DINOSAURS

Introduction

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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.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

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.

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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.

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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

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Before Richard Owen introduced the term Dinosauria in 1842, there was no concept of anything
even like a dinosaur. Large fossilized bones quite probably had been observed long before that time,
but there is little record—and no existing specimens—of such findings much before 1818. In any
case, people could not have been expected to understand what dinosaurs were even if they found
their remains. For example, some classical scholars now conclude that the Greco-Roman legends of
griffins from the 7th century bce were inspired by discoveries of protoceratopsian dinosaurs in the
Altai region of Mongolia. In 1676 Robert Plot of the University of Oxford included, in a work of
natural history, a drawing of what was apparently the knee-end of the thighbone of a dinosaur,
which he thought might have come from an elephant taken to Britain in Roman times. Fossil bones
of what were undoubtedly dinosaurs were discovered in New Jersey in the late 1700s and were
probably discussed at the meetings of the American Philosophical Society in Philadelphia. Soon
thereafter, Lewis and Clark’s expedition encountered dinosaur fossils in the western United States.

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.

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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

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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.

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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.

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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

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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.
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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.

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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

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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.

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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

Lawrence Berkeley National LaboratoryLawrence Berkeley National Laboratory

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

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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.

Courtesy of the American Museum of Natural History, New YorkCourtesy of the American
Museum of Natural History, New York

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, 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.

Adapted from C.R. Scotese, The University of Texas at ArlingtonAdapted from C.R. Scotese, The
University of Texas at Arlington

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

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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.

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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 abundant conifers and palm trees. Such a cellulose-heavy
diet would have required an unusual bacterial population in the intestines to break down the fibre. A
digestive tract with one or more crop chambers containing stones might have aided in the food-
pulverizing process, but such gastroliths, or “stomach-stones,” are only rarely found in association
with dinosaur skeletons. (A Seismosaurus specimen found with several hundred such stones is an
important exception.)

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

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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

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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.
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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
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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.

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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.
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© Index Open© Index Open

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

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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.

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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.

Courtesy of Paul C. Sereno (1997), University of ChicagoCourtesy of Paul C. Sereno (1997),

University of Chicago

The classification shows how the groups are subdivided. This classification is based on their
relationships to each other, as far as they are known. Fossil remains are often difficult to interpret,
especially when only a few fragmentary specimens of a type have been found. No universally
accepted classification of dinosaurs exists. Occasionally, for example, the Sauropodomorpha have
been divided into more or fewer lower-rank categories (e.g., families, subfamilies), and the suborder
Theropoda has been divided into two infraorders, the Carnosauria and the Coelurosauria.
Increasingly, taxonomists have abandoned the traditional Linnaean ranks of family, order, and so on
because they are cumbersome and not comparable among different kinds of organisms. Instead, the
names of the groups alone are used without denoting a category. Generally, a phylogeny such as the
accompanying diagram clearly shows which groups are subsumed under others. Additionally,
words with similar roots but different endings may indicate more or less inclusive groups.
Ornithomimosauria, for example, denotes a more inclusive group than Ornithomimidae. Because
the results of different phylogenetic analyses vary among researchers, and will continue to change
as new specimens and taxa are discovered, the classification can be expected to change accordingly.
This is a normal part of scientific activity and reflects continuing growth of knowledge and
reappraisal of current understanding.

Saurischia
Saurischians are known from specimens ranging from the Late Triassic to the present day, because,
as will be seen, birds are highly derived saurischian dinosaurs. Two distinctly different groups are
traditionally included in the saurischians—the Sauropodomorpha (herbivorous sauropods and
prosauropods) and the Theropoda (carnivorous dinosaurs). These groups are placed together on the
basis of a suite of features that they share uniquely. These include elongated posterior neck
vertebrae, accessory articulations on the trunk vertebrae, and a hand that is nearly half as long as the
rest of the arm (or longer). In addition, the second finger of the hand (not the third, as in other
animals) is invariably the longest; the thumb is borne on a short metacarpal bone that is offset at its
far end, so that the thumb diverges somewhat from the other fingers. The first joint of the thumb,
which bears a robust claw, is longer than any other joint in the hand.

Sauropodomorpha

Included in this group are the well-known sauropods, or “brontosaur” types, and their probable
ancestral group, the prosauropods. All were plant eaters, though their relationship to theropods,
along with the fact that the closest relatives of dinosaurs were evidently carnivorous, suggests that
they evolved from meat eaters. Sauropodomorpha are distinguished by leaf-shaped tooth crowns, a
small head, and a neck that is at least as long as the trunk of the body and longer than the limbs.

Prosauropoda

Most generalized of the Sauropodomorpha were the so-called prosauropods. Found from the Late
Triassic to Early Jurassic periods (229 million to 176 million years ago), their remains are probably
the most ubiquitous of all Triassic dinosaurs. They have been found in Europe (Germany), North
America (New England, Arizona, and New Mexico), South America (Argentina), Africa (South
Africa, Lesotho, Zimbabwe), China (Yunnan), and Antarctica. The best-known examples include
Plateosaurus of Germany and Massospondylus of South Africa. Prosauropods were not especially
large; they ranged from less than 2 metres (7 feet) in length up to about 8 metres (26 feet) and up to
several tons in maximum weight. Many of these animals are known from very complete skeletons
(especially the smaller, more lightly built forms). Because their forelimbs are conspicuously shorter
than their hind limbs, they have often been reconstructed poised on their hind legs in a bipedal
stance. Their anatomy, however, clearly indicates that some of them could assume a quadrupedal
(four-footed) position. Footprints generally attributed to prosauropods appear to substantiate both
forms of locomotion.

Prosauropods have long been seen as including the first direct ancestors of the giant sauropods,
probably among the melanorosaurids. That view has long prevailed largely because of their
distinctly primitive sauropod-like appearance and also because of their Late Triassic–Early Jurassic
occurrence. No better candidate has been discovered, and the first true sauropods are not found until
the Early Jurassic, so the transition between prosauropods and sauropods has been generally
accepted. In the 1990s, however, several studies have suggested that prosauropods may be a distinct
group that shared common ancestors with sauropods earlier in the Triassic. If this view is correct, it
is mystifying why the smaller prosauropods are so widespread throughout the Late Triassic, yet
none of the larger and more conspicuous sauropods have been found from that period.

