, and mid-Cretaceous.
The K–T boundary event
It was not only the dinosaurs that disappeared 66 million years ago at the Cretaceous–Tertiary, or K–T, boundary (also referred to as the Cretaceous–Paleogene, or K–Pg, boundary). Many other organisms became extinct or were greatly reduced in abundance and diversity, and the extinctions were quite different between, and even among, marine and terrestrial organisms. Land plants did not respond in the same way as land animals, and not all marine organisms showed the same patterns of extinction. Some groups died out well before the K–T boundary, including flying reptiles (pterosaurs) and sea reptiles (plesiosaurs, mosasaurs, and ichthyosaurs). Strangely, turtles, crocodilians, lizards, and snakes were either not affected or affected only slightly. Effects on amphibians and mammals were mild. These patterns seem odd, considering how environmentally sensitive and habitat-restricted many of these groups are today. Many marine groups—such as the molluscan ammonites, the belemnites, and certain bivalves—were decimated. Other greatly affected groups were the moss animals (phylum Bryozoa), the crinoids, and a number of planktonic life-forms such as foraminifers, radiolarians, coccolithophores, and diatoms.
Whatever factors caused it, there was undeniably a major, worldwide biotic change near the end of the Cretaceous. But the extermination of the dinosaurs is the best-known change by far, and it has been a puzzle to paleontologists, geologists, and biologists for two centuries. Many hypotheses have been offered over the years to explain dinosaur extinction, but only a few have received serious consideration. Proposed causes have included everything from disease to heat waves and resulting sterility, freezing cold spells, the rise of egg-eating mammals, and X-rays from a nearby exploding supernova. Since the early 1980s, attention has focused on the so-called asteroid theory put forward by the American geologist Walter Alvarez, his father, physicist Luis Alvarez, and their coworkers. This theory is consistent with the timing and magnitude of some extinctions, especially in the oceans, but it does not fully explain the patterns on land and does not eliminate the possibility that other factors were at work on land as well as in the seas.
One important question is whether the extinctions were simultaneous and instantaneous or whether they were nonsynchronous and spread over a long time. The precision with which geologic time can be measured leaves much to be desired no matter what means are used (radiometric, paleomagnetic, or the more traditional measuring of fossil content of stratigraphic layers). Only rarely does an “instantaneous” event leave a worldwide—or even regional—signature in the geologic record in the way that a volcanic eruption does locally. Attempts to pinpoint the K–T boundary event, even by using the best radiometric dating techniques, result in a margin of error on the order of 50,000 years. Consequently, the actual time involved in this, or any of the preceding or subsequent extinctions, has remained undetermined.
The asteroid theory
The discovery of an abnormally high concentration of the rare metal iridium at, or very close to, the K–T boundary provides what has been recognized as one of those rare instantaneous geologic time markers that seem to be worldwide. This iridium anomaly, or spike, was first found by Walter Alvarez in the Cretaceous–Tertiary stratigraphic sequence at Gubbio, Italy, in the 1970s. The spike has subsequently been detected at hundreds of localities in Denmark and elsewhere, both in rock outcrops on land and in core samples drilled from ocean floors. Iridium normally is a rare substance in rocks of Earth’s crust (about 0.3 part per billion). At Gubbio the iridium concentration is more than 20 times greater (6.3 parts per billion), and it exceeds this concentration at other sites.
Because the levels of iridium are higher in meteorites than on Earth, the Gubbio anomaly is thought to have an extraterrestrial explanation. If this is true, such extraterrestrial signatures will have a growing influence on the precision with which geologic time boundaries can be specified. The level of iridium in meteorites has been accepted as representing the average level throughout the solar system and, by extension, the universe. Accordingly, the iridium concentration at the K–T boundary is widely attributed to a collision between Earth and a huge meteor or asteroid. The size of the object is estimated at about 10 km (6.2 miles) in diameter and one quadrillion metric tons in weight; the velocity at the time of impact is reckoned to have been several hundreds of thousands of kilometres per hour. The crater resulting from such a collision would be some 100 km or more in diameter. Such an impact site (called an astrobleme) is the Chicxulub crater, in the Yucatán Peninsula. A second, smaller impact site, which predates the Chicxulub site by about 2,000 to 5,000 years, appears at Boltysh in Ukraine. Its existence raises the possibility that the K–T boundary event resulted from multiple extraterrestrial impacts.
