Ceratopsia
The first ceratopsian dinosaur remains were found in the 1870s by E.D. Cope, who named the animal Agathaumus, but the material was so fragmentary that its unusual design was not at once recognized. The first inkling that there had been horned dinosaurs did not emerge until the late 1880s with the discovery of a large horn core, first mistaken for that of a bison. Shortly afterward, dozens of large skulls with horns were found‹the first of many specimens of Triceratops .
Ceratopsians were a relatively short-lived dinosaur group, flourishing from the Middle Cretaceous to the 'great extinction' at the end of the Cretaceous Period. Triceratops, together with Tyrannosaurus, was one of the very last of all dinosaurs. Ceratopsians had a peculiar geographic distribution: the earliest and most primitive kinds, Psittacosaurus and Protoceratops, are known only from Asia - Mongolia and China specifically; all the advanced ceratopsians, with the exception of a few fragmentary and doubtful specimens, have been found only in North America.
Sizes ranged from relatively small animals the size of a collie to the nine-metre-long, four- to five-ton Triceratops. Although commonly compared to the modern rhinoceros, Triceratops grew to a weight and bulk several times that of the largest living rhinoceros, and its behaviour probably was correspondingly different. The most distinctive feature of nearly all members of the group was the horns on the head, giving them the name ceratops (horn face). Correlated with the various arrays of head horns in the different taxa was the unusually large size of ceratopsian heads. Great bony growths extended from the back of the skull, reaching well over the neck and shoulders. This neck shield, or frill, resulted in the biggest head that ever adorned any land animal; the length of the Torosaurus skull was almost three metres, longer than a whole adult Protoceratops.
Several theories have been proposed to explain this frill structure: a protective shield to cover the neck region, an attachment site of greatly enlarged jaw muscles, an attachment site of powerful neck muscles for wielding the head horns, or a sort of ornament to present a huge, frightening head-on profile to potential attackers. Most probably, all these functions were involved at some stage in the development of the frill. The most unusual thought is that the structure was none of these, but rather acted as a giant heat-control apparatus, with its entire upper surface covered in a vast network of blood vessels pulsing with overheated blood or absorbing solar heat. The idea is interesting but fails to account for several important associated features -the massive coronoid process of the lower jaw, for example - that are better explained as mechanical and muscular adaptations.
The infraorder is usually divided into three families: the primitive psittacosaurids, including the putative ceratopsian ancestor, Psittacosaurus; the protoceratopsids, including Protoceratops of Asia and Leptoceratops of North America; and the ceratopsids, encompassing all the advanced and better-known kinds such as Triceratops, Torosaurus, and Monoclonius, all from North America.
Psittacosaurus has sometimes been classified as an ornithopod, largely because of its relatively long hind limbs and short front limbs, probably resulting in bipedal stance and locomotion. Resembling ornithopods in body form, it had a shorter neck and tail and was much smaller (only two metres long) than the most advanced ornithopods like the iguanodonts and hadrosaurs. Psittacosaurus, however, possessed a ceratopsian-like beak, the beginnings of a characteristic neck frill at the back of the skull, and teeth that prefigured those of the more advanced ceratopsians. Also, its remains have been found in rocks that predate those containing the ceratopsids.
The best-known of the protoceratopsids is the genus Protoceratops. Dozens of skeletal specimens have been found and studied, ranging from near hatchlings to full-size adults. This rare treasure, the first to include very young individuals unmistakably associated with mature individuals, was the result of the series of American Museum of Natural History expeditions in the 1920s to the Gobi. Their collection provided the first good growth series of any dinosaur. Even more momentous was their discovery of several nests of eggs loosely associated with Protoceratops skeletons‹the first certifiable dinosaur eggs. Additional specimens of Protoceratops and its nests of eggs were recovered on subsequent expeditions by Soviets, Poles, and Chinese.