In general body form, prosauropods were mostly rather stocky, with a long, moderately flexible
neck containing surprisingly long and flexible cervical ribs. The head was small in comparison with
the body. The jaw was long and contained rows of thin, leaflike teeth suited for chopping up (but
not grinding or crushing) plant tissues, although there is an indication of direct tooth-on-tooth
occlusion.
Prosauropod forelimbs were stout, with five complete digits. The hind limbs were about 50 percent
longer than the forelimbs and even more heavily built. The foot was of primitive design, and its
five-toed configuration could be interpreted as a forerunner of the sauropod foot. Walking
apparently was done partly on the toes (semidigitigrade), with the metatarsus held well off the
ground. The vertebral column was unspecialized and bore little indication of the cavernous
excavations that were to come in later sauropod vertebrae, nor did it show projections that were to
buttress the sauropod vertebral column. The long tail probably served as a counterweight or
stabilizer whenever the animal assumed a bipedal position.

Sauropoda

The more widely known sauropods—the huge “brontosaurs” and their relatives—varied in length
from 6 or 7 metres (about 20 feet) in the primitive ancestral sauropod Vulcanodon of Africa,
Barapasaurus of India, and Ohmdenosaurus of Germany, up to 28 to 30 metres (90 to 100 feet) or
more in Late Jurassic North American forms such as Apatosaurus (formerly known as
Brontosaurus), Diplodocus, Seismosaurus, and Sauroposeidon. Weights ranged from about 20 tons
or less in Barapasaurus to 80 tons or more for the gigantic Brachiosaurus of Africa and North
America. Sauropods were worldwide in distribution but have not as yet been found in Antarctica. In
geologic time they ranged from the Late Triassic Riojasaurus to the Late Cretaceous Alamosaurus
of North America and Laplatasaurus of South America. Their greatest diversity and abundance
took place 150 million–120 million years ago, during the Late Jurassic and Early Cretaceous
periods.

Sauropods are notable for their body form as well as their enormous size. Their large bodies were
heart-shaped in cross section, like elephants, with long (sometimes extremely long) necks and tails.
Their columnar legs, again like those of elephants, had little freedom to bend at the knee and elbow.
The legs were maintained in a nearly vertical position beneath the shoulder and hip sockets.
Because of their great bulk, sauropods unquestionably were obligate quadrupeds.

The sauropod limb bones were heavy and solid. The feet were broad, close to plantigrade (adapted
for walking on the soles), and graviportal (adapted for bearing great weight). The toes were
generally short, blunt, and broad, but some sauropods had a large straight claw on the first digit of
the forefoot and the first and second toes of the hind foot. These animals must have moved
relatively slowly and with only short steps because of the comparative inflexibility of the limbs.
Running must have been stiff-legged at no better than an elephantine pace of 16 km (10 miles) per
hour, if that. Their tremendous bulk placed them out of the reach of predators and eliminated any
need for speed. Evidently their fast growth was adaptive to predator avoidance.

The vertebrae of the backbone were highly modified, with numerous excavations and struts to
reduce bone weight. Complex spines and projections for muscle and ligament attachment
compensated for any loss of skeletal strength that resulted from reductions in bone density and
mass. The long and sometimes massive tail, characteristic of so many sauropods, would appear to
have been carried well off the ground. Tail drag marks associated with sauropod trackways are not
known, and damaged (stepped-on) tails are also not known, even though these animals apparently
traveled in herds (albeit of undetermined density). Another possible use of the tail, like the neck,
may have been thermal regulation, as improved heat loss through its large surface area could have
been a result. The tail was also the critical anchor of the large, powerful hind leg muscles that
produced most of the walking force required for moving the many tons of sauropod weight. The
muscle arrangement of the tail was precisely that of modern alligators and lizards.
The most important part of any skeleton is the skull because it provides the most information about
an animal’s mode of life and general biology. Sauropod skulls were of several main types, including
the high, boxy Camarasaurus type (often incorrectly associated with Apatosaurus); the shoe-shaped
Brachiosaurus type, with its large, delicately arched nasal bones; and the low, narrow, streamlined,
almost horselike Diplodocus type. The first had broad, spatulate teeth, while the latter two had
narrow, pencil-shaped teeth largely confined to the front parts of the jaws, especially in diplodocids.

Until recently, sauropods were visualized as swamp or lake dwellers because their legs were
thought to be incapable of supporting their great weights or because such huge creatures would
naturally prefer the buoyancy of watery surroundings. The 19th-century English biologist Richard
Owen, in fact, identified the first known sauropods as giant aquatic crocodiles and called them
cetiosaurs (whale lizards) because they were so large and because they were found in aquatic
sediments. Eventually enough skeletal remains were discovered to show that these animals were
neither crocodiles nor aquatic. However, the image of amphibious habits, thought necessary to
support the great weights of sauropods, persisted for a long time, however incorrectly. Experiments
with fresh bone samples have shown that bone of the type that composed the sauropods’ limb bones
could easily have supported their estimated weights. Moreover, there is no feature in their skeletons
that suggests an aquatic, or even amphibious, existence. In addition, numerous trackway sites
clearly prove that sauropods could navigate on land, or at least where the water was too shallow to
buoy up their weight. Accordingly, newer interpretations see these animals as floodplain and forest
inhabitants.

Still another blow has been dealt to the old swamp image by the physical laws of hydrostatic
pressure, which prohibit the explanation that the long neck enabled a submerged animal to raise its
head to the surface for a breath of fresh air. The depth at which the lungs would be submerged
would not allow them to be expanded by normal atmospheric pressure, the only force that fills the
lungs. Consequently, the long necks of sauropods must be explained in terms of terrestrial functions
such as elevating the feeding apparatus or the eyes. On all counts, sauropods are best seen as
successful giraffelike browsers and only occasional waders.