Although the amount of iridium dispersed worldwide was more consistent with the impact of a smaller object, such as a comet, the asteroid theory is widely accepted as the most probable explanation of the K–T iridium anomaly. The asteroid theory does not, however, appear to account for all the paleontological data. An impact explosion of this kind would have ejected an enormous volume of terrestrial and asteroid material into the atmosphere, producing a cloud of dust and solid particles that would have encircled Earth and blocked out sunlight for many months, possibly years. The loss of sunlight could have eliminated photosynthesis and resulted in the death of plants and the subsequent extinction of herbivores, their predators, and scavengers.
The K–T mass extinctions, however, do not seem to be fully explained by this hypothesis. The stratigraphic record is most complete for extinctions of marine life—foraminifers, ammonites, coccolithophores, and the like. These apparently died out suddenly and simultaneously, and their extinction accords best with the asteroid theory. The fossil evidence of land dwellers, however, suggests a gradual rather than a sudden decline in dinosaurian diversity (and possibly abundance). Alterations in terrestrial life seem to be best accounted for by environmental factors, such as the consequences of seafloor spreading and continental drift, resulting in continental fragmentation, climatic deterioration, increased seasonality, and perhaps changes in the distributions and compositions of terrestrial communities. But one phenomenon does not preclude another. It is entirely possible that a culmination of ordinary biological changes and some catastrophic events, including increased volcanic activity, took place around the end of the Cretaceous.
Dinosaur descendants
Contrary to the commonly held belief that the dinosaurs left no descendants, Archaeopteryx, which was first discovered in 1861, and Xiaotingia, which was formally classified in 2011, provide compelling evidence that birds (class Aves) evolved from small theropod dinosaurs. Following the principles of genealogy that are applied to humans as much as to other organisms, organisms are classified at a higher level within the groups from which they evolved. Archaeopteryx and Xiaotingia—the oldest birds known—are therefore classified as both dinosaurs and birds, just as humans are both primates and mammals.
The specimens of Archaeopteryx contain particular anatomic features that also are exclusively present in certain theropods (Oviraptor, Velociraptor, Deinonychus, and Troodon, among others). These animals share long arms and hands, a somewhat shorter, stiffened tail, a similar pelvis, and an unusual wrist joint in which the hand is allowed to flex sideways instead of up and down. This wrist motion is virtually identical to the motion used by birds (and bats) in flight, though in these small dinosaurs its initial primary function was probably in catching prey.
Beginning in the 1990s, several specimens of small theropod dinosaurs from the Early Cretaceous of Liaoning province, China, were unearthed. These fossils are remarkably well preserved, and because they include impressions of featherlike, filamentous structures that covered the body, they have shed much light on the relationship between birds and Mesozoic dinosaurs. Such structures are now known in a compsognathid (Sinosauropteryx), a therizinosaurid (Beipiaosaurus), a dromaeosaur (Sinornithosaurus), and an alvarezsaurid (Shuvuuia). The filamentous structures on the skin of Sinosauropteryx are similar to the barbs of feathers, which suggests that feathers evolved from a much simpler structure that probably functioned as an insulator. True feathers of several types, including contour and body feathers, have been found in the 125-million-year-old feathered oviraptorid Caudipteryx and the apparently related Protarchaeopteryx. Because these animals were not birds and did not fly, it is now evident that true feathers neither evolved first in birds nor developed for the purpose of flight. Instead, feathers may have evolved for insulation, display, camouflage, species recognition, or some combination of these functions and only later became adapted for flight. In the case of Caudipteryx, for example, it has been established that these animals not only sat on nests but probably protected the eggs with their feathers.