The skeletal anatomy of the protoceratopsids foreshadowed that of the more advanced ceratopsids. The ceratopsian skull was disproportionately large for the rest of the animal, comprising about one-fifth of the total body length in Protoceratops and at least one-third in Torosaurus . The head frill of Protoceratops was a modest backward extension of the upper temporal fenestral arches but became the enormous fan-shaped ornament of later forms. Protoceratops also displayed a short but stout horn on the snout, a development of the nasal bones; this too was a precursor of the prominent nasal horns of ceratopsids like Monoclonius , Centrosaurus, Chasmosaurus, Styracosaurus, Torosaurus, and Triceratops. The last two genera developed two additional, larger horns above the eyes. These horns undoubtedly were covered by horny sheaths or soft tissue, as is evidenced by impressions on them of superficial vascular channels for the nourishing blood vessels.
Ceratopsian jaws were highly specialized. The lower jaw was massive and solid to support a large battery of teeth similar to those of the duckbills. The front of the lower jawbones met in a long, strong symphysis capped by a stout beak formed of the toothless predentary. This structure itself must have been covered by a sharp, horny, turtlelike beak. Continuous dental surfaces extended over the rear two-thirds of the jaw. The tooth batteries, however, differed from those of the hadrosaurs in forming long, vertical slicing surfaces as upper and lower batteries met, operating much like self-sharpening shears.
From the lower jaw a massive coronoid process jutted up toward a relatively small upper temporal fenestra, providing a strong attachment point for jaw muscles and powerful leverage for their action. Because the upper fenestra was so small, and the teeth so specialized for cutting, it has generally been accepted that the powerful jaw muscles were attached to the upper frill surface and connected to the coronoid process beneath by means of a stout tendon passing through the upper fenestra. At least one factor in the expansion of the skull frill would seem to have been the size of the required jaw muscles. (That, however, would not have precluded other functions for the frill as well‹shielding the neck, enlarging the head-on profile, or even controlling temperature.)
As in the hadrosaurs, each dental battery consisted of about two dozen or more tooth positions compressed together into a single large block of teeth. At each tooth position there was one functional, or occluding, tooth (the duckbills had two or three) along with several more unerupted replacement teeth beneath. (All toothed reptiles, living and extinct, have a lifelong supply of replacement teeth.) An intriguing question arises: what did the ceratopsians feed on that required such an unusual and powerful feeding mechanism? The answer remains hidden, but the suggestion is that they fed on something exceedingly tough and fibrous, like the fronds of palms or cycads, both of which were plentiful during late Mesozoic times.
Except for Psittacosaurus, all ceratopsians were obligate quadrupeds with a heavy, ponderous build. The leg bones were stout and the legs themselves muscular, the feet were semiplantigrade for graviportal stance and progression, and all the toes ended in ³hooves² rather than claws. As in most other four-legged animals, the rear legs were significantly longer than the front legs (again suggesting to some experts that such animals had a bipedal ancestor). The hind legs were positioned directly beneath the hip sockets and held almost straight and vertical. The front legs, on the other hand, projected out to each side from the shoulder sockets in a ³push-up² position. Consequently, the head was carried low and close to the ground. This mixed posture was probably related to the large horned head and its role in combat, the bent forelegs providing a wide stance and stable base for directing the horns at an opponent and resisting attack.
The first four neck vertebrae of ceratopsians were coossified, presumably for strength and impact resistance during hostile engagements. The first joint of the neck was unusual in that the condyle of the skull formed a nearly perfect sphere that fitted into a cuplike socket of the fused neck vertebrae. Such an arrangement would seem to have provided solid connections along with maximum freedom of the head to pivot in any direction without having to turn the body. There can be no doubt that ceratopsians used their head horns in an aggressive manner, but whether they used them as defense against possible predators or in rutting combat with other male ceratopsians, or both, is not so clear. Evidence of puncture wounds in some specimens suggests rutting encounters, but the fact that both sexes apparently had horns seems to indicate defense as their primary use.
Ornithischia
Thyreophora
The suborder Thyreophora is divided into two infraorders: Stegosauria, the plated dinosaurs, and Ankylosauria, the armoured dinosaurs.