Theropoda

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This group includes all the known carnivorous dinosaurs as well as the birds. No obviously adapted
herbivores are recognized in the group, but some theropods, notably the toothless oviraptorids and
ornithomimids, may well have been relatively omnivorous like today’s ostriches. Mesozoic Era
theropods ranged in size from the smallest known adult Mesozoic nonavian dinosaur, the crow-
sized Microraptor, up to the great Tyrannosaurus and Giganotosaurus, which were 15 or more
metres (50 feet) long, more than 5 metres (16 to 18 feet) tall, and weighed 6 tons or more.
Theropods have been recovered from deposits of the Late Triassic through the latest Cretaceous and
from all continents.
Theropods may be defined as birds and all saurischians more closely related to birds than to
sauropods. They have a carnivorous dentition and large, recurved claws on the fingers. They also
share many other characteristics, such as a distinctive joint in the lower jaw, epipophyses on the
neck vertebrae, and a unique “transition point” in the tail where the vertebrae become longer and
more lightly built. Other similarities include the reduction or loss of the outer two fingers, long end
joints of the fingers, and a straplike fibula attached to a crest on the side of the tibia.

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Herrerasaurus and several fragmentary taxa from South America, including Staurikosaurus and
Ischisaurus, from the Middle to Late Triassic of Argentina are carnivores that have often been
included in the Dinosauria, specifically in Theropoda. Whereas these animals closely resemble
dinosaurs and have many carnivorous features, they also lack a number of features present in
dinosaurs, saurischians, and theropods. For example, they have only two sacral vertebrae, unlike
dinosaurs; their hips are more primitive than those of saurischians, as are their wrists; and the
second finger is not the longest, unlike those of all saurischians. It remains probable that the
features they seem to share with theropod dinosaurs are simply primitive and related to carnivory,
the general habit of archosaurs. Future discoveries and analyses may help to resolve these questions.

In all theropods the hind leg bones were hollow to varying degrees—extremely hollow and lightly
built in small to medium-size members (Compsognathus, Coelurus, and Ornitholestes, among
others) and more solid in the larger forms (such as Allosaurus, Daspletosaurus, and Tarbosaurus).

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In stance and gait, theropods were obligatory bipeds. Their bodies conformed to a common shape in
which the hind legs were dominant and designed for support and locomotion. The forelimbs, on the
other hand, had been modified from the primitive design and entirely divested of the functions of
locomotion and body support. Hind limbs were either very robust and of graviportal (weight-
bearing) proportions, as in Allosaurus, Megalosaurus, and the tyrannosaurids, or very slender,
elongated, and of cursorial (adapted for running) proportions, as in Coelurus, Coelophysis,
Ornitholestes, and the ornithomimids. Theropod feet, despite the group’s name, which means “beast
(i.e., mammal) foot,” usually looked much like those of birds, which is not surprising, because birds
inherited their foot structure from these dinosaurs. Three main toes were directed forward and
splayed in a V-shaped arrangement; an additional inside toe was directed medially or backward.
The whole foot was supported by the toes (digitigrade), with the “heel” elevated well above the
ground. Toes usually bore sharp, somewhat curved claws.

The forelimbs varied widely from the slender, elongated ones of Struthiomimus, for example, to
shorter, more massively constructed grasping appendages like those of Allosaurus, to the greatly
abbreviated arms and hands of Tyrannosaurus, to the abbreviated, stout limb and single finger of
Mononykus, to the range of wings now seen in birds. The hands typically featured long, flexible
fingers with pronounced, often strongly curved claws, which bore sharp piercing talons. Early
theropods such as Coelophysis had four fingers, with the fifth reduced to a nubbin of the metacarpal
and the fourth greatly reduced. Most theropods were three-fingered, having lost all remnants of the
fourth and fifth fingers. Tyrannosaurids (including Albertosaurus, Daspletosaurus, Tarbosaurus,
and Tyrannosaurus) were notable for their two-fingered hands and unusually short arms; they had
lost the third finger. The odd Mononykus lost even its second finger, retaining only a bizarre thumb.
This separation of function between fore and hind limbs was a feature of the first dinosaurs.
Although the first theropods, sauropodomorphs, and ornithischians were all bipedal, only theropods
remained exclusively so.

The jaws of theropods are noted for their complement of sharp, bladelike teeth. In nearly all
theropods these laterally compressed blades had serrations along the rear edge and often along the
front edge as well. Tyrannosaur teeth differed in having a rounder, less-compressed cross section,
better adapted to puncture flesh and tear it from bone. Troodontid teeth had recurved serrations
slightly larger than those typical of theropods. Archaeopteryx and other basal birds had narrow-
waisted teeth with greatly reduced serrations or none at all. Some theropods, such as most
ornithomimids and oviraptorids, had lost most or all of their teeth.

In recent years a series of unusually well-preserved theropod dinosaurs have been discovered in
deposits from the Early Cretaceous Period (146 million to 100 million years ago) in Liaoning
province, China. These theropods have filamentous integumentary structures of several kinds that
resemble feathers. Such structures indicate that today’s birds very likely evolved from theropod
dinosaurs. See Dinosaur descendants.

Ceratosauria

Ceratosauria includes Ceratosaurus and all theropods more closely related to it than to birds. This
group includes basal theropods such as Dilophosaurus and Coelophysis. It may also include the
abelisaurids of South America and elsewhere, but this is not certain. Originally thought to be a
natural group, Ceratosauria, as traditionally constituted, may represent a more general grouping of
basal theropods, including the ancestral stock of most later theropods. The Late Triassic
Coelophysis, about 1.5 meters long, is generally regarded as an archetypal primitive theropod. It has
a long neck and a long, low head with numerous small, sharp, recurved teeth. The legs were long,
the arms relatively short, and the tail very long. Dilophosaurus, from the Early Jurassic Period (200
million to 176 million years ago), is considerably larger (about 4 metres total length) and is
distinguished by a pair of thin bony crests running along the top of the skull. Because no other
theropod had such structures, these were apparently not necessary for any physiological function
and so are thought to have been for display or species recognition. There is no evidence that
Dilophosaurus spat venom.

Tetanurae

These comprise birds and all the theropods closer to birds than to Ceratosaurus. They would
include the true carnosaurs and coelurosaurs described below as well as a few relatively large
carnivorous basal forms (such as Torvosaurus, Spinosaurus, Baryonyx, Afrovenator, and
Megalosaurus). The tetanuran theropods are distinguished by several features, including the
complete loss of digits four and five of the hand, an upper tooth row extending backward only to the
eye, and a fibula that is reduced and clasped by the tibia. The name Tetanurae, or “stiff tails,” refers
to another unusual feature, a transition point in the tail sequence where the vertebrae change form in
a distinctive way.