Until comparatively recent times, the two groups of birds from Cretaceous time that received the most attention because of their strange form were the divers, such as Hesperornis, and the strong-winged Ichthyornis, a more ternlike form. Because they were the first well-known Cretaceous birds, having been described by American paleontologist O.C. Marsh in 1880, they were thought to represent typical Cretaceous birds. Recent discoveries, however, have changed this view. For example, members of one Early Cretaceous bird group, the Confuciusornithidae, showed very little advancement compared with Archaeopteryx and the Enantiornithes (a major group of birds widely distributed around the world through most of the Cretaceous Period). Because representatives of living bird groups have long been known among the fossil species from the Paleocene and Eocene epochs (66 million to 33.9 million years ago), it has seemed evident that bird groups other than those including Hesperornis and Ichthyornis must have existed during the Cretaceous. Knowledge of these, based on fragments of fossil bone, has slowly come to light, and there is now a fairly definite record from Cretaceous rock strata of other ancestral birds related to the living groups of loons, grebes, flamingos, " style="box-sizing: border-box; color: #14599d; font-family: Georgia, serif; font-size: 18px; text-decoration-line: none;">cranes, parrots, and shorebirds—and thus indication of early avian diversity. Therefore, it is clear that birds did not go through a “bottleneck” of extinction at the end of the Cretaceous that separated the archaic groups from the extant groups. Rather, the living groups were mostly present by the latest Cretaceous, and by this time the archaic groups seem to have died out.
Natural history
Habitats
Dinosaurs lived in many kinds of terrestrial environments, and although some remains, such as footprints, indicate where dinosaurs actually lived, their bones tell us only where they died (assuming that they have not been scattered or washed far from their place of death). Not all environments are equally well preserved in the fossil record. Upland environments, forests, and plains tend to experience erosion or decomposition of organic remains, so remains from these environments are rarely preserved in the geologic record. As a result, most dinosaur fossils are known from lowland environments, usually floodplains, deltas, lake beds, stream bottoms, and even some marine environments, where their bones apparently washed in after death. Much about the environments dinosaurs lived in can be learned from studying the pollen and plant remains preserved with them and from geochemical isotopes that indicate temperature and precipitation levels. These climates, although free from the extensive ice caps of today and generally more equable, suffered extreme monsoon seasons and made much of the globe arid.
Only a few specimens represent the meagre beginning of the dinosaurian reign. This is probably because of a highly incomplete fossil record. Before dinosaurs appeared, all the continents of the world were joined to form one very large supercontinent called Pangea. Movements of the Earth’s great crustal plates then began changing Earth’s geography. By the Early Triassic Period (252.2 million to 247.2 million years ago), as dinosaurs were beginning to gain a foothold, Pangea had started to split apart at a rate averaging a few centimetres a year.
As the dinosaur line arose and experienced its initial diversification during the Late Triassic Period (235 million to 201.3 million years ago), the land areas of the world were in motion and drifting apart. Their respective inhabitants were consequently isolated from each other. Throughout the remainder of the Mesozoic Era, ocean barriers grew wider and the separate faunas became increasingly different. As the continents drifted apart, successive assemblages arose on each landmass and then diversified, waned, and disappeared, to be replaced by new fauna. By the Late Cretaceous Period (100.5 million to 66 million years ago), each continent occupied its own unique geographic position and climatic zone, and its fauna reflected that separation.
Food and feeding
The plant eaters
From the Triassic through the Jurassic and into the Cretaceous, the Earth’s vegetation changed slowly but fundamentally from forests rich in gymnosperms (cycadeoids, cycads, and conifers) to angiosperm-dominated forests of palmlike trees and magnolia-like hardwoods. Although conifers continued to flourish at high latitudes, palms were increasingly confined to subtropical and tropical regions. These forms of plant life, the vast majority of them low in calories and proteins and made largely of hard-to-digest cellulose, became the foods of changing dinosaur communities. Accordingly, certain groups of dinosaurs, such as the ornithopods, included a succession of types that were increasingly adapted for efficient food processing. At the peak of the ornithopod lineage, the hadrosaurs (duck-billed dinosaurs of the Late Cretaceous) featured large dental batteries in both the upper and lower jaws, which consisted of many tightly compressed teeth that formed a long crushing or grinding surface. The preferred food of the duckbills cannot be certified, but at least one specimen found in Wyoming offers an intriguing clue: fossil plant remains in the stomach region have been identified as pine needles.The hadrosaurs’ Late Cretaceous contemporaries, the ceratopsians (horned dinosaurs), had similar dental batteries that consisted of dozens of teeth. In this group the upper and lower batteries came together and acted as serrated shearing blades rather than crushing or grinding surfaces. Ordinarily, slicing teeth are found only in flesh-eating animals, but the bulky bodies and the unclawed, hooflike feet of dinosaurs such as Triceratops clearly are those of plant eaters. The sharp beaks and specialized shearing dentition of the ceratopsians suggest that they probably fed on tough, fibrous plant tissues, perhaps palm or cycad fronds.
