dinosaurs The name ‘dinosaur’ (‘terrible reptile’) was first coined by Sir Richard Owen, a British anatomist, in 1841. He used the term to denote the large reptiles Megalosaurus and Iguanodon that had been collected and described in the 1820s. A considerable amount of work has been done on dinosaurs since then and we now recognize them as a monophyletic (ancestor and all descendants) group of archosaurian reptiles that are closely related to the flying reptiles, the pterosaurs (Fig. 1). Dinosaurs are characterized by having erect limbs and a pelvis that incorporates at least five vertebrae, characters (using that word in its biological sense) that are related to their active terrestrial lifestyle. The earliest dinosaurs are known from the late Middle Triassic of South America and have been studied and described by Paul Sereno from the University of Chicago. Forms such as Herrerasaurus and Staurikosaurus were small bipedal (moving on two legs) carnivores showing a variety of advanced characters that quickly allowed them to gain ascendancy over the then dominant terrestrial vertebrates, the mammal-like reptiles. In these animals the hind leg was elongated and the foot was functionally three-toed and digitigrade, that is, the animal stood on its toes. The acetabulum (hip-socket) was partly open and the upper rim was buttressed to help support the body on the vertical legs. In addition, the two outer fingers were reduced and the thumb had an offset to bring it into opposition with the other fingers, thus providing a grasping hand. During the Late Triassic, the two major groups of dinosaurs developed. They are separated by characteristics of the structure of the pelvis (Fig. 2). In the Saurischia (lizard-hipped) the pubis points forwards, while in the Ornithischia (bird-hipped) the pubis has rotated backwards to lie parallel to the ischium.

Ornithischia

The ornithischian dinosaurs were an extremely diverse group of terrestrial herbivores. Apart from the structure of the pelvis they characteristically had an additional bone at the front of the upper jaw, the predentary, which bore no teeth but acted like a beak for cropping vegetation. Their teeth were rather leaf-like in shape, similar to those of the modern Iguana, and they had ossified tendons running along the back and tail. This group includes such familiar forms as the stegosaurs (plated dinosaurs), ankylosaurs (armoured dinosaurs), ornithopods (bird-footed dinosaurs), and ceratopids (horned dinosaurs) (Fig. 1). They were all large animals, between 3 and 9 m long. Whereas the ornithopods were bipedal the other groups were secondarily quadrupedal.

A number of interesting structural adaptations were developed within the ornithischians. The ornithopods included the hadrosaurs or duck-billed dinosaurs, in which the front of the mouth was enlarged to form a broad flat beak and a variety of bony crests were developed on top of the head. The crests were formed by enlargement of the nasal bones so that the nasal passages extended through them. Although at one time it was thought that the crests might have been an adaptation for storing air while swimming and diving, it is now thought that they provided a resonating chamber for the production of loud calls. Stegosaurs bore a double row of diamond-shaped plates along their back and a pair of spikes at the tip of the tail. Although the spikes were clearly defensive, the plates were probably for thermoregulation, enabling the animal to control its internal temperature. Ankylosaurs were broad, short-legged dinosaurs, covered by a mosaic of defensive bony plates while the tail bore a large bony club. Because of this defensive armour they have been called ‘dinosaurian tanks’. In the ceratopians, posterior skull bones extended back to form a large neck, while most also had horns on the nose and above the eyes. Triceratops is a typical example of these forms. The extension of the skull bones provided additional area for the attachment of jaw muscles, and together with the extensive battery of teeth made these animals extremely efficient at shearing and grinding tough vegetation.

Saurischia

The saurischian dinosaurs include both carnivores, the theropods, and herbivores, the sauropods. The theropods were all bipedal and ranged in length from less than 1 m up to 12 m. The smaller carnivores, such as Coelophysis, had small heads, long necks, long arms with grasping hands, and long hind limbs. They were lightly built and were probably opportunistic feeders, eating insects as well as small vertebrates. At the other end of the scale, forms such as Tyrannosaurus were the largest land predators that have ever lived. They possessed very large heads with serrated, blade-like teeth; the hind limbs were long and powerful, but the forelimbs were very much reduced and in some forms the hand had only two fingers. They were probably adapted as ambush predators of large herbivores and were capable of moving very quickly for short distances. A third group of theropods includes Deinonychus, a medium-sized predator in which one hind claw was greatly enlarged for use as an offensive weapon. It is from this group that the birds are thought to have developed.