Stegosauria With their unique bony back plates, the stegosaurs were one of the most distinctive dinosaur varieties. Relatively few specimens have been found, chiefly from North America, Africa, Europe, and Asia. Stegosaurian remains have appeared in Early Jurassic to Early Cretaceous strata. Their immediate ancestry is uncertain, and no descendants are known. The most familiar genus is Stegosaurus , found in the Morrison Formation (Late Jurassic) of western North America. Stegosaurus was 3.7 metres in height and 9 metres in length, probably weighed two tons, and had a broad, deep body. Not all varieties of the infraorder were this large; for example, Kentrosaurus , from eastern Africa, was less than 2 metres high and 3.5 metres long.
All stegosaurs were graviportal and undoubted quadrupeds, although the massive legs were of greatly disparate lengths - the hind legs more than twice as long as the forelegs. Some authorities have interpreted the short front legs as evidence that stegosaurs were secondarily quadrupedal and had a bipedal ancestry, probably some ornithopod. This theory of ancestry may be correct, but such a precursor need not have been bipedal. In fact, among virtually all quadrupedal animals, it is the rule rather than the exception that the front legs are shorter than the hind - often much shorter. (Notable exceptions are the giraffe and, among dinosaurs, the giant sauropod Brachiosaurus, whose front legs were much longer than its rear legs.)
Whatever walking and running skills were possessed by the stegosaurs, their limb proportions must have made them move extremely slowly. The humerus of the upper arm was longer than the bones of the forearm, the femur was much longer than the shinbones, and the metapodial bones of the feet were all very short, which means that the stride must have been short. The feet were graviportal in design, with no cursorial adaptations. The stegosaurian skull was notably small, long, low, and narrow with weakly developed dentition of small, laterally compressed, leaf-shaped teeth in short, straight rows. There was no significant coronoid process on the lower jaw, nor was there much space for sizable jaw muscles. This combination of features seems entirely incompatible with the large, bulky body, especially in view of the apparent absence of Jurassic vegetation of suitable nature. The weak dentition would indicate that the food eaten must have required little preparation by the teeth and yet have provided adequate nourishment.
The best explanation is that the digestive tract may have contained a flora of fermenting bacteria capable of breaking down the cellulose-rich Jurassic plant tissues. Perhaps that process was assisted by a crop or gizzard full of pulverizing stomach stones (gastroliths), although none have so far been discovered in stegosaurian specimens. Even so, such features would still not explain how these animals, with such small mouths and dentition, could feed themselves adequately to sustain their great bulk. (The same problem has been encountered in speculations about the feeding habits of sauropods.)
The most distinctive stegosaurian feature was the row, or rows, of large, diamond-shaped bony plates on the back. A controversy has raged ever since the first specimen was collected, in 1877 in Colorado, U.S., as to their purpose and how they were arranged. The evidence and a general consensus argue in favour of the traditional idea that the plates projected upward and were set in two parallel rows in a staggered, or alternating, arrangement rather than side by side. Others, however, suggest that they did not project above the back at all, but lay flat to form flank armour. A few others maintain that the plates were set in a single midline row along the length of the back, beginning with small ones just behind the head, increasing in size to the hips, and then diminishing along the tail. The end of the tail bore at least two pairs of long bony spikes, indicating some sort of defensive role for the tail but not necessarily for the back plates.
The discovery in 1976 that the bony plates of Stegosaurus were highly vascularized, fed by large, hose-size blood vessels, led to the notion that these fins were large ³radiators,² or cooling vanes, to dissipate excess body heat in much the same manner as does the large surface area of an elephant's ears. The staggered arrangement in parallel rows might have maximized the area of cooling surface by minimizing any leeward breeze ³shadow² that would have resulted from a paired configuration. Asymmetry is a bizarre anatomic condition, and‹right or wrong‹this certainly is an imaginative explanation of its presence in this animal. No other stegosaur, however, had such a peculiar feature. Rather, all other taxa had a variety of paired body spikes that seem best explained as passive defense or display adaptations rather than cooling mechanisms.