The giant sauropods such as Diplodocus and Apatosaurus must have required large quantities of plant food, but there is no direct evidence as to the particular plants they preferred. Because angiosperms rich in calories and proteins did not exist during most of the Mesozoic Era, it must be assumed that these sauropods fed on the
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The food preference of herbivorous dinosaurs can be inferred to some extent from their general body plan and from their teeth. It is probable, for example, that low-built animals such as the ankylosaurs, stegosaurs, and ceratopsians fed on low shrubbery. The tall ornithopods, especially the duckbills, and the long-necked sauropods probably browsed on high branches and treetops. No dinosaurs could have fed on grasses (family Poaceae), as these plants had not yet evolved.
The flesh eaters
The flesh-eating dinosaurs came in all shapes and sizes and account for about 40 percent of the diversity of Mesozoic dinosaurs. They must have eaten anything they could catch, because predation is a highly opportunistic lifestyle. In several instances the prey victim of a particular carnivore has been established beyond much doubt. Remains were found of the small predator Compsognathus containing a tiny skeleton of the lizard Bavarisaurus in its stomach region. In Mongolia two different dinosaur skeletons were found together, a nearly adult-size Protoceratops in the clutches of its predator Velociraptor. Two of the many skeletons of Coelophysis discovered at Ghost Ranch in New Mexico, U.S., contained bones of several half-grown Coelophysis, apparently an early Mesozoic example of cannibalism. Fossilized feces (coprolites) from a large tyrannosaur contained crushed bone of another dinosaur. Skeletons of Deinonychus unearthed in Montana, U.S., were mixed with fragmentary bones of a much larger victim, the herbivore Tenontosaurus. This last example is significant because the multiple remains of the predator Deinonychus, associated with the bones of a single large prey animal, Tenontosaurus, strongly suggest that Deinonychus hunted in packs.
Herding behaviour
It should not come as a surprise that Deinonychus was a social animal, because many animals today are gregarious and form groups. Fossil evidence documents similar herding behaviour in a variety of dinosaurs. The mass assemblage in Bernissart, Belgium, for example, held at least three groups of Iguanodon. Group association and activity is also indicated by the dozens of Coelophysis skeletons of all ages recovered in New Mexico, U.S. The many specimens of Allosaurus at the Cleveland-Lloyd Quarry in Utah, U.S., may denote a herd of animals attracted to the site for the common purpose of scavenging. In the last two decades, several assemblages of ceratopsians and duckbills containing thousands of individuals have been found. Even Tyrannosaurus rex is now known from sites where a group has been preserved together.
These rare occurrences of multiple skeletal remains have repeatedly been reinforced by dinosaur footprints as evidence of herding. Trackways were first noted by Roland T. Bird in the early 1940s along the Paluxy riverbed in central Texas, U.S., where numerous washbasin-size depressions proved to be a series of giant sauropod footsteps preserved in limestone of the Early Cretaceous Period (145 million to 100.5 million years ago). Because the tracks are nearly parallel and all progress in the same direction, Bird concluded that “all were headed toward a common objective” and suggested that the sauropod trackmakers “passed in a single herd.” Large trackway sites also exist in the eastern and western United States, Canada, Australia, England, Argentina, South Africa, and China, among other places. These sites, dating from the Late Triassic Period (235 million to 201.3 million years ago) to the latest Cretaceous (66 million years ago), document herding as common behaviour among a variety of dinosaur types.
Some dinosaur trackways record hundreds, perhaps even thousands, of animals, possibly indicating mass migrations. The existence of so many trackways suggests the presence of great populations of sauropods, prosauropods, ornithopods, and probably most other kinds of dinosaurs. The majority must have been herbivores, and many of them were huge, weighing several tons or more. The impact of such large herds on the plant life of the time must have been great, suggesting constant migration in search of food.
Nesting sites discovered in the late 20th century also establish herding among dinosaurs. Nests and eggs numbering from dozens to thousands are preserved at sites that were possibly used for thousands of years by the same evolving populations of dinosaurs.