Carnosauria includes Allosaurus and all theropods more closely related to it than to birds, including
forms such as Acrocanthosaurus, Sinraptor, and Giganotosaurus. The first known members appear
in the Late Jurassic and persist into the Cretaceous. Originally, this group was designed to include
all the big predatory dinosaurs, but it was recently recognized that only size, not their relationships,
was the trait unifying this group. Some, such as Dilophosaurus and Carnotaurus, were probably
more closely related to basal ceratosaurs. Others, such as Baryonyx and Spinosaurus, represented an
unusual diversification of fish-eating forms that were almost crocodilian in some of their habits.
Still others, such as Tyrannosaurus and its relatives, the albertosaurs and daspletosaurs, were
probably just giant coelurosaurs, as had been hypothesized by German paleontologist Friedrich von
Huene early in the 20th century. As these groups were removed from the original Carnosauria, only
Allosaurus and its relatives of the great Late Jurassic and Early Cretaceous diversification were left.
Along with Torvosaurus and the megalosaurs, they must have been among the most deadly and
rapacious large predators of their time. They are distinguished by relatively few characteristics. It is
commonly thought that carnosaurs had very short limbs, but this is not particularly true—they were
proportionally much shorter in tyrannosaurs, which are no longer considered carnosaurs. True
carnosaurs had limbs comparable in size to those of more basal theropods. Sauropod vertebrae have
been found with carnosaur tooth marks in them, which attests to the predatory habits of these
dinosaurs.

The coelurosaurs (“hollow-tailed reptiles”) include generally small to medium-size theropods,

though the recent inclusion of tyrannosaurs would seem to discount this generalization.
Coelurosauria is defined as birds and all tetanurans more closely related to birds than to the
carnosaurs. The first known members, including birds, appear in the Late Jurassic; the great
Cretaceous diversification of the other coelurosaurs ended with the Cretaceous extinctions.

In coelurosaurs the pelvis is modified so that the ischium is reduced to two-thirds or less the size of
the pubis; the eyes are larger, and no more than 15 tail vertebrae bear transverse projections. Each
of the various coelurosaurian groups has very distinct features that sets it apart from the others. The
most basal known form, the Late Jurassic Compsognathus, was the size of a chicken and
contemporaneous with the first known bird, Archaeopteryx. However, the two animals were not as
closely related as some other coelurosaurs were to birds.

Tyrannosaurs and the related albertosaurs were the largest of the Late Cretaceous theropods of the
northern continents. They are distinguished by an exceptionally large, high skull and teeth with a
much more rounded cross section than the typical daggerlike teeth of other theropods. Their
forelimbs are very short, and the third finger is reduced to a splint or lost entirely. Tyrannosaurs are
thought to have migrated to North America from Asia, because early relatives first appear on the
latter continent. Although there has been some debate about whether tyrannosaurs were active
predators or more passive scavengers, the distinction is not usually strong in living predatory
animals, and frequently larger carnivores will chase smaller ones away from fresh kills. However,
some skeletons of plant-eating dinosaurs evidently have healed wounds caused by tyrannosaur
bites, so active predation appears to be sustained.
Ornithomimids were medium-size to large theropods. Almost all of them were toothless, and
apparently their jaws were covered by a horny beak; they also had very long legs and arms. A well-
known example is Struthiomimus. Most were ostrich-sized and were adapted for fast running, with
particularly long foot bones, or metatarsals. The largest was Deinocheirus from Asia, known only
from one specimen consisting of complete arms and hands almost 3 metres (10 feet) long—nearly
four times longer than those of Struthiomimus. These animals’ speed, toothlessness, and long hands
with relatively symmetrical fingers leave their lifestyle and feeding habits unclear, but they may
have been fairly omnivorous like ostriches, although they are not directly related.

Oviraptorids, therizinosaurids, and caenagnathids appear to form a clade slightly more related to
birds than to the coelurosaurs. Oviraptorids, known from the Late Cretaceous of Mongolia, had very
strange skulls, often with high crests and a reduced dentition in an oddly curved jaw. The name
oviraptor means “egg stealer,” and it was given because remains of this carnivorous dinosaur were
found along with fossil eggs presumed to belong to a small ceratopsian, Protoceratops, which lay
nearby. Recent discoveries in Mongolia of oviraptorids sitting in birdlike positions on nests of eggs
formerly thought to belong to Protoceratops reveal that the parentage was misplaced and that
oviraptorids, like their bird relatives, apparently tended their young. Therizinosaurids, or
segnosaurs, were medium-size Asian theropods known only from a few examples. The mouth had
bladelike teeth at the back but apparently no teeth at the front. The pelvis differed markedly from
the normal saurischian design. They are very inadequately understood but seem to have been unlike
all other theropods. Caenagnathids are not well known either but appear to have had rounded jaws
that, lacking or bearing reduced teeth, are sometimes mistaken for the jaws of birds.

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The maniraptorans comprise birds, dromaeosaurs, and troodontids. Dromaeosaurs were medium-
size predators with long, grasping arms and hands, moderately long legs, and a specialized stiffened
tail that could be used for active balance control. Their feet bore large talons on one toe that were
evidently used for raking and slicing prey. A famous discovery known as the “fighting dinosaurs of
Mongolia” features a small dromaeosaur, Velociraptor, locked in petrified combat with a small
protoceratopsian. The hands of the dromaeosaur are grasping the beaked dinosaur’s frill, and the
foot talons are apparently lodged in its throat. The best-known examples are Deinonychus of North
America and Velociraptor of Asia.

Ornithischia
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The Ornithischia were all plant eaters, as far as is known. In addition to a common pelvic structure,
they share a number of other unique features, including a bone that joined the two lower jaws and
distinctive leaf-shaped teeth crenulated along the upper edges. They had at least one palpebral, or
“eyelid,” bone, reduced skull openings near the eyes and in the lower jaw (antorbital and
mandibular), five or more sacral vertebrae, and a pubis whose main shaft points backward and
down, parallel to the ischium. The earliest and most basal form is the incompletely known
Pisanosaurus, from the Late Triassic of Argentina. Some teeth and footprints and some fragmentary
skeletal material of ornithischians are known from Late Triassic sediments, but it is only in the
Early Jurassic that they become well known. Basal Jurassic forms include Lesothosaurus and other
fabrosaurids, small animals that are the best-known basal ornithischians. They have the
ornithischian features mentioned above but few specializations beyond these. Otherwise, the two
main ornithischian lineages are the Cerapoda and Thyreophora.