The sauropods include the largest land animals that have ever existed; they may have reached 24 m in length. They had small heads with weak teeth at the front of the mouth, long necks and tails, and short barrel-shaped bodies supported on massive vertical limbs. At one time they were considered to have been swamp-dwellers, too massive to have been able to move on land, but further analysis has shown that they were terrestrial animals adapted to browse in high trees. As there were no crushing teeth in the mouth, vegetation must have been swallowed and then crushed in a gizzard similar to that found in many birds. The presence of gastroliths (gizzard stones) in the rib cages of some specimens shows that this view is correct.

Dinosaur metabolism

It was long considered that dinosaurs had an essentially reptilian metabolism; that is, they controlled their internal body temperature by activities such as moving into or out of the sun. This is termed ectothermy and differs from the mammalian system in which control is internal or endothermic. In accordance with this view, dinosaurs were often reconstructed as large sprawling lizards with activity levels similar to those of modern reptiles. In 1970 John Ostrom of Yale and Robert Bakker of Harvard University suggested that dinosaur metabolisms may have been more mammalian, thus initiating an argument that still continues. Bakker pointed out that dinosaurs' skeletons show that they had upright postures and gaits similar to those of modern endotherms. In particular, the combination of long hind limbs and flexible limb joints, together with long rigid tails for balance suggests that bipedal dinosaurs were fast and agile. The bone structure of dinosaurs is also similar to that seen in modern endotherms and unlike that of ectotherms. More recent evidence includes the fact that dinosaurs are found in localities much further north or south than ectotherms inhabit now, again suggesting that they were endothermic. Bakker also suggested that fossil predator–prey ratios supported endothermy in dinosaurs. These ratios are based on the fact that endothermic predators consume more than ectothermic predators in order to support their higher metabolic rates, and hence there are fewer of them in proportion to prey organisms. Although some analyses suggest that dinosaurs showed an endothermic ratio, these have been challenged as unreliable because of problems such as preservational biases. In fact, the case for endothermy in all dinosaurs is still not proved, although it is generally accepted for small carnivorous dinosaurs. Other dinosaurs, particularly large herbivores, would have been more efficient as ectotherms because the temperatures of their large bodies would not have fluctuated easily, and they would have needed less food to support an ectothermic metabolism.

Dinosaur behaviour

Information about the behaviour of dinosaurs has been gleaned from trace fossils, mainly footprints, analysis of various skeletal structures, and the presence of eggs and nests. These suggest that many dinosaurs exhibited group behaviour like that seen in modern large mammals rather than the essentially solitary behaviour observed in modern reptiles. Trackways of dinosaurs commonly show multiple tracks moving in the same direction, indicating herding behaviour. Many finds of dinosaur eggs and nests, particularly those made in Montana by Jack Horner of the Museum of the Rockies in Bozeman, Montana, show that the young must have been cared for by the adults for an extended period, for they hatched out at a stage at which they were not self-sufficient. This indicates co-operative group behaviour of the adults at group nesting grounds on a level similar to that seen in birds, and far beyond that seen in any modern reptiles. Finally, the presence of possible display structures, particularly crests on hadrosaur skulls, suggests sociality among groups of dinosaurs, for such structures would have aided the recognition of potential mates or opponents within a social group.