Growth and life span
Much attention has been devoted to dinosaurs as living animals—moving, eating, growing, reproducing biological machines. But how fast did they grow? How long did they live? How did they reproduce? The evidence concerning growth and life expectancy is sparse but growing. In the 1990s histological studies of fossilized bone by Armand de Ricqlès in Paris and R.E.H. Reid in Ireland showed that dinosaur skeletons grew quite rapidly. The time required for full growth has not been quantified for most dinosaurs, but de Ricqlès and his colleagues have shown that duckbills (hadrosaurs) such as Hypacrosaurus and Maiasaura reached adult size in seven or eight years and that the giant sauropods reached nearly full size in as little as 12 years. How long dinosaurs lived after reaching adult size is difficult to determine, but it is thought that the majority of known skeletons are not fully grown, because their bone ends and arches are very often not fused; in mature individuals these features would be fused.
Reproduction
The idea that dinosaurs, like most living reptiles and birds, built nests and laid eggs had been widely debated even before the 1920s, when a team of scientists from the American Museum of Natural History, New York, made an expedition to Mongolia. Their discovery of dinosaur eggs in the Gobi Desert proved conclusively that at least one kind of dinosaur had been an egg layer and nest builder. These eggs were at first attributed to Protoceratops, but they are now known to have been those of Oviraptor. In 1978 John R. Horner and his field crews from Princeton University discovered dinosaur nests in western Montana. A few other finds, mostly of eggshell fragments from a number of sites, established oviparity as the only known mode of reproduction. In recent years an increasing number of dinosaur eggshells have been found and identified with the dinosaurs that laid them, and embryos have been found inside some eggs.
The almost complete absence of juvenile dinosaur remains was puzzling until the 1980s. Horner, having moved to Montana State University, demonstrated that most paleontologists simply had not been exploring the right territory. After a series of intensive searches for the remains of immature dinosaurs, he succeeded beyond all expectations. The first such bones were unearthed near Choteau, Montana, and thereafter Horner and his crews discovered hundreds of nests, eggs, and newly hatched dinosaurs (mostly duckbills). Horner observed that previous explorations had usually concentrated on lowland areas, where sediments were commonly deposited and where most fossil remains were preserved. He recognized that such regions were not likely to produce dinosaur nests and young because they would have been hazardous places for nesting and raising the hatchlings. Upland regions would have been safer, but they were subject to erosion rather than deposition and were therefore less likely to preserve nests and eggs. However, it was exactly in such upland areas, close to the young and still-rising Rocky Mountains, that Horner made his discoveries.
Egg Mountain, as the area was named, produced some of the most important clues to dinosaurian habits yet found. For example, the sites show that a number of different dinosaur species made annual treks to this same nesting ground (though perhaps not all at the same time). Because of the succession of similar nests and eggs lying one on top of the other, it is thought that particular species returned to the same site year after year to lay their clutches. As Horner concluded, “site fidelity” was an instinctive part of dinosaurian reproductive strategy. This was confirmed more recently with the discovery of sauropod nests and eggs spread over many square kilometres in Patagonia, Argentina.
Body temperature
Beyond eating, digestion, assimilation, reproduction, and nesting, many other processes and activities went into making the dinosaur a successful biological machine. Breathing, fluid balance, temperature regulation, and other such capabilities are also required. Dinosaurian body temperature regulation, or lack thereof, has been a hotly debated topic among students of dinosaur biology. Because it is obviously not possible to take an extinct dinosaur’s temperature, all aspects of their metabolism and thermophysiology can be assessed only indirectly.
Ectothermy and endothermy
All animals thermoregulate. The internal environment of the body is under the influence of both external and internal conditions. Land animals thermoregulate in several ways. They do so behaviorally, by moving to a colder or warmer place, by exercising to generate body heat, or by panting or sweating to lose it. They also thermoregulate physiologically, by activating internal metabolic processes that warm or cool the blood. But these efforts have limits, and, as a result, external temperatures and climatic conditions are among the most important factors controlling the geographic distribution of animals.
Today’s so-called warm-blooded animals are the mammals and birds; reptiles, amphibians, and most fishes are called cold-blooded. These two terms, however, are imprecise and misleading. Some “cold-blooded” lizards have higher normal body temperatures than do some mammals, for instance. Another pair of terms, ectothermy and endothermy, describes whether most of an animal’s heat is absorbed from the environment (“ecto-”) or generated by internal processes (“endo-”). A third pair of terms, poikilothermy and homeothermy, describes whether the body temperature tends to vary with that of the immediate environment or remains relatively constant.