Cerapoda

Cerapoda is divided into three groups: Ornithopoda, Pachycephalosauria, and Ceratopsia. The latter
two are sometimes grouped together as Marginocephalia because they share a few features,
including a bony shelf on the back of the skull.

Ornithopoda

Ornithopods include heterodontosaurs, known from southern Africa; the slightly larger
hypsilophodontids, about three metres in length; the much larger iguanodontids, about nine metres
long, mostly from North America and Europe; and the large duck-billed hadrosaurs of North
America and Eurasia. In all these forms, the front teeth are set slightly lower than the cheek teeth;
the jaw joint is set lower than where the teeth meet in the jaws (the occlusal plane); and the nasal
bone is excluded by a separate bone (the premaxilla) from contacting the upper jaw (maxilla).

The postcranial anatomy of the ornithopods reflects the bipedal ancestry of the group, but the giant
hadrosaurs and some iguanodontids may have been as comfortable on four legs as on two,
especially while feeding on low vegetation. All members had hind legs that were much longer and
sturdier than their forelegs. The thighbone (femur) was nearly always shorter than the shinbones
(tibia and fibula), especially in all but the largest forms, and it usually bore a prominent process,
called the fourth trochanter, just above mid-length for the attachment of the retractor, or walking,
muscles. The pelvis was expanded, usually with an elongated and broad blade of the ilium for the
attachment of the protractor, or recovery, leg muscles. The pubis, as in all ornithischians, had
migrated backward to lie parallel to the ischium, as described above; but in all but the most basal
forms, a new prepubic process began to grow forward from the pubis, eventually reaching far in
front of the forward edge of the ilium and becoming expanded into a paddlelike shape in
hadrosaurs. It is generally thought that this process supported abdominal muscles and connective
tissues of internal organs, but little is demonstrably known. The tail was long and sometimes quite
deep and flat-sided. The vertebral spines of the tail and trunk region were reinforced by a
rhomboidal latticework of bony (ossified) tendons running in criss-cross fashion between adjacent
spines. They suggest a certain degree of stiffening of the tail and backbone, which were balanced
over the massive hips.

Ornithopod feet were modified from the primitive five-toed pattern in a way that resembled similar
modifications in theropod feet. The three middle toes served as the functional foot; the inside toe
was shortened and often held off the ground, and the outside toe was greatly reduced or absent
altogether. The resemblance to theropod feet is so strong that the footprints of the two groups are
easily confused, especially if poorly preserved. The toes of all but the most basal ornithopods
terminated in broad, almost hooflike bones, especially in the duckbills, as opposed to the sharp
claws of theropods, and this is one way to distinguish their footprints. The hand reflected the
primitive five-digit design, and, as was generally true in archosaurs, the fourth and fifth digits were
shorter than the other three, with the third being longest. In iguanodontids and hadrosaurs, the
fingers ended in broad, blunt bones rather than in claws, much like the toes. It is thought that these
middle fingers and toes were covered by blunt, hooflike structures. In the duckbills the fingers
apparently were encased in a mittenlike structure that could have broadened the hand for better
support of the animal’s weight on soft ground.

The Ornithopoda differ from one another mainly in the structure of their skulls, their jaws and teeth,
their hands and feet, and their pelvises. Ornithopods constitute an excellent case study in evolution
because, as the various lineages arise and die out from the latest Triassic to the latest Cretaceous,
trends in size, complications and elaborations of teeth and chewing mechanisms, adaptations for
quadrupedal posture in some forms, and other changes emerge clearly from their phylogenetic
patterns.

In the fabrosaurids the teeth were simple leaf-shaped, laterally compressed elements arranged in a
single front-to-back row in each jaw. They were not set in from the outer cheek surface as in most
ornithopods. Small incisor-like teeth were borne on the premaxillary bones above, but (as always)
no teeth were present on the predentary below. One pair of incisors had been lost. The lower jaw
had no coronoid process for large muscle attachment, and the upper temporal opening (the jaw
muscle site), like the mandibular opening, was relatively smaller than in theropods and other
archosaurs. Upper and lower teeth alternated in position when the jaw was closed; they did not
occlude directly.

In heterodontosaurs the cheek teeth were crowded together into long rows and set inward slightly
from the outer cheek surface. The inset, which persisted through all later ornithopods, has been
interpreted to suggest the presence of cheeks that may have held plant food in the mouth for further
processing by the cheek teeth. They occluded directly to form distinct chisel-like cutting edges with
a self-sharpening mechanism maintained by hard enamel on the outer side of the upper teeth and the
inner side of the lower. There were prominent upper and lower tusklike teeth at the front of the
mouth (the upper set in the premaxillary bones, the lower on the dentary bones). At least two pairs
of incisors seem to have been retained. Certain features of the skull suggest much larger jaw
muscles in heterodontosaurs than in the fabrosaurids.
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The hypsilophodonts had cheek teeth arranged in tightly packed rows set well inward from the outer
cheek surfaces. The teeth occluded directly, and the opposing rows formed a long shearing edge
similar to that of the heterodontosaurs. There was, however, no “tusk” either above or below. The
premaxillaries had small simple incisor-like teeth above the beak-covered, toothless predentary.
Strong projections of bone extended up from the lower jaw toward the moderate-size upper
temporal fenestrae.

The skulls of iguanodonts accommodated still larger jaw muscles, but the cheek teeth were less
regular and compacted than in the primitive ornithopods and consequently did not occlude as
uniformly. Both the premaxillaries and the predentary were toothless but probably were sheathed in
horny beaks.