Another variety of dinosaur may be mentioned here, although its correct systematic assignment is still under review, and it is known from very few specimens. The animal has been named Scutellosaurus after one of its most distinctive features, an armour of small bony scutes. The specimens were recovered from very late Triassic or the earliest Jurassic strata of the American Southwest. In general body form, limb proportions, and tooth form, Scutellosaurus resembled the early ornithopods and was at least preferentially bipedal. Its body armour, however, was unlike anything known in other ornithopods. An array of small to moderate-size, separated bony scutes, set in multiple rows, covered the back and flanks. These attachments apparently corresponded in location with the ribs and with the vertebrae along the entire length of the backbone.
Ornithischia
Thyreophora
Ankylosauria The ankylosaurs lived during the Cretaceous Period and, like the stegosaurs, were a relatively brief dinosaurian experiment. They are called armoured dinosaurs after their most distinctive feature, an extensive mosaic of small and large interlocking bony plates that completely encased the back and flanks. Most varieties, like Euoplocephalus , Nodosaurus , and Palaeoscincus were relatively low and broad in body form and built close to the ground, with short, stocky legs and a quadrupedal stance. Again, the hind legs were longer than the front legs but not so extremely disproportionate as those of Stegosaurus. Like the stegosaurs, however, their limbs were stout and columnar, the thighbone and upper arm were longer than the shin and forearm, and the metapodials were stubby. These features point to a slow, graviportal mode of locomotion. The feet were semiplantigrade and possibly supported from beneath by pads of cartilage. The terminal phalanges of both fore and hind feet were broad and hooflike elements rather than claws.
The ankylosaur skull was a low, broad, boxlike affair with dermal scutes, or osteoderms, that often fused with the underlying skull bones. In one form, Euoplocephalus, even the eyelid seems to have developed a protective bony covering. The jaws were weak, with a very small predentary and no significant coronoid process for jaw muscle attachment. The small jaw muscle chamber was largely covered over by dermal bones rather than fenestrated. The teeth were small, loosely spaced leaf-shaped structures reminiscent of the earliest primitive ornithischian teeth. All taxa had very few teeth in either jaw, a marked contrast to the highly specialized, numerous teeth of other ornithischians. These features of the jaws and teeth lead to the impression that the animals must have fed on some sort of soft, pulpy plant food.
Not a particularly diverse or abundant group, the ankylosaurs are known only from North America, Europe, and Asia. They are divided into two families, the primitive Nodosauridae and the advanced Ankylosauridae. The most conspicuous difference between the two families is the presence of a massive bony club at the end of the tail in the advanced ankylosaurs; no such tail structure is present in the nodosaurs.
Dinosaurs have broad public, as well as scientific, interest partly because they are extinct. It is widely believed that all dinosaurs died out at the same time‹apparently quite suddenly at the end of the Cretaceous Period. This belief is not entirely correct, however. It is also usually supposed that dinosaurs left no direct descendants, a view that has been challenged and is now a matter of intense reexamination by paleontologists and evolutionary biologists.
Faunal changes
During the 150 million years or so in which dinosaurs existed, there were repeated changes in the dinosaur communities. The stratigraphic record is too incomplete to establish whether these faunal turnovers were uniform, taking place at a steady rate, or episodic, but it seems to indicate the latter. The evidence shows a moderately rich Late Triassic fauna of plateosaurs and other prosauropods, primitive ornithopods, and theropods. Most of these kinds of dinosaurs are not represented in Early Jurassic strata, and by Late Jurassic time the fauna was very different, with sauropods, more advanced ornithopods, stegosaurs, and a variety of theropods predominating.
Early Cretaceous strata contain few sauropods (all new), a few stegosaurian holdovers, new kinds of theropods and ornithopods, and the first ankylosaurs. By Late Cretaceous time, sauropods apparently were rare, 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 kinds that were prominent in the North American and Asian faunas. A totally new group of dinosaurs, the horned ceratopsians, had appeared in Asia and had successfully colonized North America. The overall picture is quite clear: throughout Mesozoic time there was an ongoing turnover, or dying out and renewal, of dinosaurian life.