Today’s mammals and birds have a high metabolism and are considered endotherms, which produce body heat internally. They possess biological temperature sensors that control heat production and switch on heat-loss mechanisms such as perspiration. Today’s reptiles and amphibians, on the other hand, are ectotherms that mostly gain heat energy from sunlight, a heated rock surface, or some other external source. The endothermic state is effective but metabolically expensive, as the body must produce heat continuously, which requires correspondingly high quantities of fuel in the form of food. On the other hand, endotherms can be more active and survive lower external temperatures. Ectotherms do not require as much fuel, but most cannot deal as well with cold surroundings.
From the time of the earliest discoveries in the 19th century, dinosaur remains were classified as reptilian because their anatomic features are typical of living reptiles such as turtles, crocodiles, and lizards. Because dinosaurs all have lower jaws constructed of several bones, a reptilian jaw joint, and a number of other nonmammalian, nonbirdlike characteristics, it was assumed that living dinosaurs were similar to living reptiles—scaly, cold-blooded, ectothermic egg layers (predominantly), not furry, warm-blooded live-bearers. A chauvinistic attitude seems to prevail that the warm-bloodedness of mammals is better than the cold-blooded reptilian state, even though turtles, snakes, and other reptiles do very well regulating their body temperature in a different way. Moreover, both birds and mammals evolved from ectothermic, poikilothermic ancestors. At what point did metabolism heat up?
Clues to dinosaurian metabolism
The question of whether any extinct dinosaur was a true endotherm or homeotherm cannot be answered, but some interesting anatomic facts suggest these “warmer” possibilities. Probably the most direct evidence of dinosaurian physiology comes from bones themselves, particularly in regard to how they grew. The long bones (such as arm and leg bones) of most dinosaurs are composed almost exclusively of a well-vascularized type of bone matrix (fibro-lamellar) also found in most mammals and large birds. This type of bone tissue always indicates rapid growth, and it is very different from the more compact, poorly vascularized, parallel-fibred bone found in crocodiles and other reptiles and amphibians. It is generally thought that well-vascularized, rapidly growing bone can be sustained only by high metabolic rates that bring a continual source of nutrients and minerals to the growing tissues. It is difficult to explain these histological features in any other metabolic terms. On the other hand, most dinosaurs retain lines of arrested growth (LAGs) in most of their long bones. LAGs are found in other reptiles, amphibians, and fishes, and they often reflect a seasonal period during which metabolism slows, usually because of environmental stresses. This slowdown produces “rest lines” as LAGs in the bones. The presence of these lines in dinosaur bones has been taken as an indication that they were metabolically incapable of growing throughout the year. However, LAGs in dinosaurs are less pronounced than in other reptiles; LAGs can also appear in different numbers in different bones of the same skeleton, and they are sometimes even completely absent. Finally, some living birds and mammals, which are clearly endotherms, have LAGs very much like those of dinosaurs, so LAGs are probably not strong indicators of metabolism in any of these animals.
Other, less direct lines of evidence may reveal other clues about dinosaurian metabolism. Two dinosaurian groups, the hadrosaurs and the ceratopsians, had highly specialized sets of teeth that were obviously effective at processing food. Both groups were herbivorous, but unlike living reptiles they chopped and ground foliage thoroughly. Such highly efficient dentitions may suggest a highly effective digestive process that would allow more energy to be extracted from the food. This feature by itself, however, may not be crucial. Pandas, for example, are not very efficient in digesting plant material, but they survive quite well on a diet of almost nothing but bamboo.
Another line of evidence is that dinosaurs had anatomic features reflecting a high capacity for activity. The first dinosaurs walked upright, holding their legs under their bodies; they could not sprawl. This indicates that, by standing and walking all day, they probably expended more energy than reptiles, which typically sit and wait for prey. As some lineages of dinosaurs grew larger, they reverted to four-legged (quadrupedal) locomotion, but their stance was still upright. They also put one foot directly in front of the other when they walked (parasagittal gait), instead of swinging the limbs to the side. Such posture and gait are present in all nonaquatic endotherms (mammals and birds) today, whereas a sprawling or semierect posture is typical of all ectotherms (reptiles and amphibians). Bipedal stance and parasagittal gait are not sustained in any living ectotherm, perhaps because they require a relatively higher level of sustained energy.