Specialization of the teeth and jaws reached a pinnacle in the hadrosaurs, or duck-billed
ornithopods. In this group a very prominent, robust projection jutted from the back of the stout
lower jaw. Large chambers housing muscles were present above this process and beneath certain
openings in the skull (the lateral and upper temporal fenestrae). These chambers are clear evidence
of powerful jaw muscles. The dentition consisted of numerous tightly compacted teeth crowded into
large grinding batteries. The battery in each jaw was composed of as many as 200 functional and
replacement teeth with distinct, well-defined wear, or grinding, surfaces that resulted from very
exact occlusion. As teeth were lost from the front of the jaws in iguanodontids and hadrosaurs, the
snouts expanded into a bulbous shape, especially in the “duck-billed” hadrosaur, and may have been
covered by a horny beak that improved feeding. These bills apparently had edges sharp enough to
shred and strip leaves or needles from low shrubs and branches. Pine needles have been identified
in duck-billed dinosaur remains and presumably represent stomach contents.

Other interesting specializations may have assisted iguanodontids and hadrosaurs in feeding. In
both groups there was a marked increase in mobility (kinesis) among the joints of the bones of the
facial region. As the jaws clamped down, some cheek bones were allowed to rotate outward
slightly, perhaps to cushion the stress of chewing tough foods. The hands were also unusually
modified in the two groups, though in different ways. In iguanodontids the wrist bones were
coalesced into a single blocky structure that was less mobile than in more primitive wrist
configurations. The joints of the thumb were similarly coalesced into a single conelike spike that
had limited mobility on the wrist. The middle three digits flexed in the normal way and bore broad
flat, spatulate claws. The fifth digit actually had two additional joints and became somewhat
opposable to the rest of the hand. It is thought that the hands may have been adapted to grasp and
strip vegetation, and the spikelike thumb has been suggested to have been an effective weapon
against predators. These features were more or less continued in hadrosaurs, except in this group the
blocky wrist was reduced and the thumb was lost completely.
Encyclopædia Britannica, Inc.Encyclopædia Britannica, Inc.

Some varieties of hadrosaurs are also noted for the peculiar crests and projections on the top of the
head. These structures were expansions of the skull composed almost entirely of the nasal bones. In
genera such as Corythosaurus, Lambeosaurus, Parasaurolophus (and a few others), the crests were
hollow, containing a series of middle and outer chambers that formed a convoluted passage from
the nostrils to the trachea. Except for passing air along to the lungs, the function of these crests is
not widely agreed upon. Sound production (honking), an improved sense of smell, and a visually
conspicuous ornament for species recognition are some suggestions. Because these animals are no
longer considered to have been amphibious, ideas such as snorkeling and extra air storage space
have generally been discarded. Besides, the crests had no opening at their ends and consequently
would not have been able to work as snorkels; even the largest crests held only an estimated 2
percent of the volume of the lungs, hardly enough to justify the construction of such an elaborate
structure.

Pachycephalosauria

In important respects the pachycephalosaurs conformed to the basic ornithopod body plan, and there
is some evidence that pachycephalosaurs actually evolved from (and are therefore members of)
ornithopods, perhaps similar to hypsilophodontids. All of them appear to have been bipedal. They
bore the typical ornithopod ossified tendons along the back, and they had simple leaf-shaped teeth,
although the teeth were enameled on both sides. The ornithischian type of pelvis was present, but a
portion of the ischium was not.

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The pachycephalosaurs are known as domeheads because of their most distinctive feature—a
marked thickening of the frontoparietal (forehead) bones of the skull. The thickness of bone was
much greater than might be expected in animals of their size. The suggestion has been made that
this forehead swelling served as protection against the impact of the type of head-butting activities
seen today in animals such as bighorn sheep, but microscopic studies of the bone structure of these
thick domes suggest that they are poorly designed to divert stresses away from the braincase. Also,
the great variety of pachycephalosaur domes—from thin, flat skull tops to pointed ridges with large
spikes and knobs facing down and back—suggests no single function in defense or combat.

Stegoceras and Pachycephalosaurus of the North American Cretaceous were, respectively, the
smallest and largest members of the group, the former attaining a length of about 2.5 metres (8 feet)
and the latter twice that. Pachycephalosaurs are known almost entirely from the Late Cretaceous
(although Yaverlandia is from the Early Cretaceous) and have been found in North America and
Asia. They are generally rare and still are relatively poorly known among dinosaur groups.

Ceratopsia

The first ceratopsian (“horn-faced”) dinosaur remains were found in the 1870s by the American
paleontologist Edward 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 first appeared in the modest form of psittacosaurids, or parrot-reptiles, in the Early
Cretaceous and survived to the “great extinction” at the end of the Cretaceous Period. Triceratops,
together with Tyrannosaurus, was one of the very last of all known Mesozoic Era dinosaurs in
North America, where the fossil record of the latest Cretaceous is best known. Ceratopsians had a
peculiar geographic distribution: the earliest and most primitive kinds, such as Psittacosaurus, are
known only from Asia—Mongolia and China, specifically. Protoceratops and its relatives are
known from both Asia and North America. All the advanced ceratopsids (chasmosaurines and
centrosaurines), with the exception of a few fragmentary and doubtful specimens, have been found
only in North America.

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Ceratopsians ranged in size from relatively small animals the size of a dog to the nearly 9-metre-
(30-foot-) 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, hence the name ceratops. 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 longest head that ever adorned any land animal; the length
of the Torosaurus skull was almost 3 metres (10 feet), longer than a whole adult Protoceratops.

Several hypotheses 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. 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.

Most of these hypotheses are difficult to test. One important fact to keep in mind was that the frill
was little more than a frame of bone, sometimes ornamented with knobs and spikes around large
openings behind and above the skull. An exception to this pattern was Triceratops, which had a
solid and relatively short frill, but Triceratops is so well known that its frill is often mistakenly
considered typical of ceratopsians. The open frill of other ceratopsians would have provided only
poor protection for the neck region and only a modest area of attachment for jaw or neck muscles. If
skin and soft tissues spanned the area framed by the bony frill, it would have created a formidable
presence when the head was lowered in threatening display. Such a large structure would naturally
have absorbed and reflected sunlight that warmed the tissue and its internal blood vessels, but it is
questionable whether this was an important or necessary function of the frill, since other dinosaurs
do not have similar structures.

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The Ceratopsia are divided into groups that mirror their evolutionary trends through time: the
primitive psittacosaurids, such as 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 the chasmosaurines Triceratops and Torosaurus as well as the
centrosaurines such as Centrosaurus (or Monoclonius)—all from North America.