It is important to note that extinction is a normal, universal occurrence. On balance, it is as commonplace as is the appearance of new species. Old life-forms decline and diminish in numbers beyond the critical threshold below which the reproduction rate can no longer sustain the population. Ecological space and opportunities are created as a result of the void left by an extinguished species.
Sometimes new forms that originate by phylogenetic diversification are suitably adapted to make use of the vacated niche. That does not always happen, however, and the niche may remain empty or be parceled out among many occupants.
In a sense, the history of animal and plant life is replete with successions -early primitive kinds replaced by new and often more advanced kinds. In most instances, the stratigraphic record gives too little information to show whether the old forms were actually displaced by the new successors or the new kinds simply expanded into the declining population's ecological niches. Nor is the stratigraphic sequence adequate to document actual evolutionary lineages except in the most general way. For example, among dinosaurs, the sauropod group is generally thought to have originated from melanorosaurid prosauropods, but the sequence of ancestral to descendant species is not known specifically. Likewise, the hadrosaurs are widely believed to have derived from an Early Cretaceous iguanodont-like ornithopod (perhaps Probactrosaurus of Asia), but again the exact lineage is unknown.
Because of such stratigraphic gaps, it is not possible to say precisely how long dinosaur species or genera actually existed. Moreover, because of the somewhat inconsistent, and thus inexact, anatomic definitions of the various dinosaur taxa, the duration of any particular kind 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 usually defines geologic boundaries. The Mesozoic ³moments² of apparently high extinction levels among dinosaurs were around the end of the Triassic (208 million years ago), the end of the Jurassic (144 million years ago), and of course the end of the Cretaceous (66.4 million years ago). Undoubtedly, there were lesser extinction peaks at other times in between, but these are poorly documented by fossil records.
The KT boundary event
It was not only the dinosaurs that disappeared at the end of the Mesozoic. Many other organisms became extinct or were greatly reduced in abundance and diversity. Among these were the flying reptiles (pterosaurs), sea reptiles (plesiosaurs and mosasaurs), and ichthyosaurs, the last disappearing slightly before the CretaceousTertiary boundary‹known as the KT boundary. Strangely, turtles, crocodilians, lizards, and snakes were not affected or were affected only slightly. Effects on amphibians and mammals were mild. But other organisms, such as the molluscan ammonites, the belemnites and certain bivalves, the bryozoans, the crinoids, and a number of planktonic life-forms‹like foraminifera, radiolarians, coccolithophores, and diatoms‹were decimated.
Whatever factor or factors caused it, there was a major, worldwide biotic change at about the end of the Cretaceous. But the extermination of the dinosaurs is the best-known change by far and has been a puzzle to paleontologists, geologists, and biologists for two centuries. Many theories have been offered over the years to explain dinosaur extinction, but few have received serious consideration. Proposed causes have included everything from disease, heat waves and resulting sterility, freezing cold spells, and the rise of egg-eating mammals, to X rays from a supernova exploding nearby. Since the early 1980s, attention has focused on the so-called asteroid theory put forward by the American geologist Walter Alvarez, his father, the physicist Luis Alvarez, and their coworkers.
The ongoing debate pits catastrophists against gradualists. Were the extinctions simultaneous and instantaneous, or were they nonsynchronous and spread over a long time period? The precision with which geologic time can be measured, by either radiometric means, paleomagnetic reversal stratigraphy, or the more traditional approach of measuring the fossil content of stratigraphic layers, leaves much to be desired. Only rarely does an ³instantaneous² 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 KT boundary event, even using the best radiometric dating techniques, result in a margin of error on the order of a half million years. Consequently, the actual time involved in the mass extinctions of that period, or in 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 KT boundary, however, 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 CretaceousTertiary stratigraphic sequence at Gubbio, Italy, in the 1970s. The spike has subsequently been detected at 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 the Earth's crust (about 0.3 parts per billion). At Gubbio, the iridium concentration is more than 20 times greater (6.3 parts per billion), and it is even greater at other sites.