The high speeds at which some dinosaurs must have traveled have also been invoked as evidence of high metabolic levels. For example, the ostrichlike dinosaurs, such as Struthiomimus, Ornithomimus, Gallimimus, and Dromiceiomimus, had long hind legs and must have been very fleet. The dromaeosaurs, such as Deinonychus, Velociraptor, and Dromaeosaurus, also were obligatory bipeds. They killed prey with talons on their feet, and one can argue that it must have taken a high level of metabolism to generate the degree of activity and agility required of such a skill. However, most ectotherms can move very rapidly in bursts of activity such as running and fighting, so this feature may not provide conclusive evidence either.
Related to the upright posture of many dinosaurs is the fact that the head was often positioned well above the level of the heart. In some sauropods (Apatosaurus, Diplodocus, Brachiosaurus, and Barosaurus, for instance), the brain must have been several metres above the heart. The physiological importance of this is that a four-chambered heart would be required for pumping freshly oxygenated blood to the brain. Brain death follows very quickly when nerve cells are deprived of oxygen, and to prevent it most dinosaurs must have required two ventricles. In a four-chambered heart, one ventricle pumps oxygen-poor venous blood at low pressure to the lungs to absorb fresh oxygen (high pressure would rupture capillaries of the lungs). A powerful second ventricle pumps freshly oxygenated blood to all other parts of the body at high pressure. To overcome the weight of the column of blood that must be moved from the heart to the elevated brain, high pressure is certainly needed. In short, like birds and mammals, many dinosaurs apparently had the required four-chambered heart necessary for an animal with a high metabolism.
The significance of thermoregulation can be seen by comparing today’s reptiles with mammals. The rate of metabolism is usually measured in terms of oxygen consumed per unit of body weight per unit of time. The resting metabolic rate for most mammals is about 10 times that of modern reptiles, and the range of metabolic rates among living mammals is about double that seen among reptiles. These differences mean that endothermic mammals have much more endurance than their cold-blooded counterparts. Some dinosaurs may have been so endowed, and although they seem to have possessed the cardiovascular system necessary for endothermy, that capacity does not conclusively prove that they were endothermic. There exists the possibility that dinosaurs were neither complete ectotherms nor complete endotherms. Rather, they may have evolved a range of metabolic strategies, much as mammals have (as is illustrated by the differences between sloths and cheetahs, bats and whales, for example).
Classification
The chief difference between the two major groups of dinosaurs is in the configuration of the pelvis. It was primarily on this distinction that the English biologist H.G. Seeley established the two dinosaurian orders and named them Saurischia (“lizard hips”) and Ornithischia (“bird hips”) in 1887; this differentiation is still maintained.
As in all four-legged animals, the dinosaurian pelvis was a paired structure consisting of three separate bones on each side that attached to the sacrum of the backbone. The ilium was attached to the spine, and the pubis and ischium were below, forming a robust bony plate. At the centre of each plate was a deep cup—the hip socket (acetabulum). The hip socket faced outward and was open at its centre for the articulation of the thighbone. The combined saurischian pelvic bones presented a triangular outline as seen from the side, with the pubis extending down and forward and the ischium projecting down and backward from the hip socket. The massive ilium formed a deep vertical plate of bone to which the muscles of the pelvis, hind leg, and tail were attached. The pubis had a stout shaft, commonly terminating in a pronounced expansion or bootlike structure (presumably for muscle attachment) that solidly joined its opposite mate. The ischium was slightly less robust than the pubis, but it too joined its mate along a midline. There were minor variations in this structure between the various saurischians.
The ornithischian pelvis was constructed of the same three bones on each side of the sacral vertebrae, to which they were attached. The lateral profile of the pelvis was quite different from that of the saurischians, which had a long but low iliac blade above the hip socket and a modified ischium-pubis structure below. Here the long, thin ischium extended backward and slightly downward from the hip socket. In the most primitive, or basal, ornithischians, the pubis had a moderately long anterior blade, but this was reduced in later ornithischians. Posteriorly it stretched out into a long, thin postpubic process lying beneath and closely parallel to the ischium. The resulting configuration superficially resembled that of birds, whose pubis is a thin process extending backward beneath the larger ischium. These anatomic dissimilarities are thought to reflect important differences in muscle arrangements in the hips and hind legs of these two orders. However, the soft parts of these dinosaurs are not well enough understood to reveal any functional or physiological basis for the differences. Other marked dissimilarities between saurischians and ornithischians are found in their jaws and teeth, their limbs, and especially their skulls. Details regarding these differences are given in the following discussions of the major dinosaur groups.
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