Like the pachycephalosaurs, the most basal ceratopsians, such as Psittacosaurus, look much like
typical ornithopods, largely because of their relatively long hind limbs and short front limbs
(probably resulting in bipedal stance and locomotion) and the persistence of upper front teeth and a
fairly unspecialized pelvis. Resembling ornithopods in body form, Psittacosaurus had a shorter
neck and tail and was much smaller (only 2 metres [6.5 feet] long) than the most advanced
ornithopods such as the iguanodonts and hadrosaurs. Psittacosaurus, however, possessed a beak,
the beginnings of a characteristic neck frill at the back of the skull, and teeth that prefigured those
of the more advanced ceratopsians. It is also recognized diagnostically as a ceratopsian by the
presence of a unique bone called the rostral, a toothless upper beak bone that opposed the lower
predentary found in all ornithischians.
Encyclopædia Britannica, Inc.Encyclopædia Britannica, Inc.

The best-known of the protoceratopsids is the genus Protoceratops. Dozens of skeletal specimens,
ranging from near hatchlings to full-size adults, have been found and studied. 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
Desert of Mongolia. Their collection provided the first valid growth series of any dinosaur. Their
discovery of several nests of eggs loosely associated with Protoceratops skeletons was the first
finding of eggs that were unquestionably dinosaurian; originally attributed to Protoceratops, the
eggs only recently were correctly attributed to the theropod Oviraptor (as noted in the section
Tetanurae).

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, constituting 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 two cranial arches, but it became the enormous
fan-shaped ornament of later forms. Protoceratops also displayed a short but stout horn on the
snout due to development of the nasal bones; this too was a precursor of the prominent nasal horns
of ceratopsids such as Centrosaurus, Chasmosaurus, Styracosaurus, Torosaurus, and Triceratops.
The last two genera evolved 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 nourishing blood vessels. These advanced ceratopsids are sometimes divided
into centrosaurines, which had a prominent nose horn but small or absent eye horns, and
chasmosaurines, which had larger eye horns but reduced nose horns.

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 lower jawbones were joined at the front and
capped by a stout beak formed of the toothless predentary bone. 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.

As in the hadrosaurs, each dental battery consisted of about two dozen or more tooth positions
compressed together into a single large block. 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 vertebrates, living and extinct, except mammals, have a lifelong supply
of replacement teeth.) The suggestion is that they fed on something exceedingly tough and fibrous,
such as the fronds of palms or cycads, both of which were plentiful during late Mesozoic times.

With the exception of the bipedal Psittacosaurus, and perhaps the facultatively bipedal
protoceratopsids, 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 “hooves” rather than claws. As in most other four-
legged animals, the rear legs were significantly longer than the front legs (which again suggests
their bipedal ancestry). 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 “push-up” position. Consequently, the head was carried low and close to the
ground. This mixed posture was perhaps 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 fused (co-ossified), presumably to support the
massive skull. The first joint of the neck was unusual in that the bone at the base of the skull formed
a nearly perfect sphere that fit 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. Presumably ceratopsians used their
horns in an aggressive manner, but whether they used them as defense against possible predators, in
rutting combat with other male ceratopsians, or in 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 or species recognition as their primary uses.

Thyreophora

The Thyreophora consist mainly of the well-known Stegosauria, the plated dinosaurs, and
Ankylosauria, the armoured dinosaurs, as well as their more basal relatives, including
Scutellosaurus and Scelidosaurus. Scutellosaurus was a small bipedal dinosaur, only about a metre
(3.3 feet ) in length, known from the Early Jurassic Period of Arizona, U.S. It was first classified as
a fabrosaurid because of its primitive skeletal structures. However, it differed from fabrosaurids in
some important respects, including the possession of small bony plates, or scutes, of various shapes
along the back and sides of its body. These scutes are also found in the slightly larger
Scelidosaurus, which was up to three metres in length and quadrupedal; this dinosaur is known
from the Early Jurassic of England and Arizona.

In the Middle and Late Jurassic, the first stegosaurs and ankylosaurs appeared. Like the previously
described forms, they are distinguished by bony scutes. Scutes are maintained and elaborated all
over the body in ankylosaurs but are reduced to a series of plates and spikes along the backbone in
stegosaurs, though their basic structure remains the same in both groups. Thyreophorans also have
low, flat skulls, simple S-shaped tooth rows with small leaf-shaped tooth crowns, and spout-shaped
snouts.

Stegosauria

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With their unique bony back plates, the stegosaurs are very distinctive. Relatively few specimens
have been found, but they were widespread, with remains being found in North America, Africa,
Europe, and Asia. Stegosaurian remains have appeared in Early Jurassic to Early Cretaceous strata.
The most familiar genus is Stegosaurus, found in the Morrison Formation (Late Jurassic) of western
North America. Stegosaurus was 3.7 metres (12 feet) in height and 9 metres in length, probably
weighed two tons, and had a broad, deep body. Not all varieties of the Stegosauria 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 undoubtedly quadrupedal, although the massive legs were of
greatly disparate lengths—the hind legs being more than twice the length of the forelegs. Whatever
walking and running skills were possessed by the stegosaurs, their limb proportions must have
made these movements extremely slow. The humerus of the upper arm was longer than the bones of
the forearm, the femur much longer than the shinbones, and certain bones of the feet very short,
which means that the stride must have been short. In addition, the feet were graviportal in design
and showed no adaptations for running.

The stegosaurian skull was notably small, long, low, and narrow, with little space for sizable jaw
muscles. The weakly developed dentition consisted of small, laterally compressed, leaf-shaped teeth
arranged in short, straight rows. This combination of features seems odd in comparison with the
large, bulky body. The weak dentition suggests that the food eaten must have required little
preparation by the teeth and yet provided adequate nourishment. Perhaps the digestive tract
contained fermenting bacteria capable of breaking down the cellulose-rich Jurassic plant tissues.
Digestion may also have been assisted by a crop or gizzard full of pulverizing stomach stones
(gastroliths), though none has yet been discovered in stegosaurian specimens. A collection of
disklike bones is found in the throat region of Stegosaurus, but these are likely to have been
embedded in the skin, not used in the gut. Even so, it is still difficult to understand how these
animals, with such small and poorly equipped mouths, could have fed 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 double row of large diamond-shaped bony plates
on the back. A controversy as to their purpose and how they were arranged has raged ever since the
first Stegosaurus specimen was collected (1877, Colorado, U.S.). The evidence and a general
consensus argue in favour of the traditional idea that the plates projected upward and were set in
two staggered (alternating) rows on either side of the backbone. In other stegosaurs, such as
Kentrosaurus, the plates are more symmetrical and may have been arranged side by side. The
suggestion that the plates did not project above the back at all, but lay flat to form flank armour, has
been rejected on the basis of studies of the microstructure of the bone of the plates, in which
attachment fibres are embedded in a manner consistent with an upright position. In Stegosaurus
itself, the end of the tail bore at least two pairs of long bony spikes, which suggests some sort of
defensive role for the tail but not necessarily for the back plates. However, other stegosaurs, such as
Kentrosaurus, had relatively small plates along the front half of the spine and spikes along the back
half of the spine and the tail.