Because the levels of iridium are higher in meteorites than on the Earth, the Gubbio anomaly is thought to have an extraterrestrial explanation. 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 KT boundary is widely attributed to a collision between the Earth and a huge meteor or asteroid. The size of the object is estimated at about 10 kilometres (6.5 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 miles per hour. The crater resulting from such a collision would be some 100 kilometres or more in diameter. No crater of this sort has been recognized, but, in view of the fact that the Earth's surface is two-thirds ocean, it is likely that such an impact site (called an astrobleme) would be hidden on the ocean floor.
The asteroid theory is widely accepted as the most probable explanation of the KT iridium anomaly, but it does not 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 the 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 and their predators and scavengers.
The KT mass extinctions, however, do not seem to be fully explained by this hypothesis. The stratigraphic record is most complete for extinctions of marine life‹foraminifera, ammonites, coccolithophores, and the like. These life-forms 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 decline in dinosaurian diversity, and possibly abundance, rather than a sudden change at the KT boundary. Alterations in terrestrial life seem to be best accounted for by environmental factors such as the consequences of seafloor spreading and continental drift. With the rearrangement of the continental masses, disrupting and deflecting oceanic current patterns and causing repeated changes in sea level, there undoubtedly occurred many climatic changes, which in turn would have affected terrestrial organisms and their distribution.
Finally, in the controversy between the gradualist and catastrophist explanations of the dinosaurs' extinction, it should be noted that one phenomenon does not preclude the other. It is entirely possible that a culmination of ordinary biological changes and some catastrophic event both took place around the end of Cretaceous time.
Dinosaur descendants
Contrary to the commonly held belief that the dinosaurs left no descendants, the rare (seven) specimens of Archaeopteryx (see photograph ) provide compelling evidence that birds are closely related to, and probably are direct descendants of, small theropod dinosaurs. In fact, today Archaeopteryx actually is classified as a dinosaur and a bird by many experts, and some even categorize all living birds as dinosaurs.
Many ornithologists do not agree with this taxonomy, although many (if not most) accept a dinosaurian origin of birds.
The specimens of Archaeopteryx contain particular anatomic features that also are exclusively present in certain theropods, such as Ornitholestes, Coelurus, Velociraptor, Deinonychus, Saurornithoides, Troodon, and Sinornithoides.
All these dinosaurs existed too late in time to have given rise to Archaeopteryx and thereby to later birds. However, it is believed that they arose from a common theropod ancestor that possessed the beginnings of these shared anatomic features found in Archaeopteryx, the oldest-known unquestionable bird.
Dinosaurs traditionally have been placed in the reptilian subclass Diapsida, reptiles with two pairs of temporal openings in the skull. As diapsids, dinosaurs are grouped with the crocodilians, thecodonts, and pterosaurs, all of which have socketed teeth and a number of other features in common. These are the so-called archosaurian reptiles. In recent years it has been suggested that dinosaurs be ranked as a class of their own, comparable to the classes Mammalia and Aves. That idea has not been universally accepted as yet. At the present time, the term Dinosauria is not used as a taxonomic category to include all dinosaurs. Instead, they are classified in their two orders, as either Saurischia or Ornithischia.
Order Saurischia
The reptile-hipped dinosaurs.
Suborder Sauropodomorpha
All the reptile-hipped herbivorous dinosaurs. Late Triassic to Late Cretaceous.
Infraorder Prosauropoda
Facultative bipeds; primitive forerunners of sauropods. Late Triassic to Early Jurassic.
Family Anchisauridae
Primitive prosauropods including Anchisaurus and Plateosaurus . Late Triassic to Early Jurassic.
Family Melanorosauridae
Advanced prosauropods such as Melanorosaurus and Riojasaurus; probably includes the sauropod ancestry. Late Triassic to Early Jurassic
Infraorder Sauropoda
Large to gigantic obligatory quadrupeds; all herbivorous. Early Jurassic to Late Cretaceous.