The discovery in 1976 that the bony plates of Stegosaurus were highly vascularized led to the
suggestion that these “fins” functioned as cooling vanes to dissipate excess body heat in much the
same way that the ears of elephants do. The staggered arrangement in parallel rows might have
maximized the area of cooling surface by minimizing any downwind “breeze shadow” 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.
Ankylosauria

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The ankylosaurs are known from the Late Jurassic and Cretaceous periods. They are called
“armoured dinosaurs” for their extensive mosaic of small and large interlocking bony plates that
completely encased the back and flanks. Most ankylosaurs, such as Euoplocephalus, Nodosaurus,
and Palaeoscincus, were relatively low and broad in body form and walked close to the ground on
short, stocky legs in a quadrupedal stance. As in stegosaurs, the hind legs were longer than the front
legs, but they were not as 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 bones at the ends of the digits (terminal phalanges) were broad and hooflike rather
than clawlike.

The ankylosaur skull was low, broad, and boxlike, with dermal scutes (osteoderms) that were often
fused to the underlying skull bones. In Euoplocephalus even the eyelid seems to have developed a
protective bony covering. The jaws were weak, with a very small predentary and no significant
projections of bone for jaw muscle attachment. The small jaw muscle chamber was largely covered
by dermal bones rather than having openings. 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, in 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.

Apparently neither very diverse nor abundant, the ankylosaurs are known only from North America,
Europe, and Asia. They are divided into the more basal Nodosauridae and the more advanced
Ankylosauridae, which may have evolved from nodosaurs. The most conspicuous difference
between the two groups 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. The patterns of the armour also
generally differ between the two groups, and ankylosaurids tend to have even broader, more bone-
encrusted skulls than did the nodosaurs.

Kevin Padian

John H. Ostrom

Additional Reading
General works
Philip J. Currie and Kevin Padian (eds.), Encyclopedia of Dinosaurs (1997), comprises articles on
topics related to dinosaur taxonomy, biology, and evolution as well as important paleontological
sites and exhibits worldwide. James O. Farlow and M.K. Brett-Surman (eds.), The Complete
Dinosaur (1997), emphasizes aspects of various groups of dinosaurs and their biology. David
Norman and John Sibbick, The Illustrated Encyclopedia of Dinosaurs: An Original and Compelling
Insight into Life in the Dinosaur Kingdom (1985, reissued 1998), provides a well-written and
lavishly illustrated treatment that is excellent for the specialist and nonspecialist alike.

Advanced textbooks on vertebrate evolution and paleontology include Michael J. Benton,

Vertebrate Palaeontology, 2nd ed. (1997, reissued 2000); and Robert L. Carroll, Vertebrate
Paleontology and Evolution (1988). David B. Weishampel, Peter Dodson, and Halszka Osmólska
(eds.), The Dinosauria (1990), primarily contains extensive reviews of the major taxonomic groups,
defining them via anatomic descriptions and drawings while also supplying fossil-site information.

The search for dinosaurs

Louie Psihoyos and John Knoebber, Hunting Dinosaurs (1994), assembles an impressive
photographic record of the discoveries and the people responsible for them. Philippe Taquet,
Dinosaur Impressions: Postcards from a Paleontologist (1998; originally published in French,
1994), vividly traces one paleontologist’s travels throughout the world over a period of 30 years.
Edwin H. Colbert, Dinosaurs: Their Discovery and Their World (1961), a landmark treatment of
the subject by a world authority of the period, includes extensive photographic and line-drawing
coverage, and Men and Dinosaurs: The Search in Field and Laboratory (1968, reissued 1971),
provides a thorough illustrated history of the discovery, collection, and study of dinosaurs. John R.
Horner and James Gorman, Digging Dinosaurs (1988, reprinted 1995), is a fascinating account of
the search for and collecting of dinosaur eggs and nests as told by the discoverers. John H. Ostrom
and John S. McIntosh, Marsh’s Dinosaurs: The Collections from Como Bluff (1966, reissued 1999),
is illustrated for technical professionals and contains a historical study of one of the most famous
dinosaur localities.

For specific information about the origin of birds from theropod dinosaurs, Lowell Dingus and
Timothy Rowe, The Mistaken Extinction: Dinosaur Evolution and the Origin of Birds (1998), is an
excellent reference source. Kevin Padian and Luis M. Chiappe, “The Origin of Birds and Their
Flight,” Scientific American, 278(2):38–47 (February 1998), provides a brief account of the
evidence; and Jennifer Ackerman, “Dinosaurs Take Wing,” National Geographic, 194(1):74–99
(July 1998), furnishes additional information and graphics.

Natural history

John R. Horner and Edwin Dobb, Dinosaur Lives: Unearthing an Evolutionary Saga (1997),
explores developments in the understanding of dinosaurian paleobiology. Peter Dodson, The
Horned Dinosaurs: A Natural History (1996), presents a case history of the ceratopsians, an
important group of dinosaurs. Kenneth Carpenter, Karl F. Hirsch, and John R. Horner (eds.),
Dinosaur Eggs and Babies (1994, reissued 1996), a technical multiauthor work, reviews many
aspects of dinosaur reproductive biology.

Extinction

The most authoritative account of the Late Cretaceous extinctions is J. David Archibald, Dinosaur
Extinction and the End of an Era: What the Fossils Say (1996), a masterful book. Walter Alvarez,
T. rex and the Crater of Doom (1998), offers a somewhat different point of view.
John H. Ostrom

Kevin Padian