Family Cetiosauridae
Primitive and poorly studied sauropods such as Cetiosaurus ; specimens mostly from the Old World; vertebrae well-excavated to lighten bone weight. Early to Late Jurassic.
Family Diplodocidae
More advanced and better-known large sauropods; highly excavated vertebrae. Diplodocus and Apatosaurus (Brontosaurus) are the best known. Late Jurassic to Late Cretaceous.
Family Brachiosauridae
Largest of all the sauropods; greatly elongated necks and highly excavated vertebrae; front legs longer than back legs. Brachiosaurus is the most famous. Middle Jurassic to Late Cretaceous.
Family Titanosauridae
Advanced sauropods, primarily from the southern continents. Examples are Titanosaurus and Alamosaurus.
Early to Late Cretaceous.
Family Camarasauridae
Moderate-size sauropods; relatively short necks and tails for this infraorder; teeth spatulate or spoon-shaped. Camarasaurus and Morosaurus are typical. Late Jurassic to Late Cretaceous.
Suborder Staurikosauria?
Primitive carnivorous dinosaurs resembling theropods; known only from South America. Classification is uncertain(?). Name bearer is Staurikosaurus. Middle to Late Triassic.
Suborder Theropoda
All the carnivorous dinosaurs except the staurikosaurs; obligatory bipeds. Late Triassic to Late Cretaceous.
Infraorder Ceratosauria
Small to medium-size, hollow-boned carnivores. Late Triassic to Late Jurassic. Includes the primitive theropods such as Ceratosaurus , Coelophysis , and Syntarsus.
Infraorder Tetanurae
All other theropods are grouped here into the five subcategories Coelurosauria, Ornithomimosauria, Maniraptora (previously Deinonychosauria), Segnosauria, and Carnosauria. Late Triassic to Late Cretaceous.
Family Compsognathidae (Coelurosauria)
Advanced; smallest of the theropods; all known specimens 2-fingered. Name bearer is Compsognathus .
Family Oviraptoridae (Maniraptora)
Small, toothless theropods with an odd skull form; perhaps related to the ornithomimids. Oviraptor is the best-known example. Late Cretaceous.
Family Dromaeosauridae (Maniraptora)
Includes Deinonychus and Velociraptor . Early to Late Cretaceous.
Family Troodontidae (Maniraptora)
Less specialized in foot structure, but close to the dromaeosaurids. Name bearer is Troodon. Late Cretaceous.
Family Megalosauridae (Carnosauria)
Primitive large theropods; often 4-fingered. Megalosaurus is the best known. Early Jurassic to Late Cretaceous
Family Allosauridae (Carnosauria)
More advanced large theropods; all 3-fingered except for Ceratosaurus. Best known is Allosaurus . Late Jurassic to Late Cretaceous
Family Tyrannosauridae (Carnosauria)
Largest of the theropods and the most advanced; all with just 2 fingers. Examples are Tyrannosaurus, Tarbosaurus, and Albertosaurus . Late Cretaceous.
Order Ornithischia
The bird-hipped, herbivorous dinosaurs; characterized by the diagnostic predentary bone of the jaw.
Suborder Cerapoda
Facultatively bipedal ornithischian dinosaurs plus the horned ceratopsian forms. Late Triassic to Late Cretaceous.
Infraorder Ornithopoda
Family Fabrosauridae Earliest and most primitive of the ornithopods; small and often hollow-boned. Best known is Fabrosaurus. Late Triassic to Early Cretaceous
Family Heterodontosauridae
More advanced small ornithopods, with the beginnings of specialized dentition. Best examples are Heterodontosaurus and the primitive Pisanosaurus . Late Triassic to Late Jurassic.
Family Hypsilophodontidae
More advanced small to medium-size ornithopods, with only a suggestion of specialized dentition. Hypsilophodon and Thescelosaurus are examples. Late Jurassic to Late Cretaceous.
Family Iguanodontidae
Medium to large ornithopods, with the first stages of specialized grinding dentition. Iguanodon and Camptosaurus are the best known. Late Jurassic to Late Cretaceous.
Family Hadrosauridae
The duck-billed ornithopods, with highly specialized grinding dentition; medium to large size. Edmontosaurus, Corythosaurus , and Lambeosaurus are well known. Late Cretaceous.
Infraorder Pachycephalosauria
The dome-headed ornithischians; closely related to the ornithopods; usually with a massively thick bony skull roof; bipedal. Stegoceras and Pachycephalosaurus are the best examples. Late Cretaceous.
Infraorder Ceratopsia The horned dinosaurs. Early to Late Cretaceous.
Family Psittacosauridae Ancestral and most primitive of the ceratopsians; represented by the hornless and bipedal Psittacosaurus. Early Cretaceous
Family Protoceratopsidae
Primitive quadrupedal ceratopsians, with short frills and very modest horns. Protoceratops and Leptoceratops are the best examples. Late Cretaceous.
Family Ceratopsidae
Advanced quadrupedal ceratopsians, with prominent horns and frills. Examples are Monoclonius, Torosaurus, and Triceratops. Late Cretaceous.
Suborder Thyreophora
The plated and armoured dinosaurs. Late Triassic or Early Jurassic to Late Cretaceous.
Infraorder Stegosauria
Family Scelidosauridae
Primitive stegosaurs, with less well-developed back plates. Scelidosaurus is the most primitive form; Scutellosaurus perhaps the most advanced. Late Triassic or Early Jurassic.
Family Stegosauridae
Advanced stegosaurs, usually with well-developed back plates and spines. Stegosaurus and Kentrosaurus are the best known. Middle Jurassic to Early Cretaceous.
Infraorder Ankylosauria
Family Nodosauridae
Primitive ankylosaurs, usually with less completely developed armour. Nodosaurus , Hylaeosaurus , and Sauropelta are well-known kinds. Early to Late Cretaceous.
Family Ankylosauridae
Advanced ankylosaurs such as Euoplocephalus and Ankylosaurus
Late Cretaceous
No universally accepted classification of dinosaurs exists. Fossil remains are often difficult to interpret, especially when only a few fragmentary specimens of a type have been found. Moreover, classifications may be constructed to serve different purposes that require different categories or organization.
Occasionally, for example, the Sauropodomorpha have been divided into more or fewer lower-rank categories (e.g., families, subfamilies); but the twofold division into the infraorders Sauropoda and Prosauropoda has stood the test of time and has been followed here.
Likewise, previous classifications divided the suborder Theropoda into two infraorders, the Carnosauria and the Coelurosauria. The former included all the larger animals and the latter all the smaller kinds. That arrangement did recognize certain distinctive anatomic features such as large heads and short necks in the Carnosauria and small heads and long necks in the Coelurosauria.
But great numbers of theropod discoveries around the world in the past several decades have blurred those anatomic distinctions and reduced the importance of size as a diagnostic criterion. Accordingly, infraordinal categories are not always used in current classifications of the Theropoda; sometimes only family groupings are listed. In the classification adopted here, the theropods are divided into two infraorders, the Ceratosauria and the Tetanurae.
The tetanuran theropods are further divided into certain subcategories - Coelurosauria, Ornithomimosauria, Maniraptora, Segnosauria, and Carnosauria - that are at a higher level than the families of this infraorder. It must be noted, however, that evolutionary affinities among all the theropod types are still being analyzed, and experts have not reached full agreement on a formal classification.
Within the order Ornithischia, two distinct subdivisions are generally given equal rank, currently as the suborders Cerapoda and Thyreophora.
A final example is the recently discovered Scutellosaurus, which has been assigned by some to the Fabrosauridae (Ornithopoda) and by others to the Stegosauria. Scutellosaurus might well represent an evolutionary link between the ornithopods and the later stegosaurs or ankylosaurs. Since its affinities are still unclear, it has here been tentatively placed with the Stegosauria.
Encyclopedia Britannica
Dinosaurs Wikipedia
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