Dinosaur extinction

Despite their success during the Mesozoic, the dinosaurs became extinct at the end of the Cretaceous. How this happened has become the focus of a highly charged scientific debate. The dinosaurs were only one of the groups that became extinct at the end of the Cretaceous; also hard-hit were the unicellular Foraminifera, the ammonites, and marine reptiles such as the mosasaurs and plesiosaurs. On land the pterosaurs and some mammalian groups became extinct, but many terrestrial organisms were unaffected. This rather selective extinction has been explained by changing climatic conditions, particularly a gradual cooling and drying of the climate during the Late Cretaceous. William Clemens of the University of California at Berkeley has pointed out that in the rare cases where there is a good record of dinosaurs up to the Cretaceous–Tertiary boundary it shows a gradual reduction in diversity that would tend to support the climatic model. However, more recent work by Louis and Walter Alvarez, also from Berkeley, has demonstrated the presence of a clay layer at the boundary in many localities. This contains high levels of elements that are usually rare at the Earth's surface, particularly iridium, together with droplets of shocked quartz which indicate a high-speed impact. They have interpreted this as evidence the Earth was hit by a large extraterrestrial body or bolide, and that the impact generated large amounts of dust, causing global darkness and ultimately extinction of many groups of organisms. Although this appears to be a simple and elegant solution to the problem, it has been criticized for a lack of connection between such a catastrophic event and the selective pattern of extinctions that is seen. No generally accepted scenario has yet been proposed and the debate will certainly continue for some time—although it has been pointed out that as the dinosaurs survived as birds perhaps they did not really become extinct.

David K. Elliott

Bibliography

Lucas, S. G. (2000) Dinosaurs: the textbook. McGraw-Hill, Boston.
Norman, D. (1985) The illustrated encyclopedia of dinosaurs. Crescent Books Ltd, New York.

Dinosaurs

The history of the Dinosauria begins with one of the dinosaur's small extinct ancestors called Petrolacosaurus. Around 270 million years ago, this animal was a member of the group of early land vertebrates called the diapsids that had skulls with two openings behind the eye socket (other evolutionary groups, like fish and amphibians, had one opening or none at all). The diapsids are believed to be the ancestors of the lepidosaurs (modern lizards and snakes) as well as of the archosaurs, the group that led to the dinosaurs. The lepidosaurs achieved great evolutionary success. The extra openings in their skulls led to the interesting structures found in modern snakes, including a light and flexible skull that allows them to catch and eat prey larger than their mouths.

Ancestry: Euparkeria

The first known archosaurs appeared in the Permian Period (319 to 286 million years ago) and they were well on their way to becoming large-sized animals by the early Triassic (about 245 million years ago). When discussing dinosaur ancestry, paleontologists prefer to examine an interesting little archosaur known as Euparkeria. Euparkeria had anatomical characteristics of most archosaurs (and eventually the dinosaurs), including deeply rooted, sharp, serrated teeth; two holes behind the eyes; and a broad space in front of the eye sockets. Their jaws had a distinctive opening that was different in shape and position from other tetrapods, and their spine had small bony plates suggestive of the beginning of armor plating. Perhaps the most important feature of Euparkeria is the arrangement of their hipbones.

As the archosaurs evolved, many species developed hipbones that allowed the angle of their hind limbs to change from a sprawling posture, such as a lizard or crocodile has, to an erect one, like a bird. The limbs came under the body instead of being spread out to the side. This leg position provided a firmer basis of support for a larger and heavier body. Dinosaurs became the largest land animals, and part of their ability to become so large was a direct result of this change in posture. When the legs are spread out to the side they can only support so much weight before the joints that attach them to the body give out. With the legs underneath the body, they form a kind of column that can support a great deal more weight. This means a larger animal can move around without being slow and sluggish or breaking its bones. This change in body posture is one of the main reasons dinosaurs were able to become so large.

One important group of archosaurs was the thecodonts, which included Euparkeria. During the Triassic period, thecodonts continued to evolve and undergo changes in body shape. By the end of the Triassic 213 million years ago), there were two groups of thecodonts, the saurischians and the ornithischians. The saurischians ate both plants and meat. Their pelvis was distinctive in that the three bones that made up the hip—the ilium, the ischium, and the pubis—were joined so that they angled away from each other in a triangular shape. There were two distinct groups of saurischians, the theropods ("beast feet") and the sauropods.

The theropods were carnivorous dinosaurs that walked upright on two feet. Some of the most famous dinosaurs we know today are theropods, including Tyrannosaurus rex, Velociraptor, and other swift and dangerous predators. The other group of saurischians, the sauropods, were herbivores and moved about on four legs. Some sauropods, including Brachiosaurus, Camarasaurus, and Ultrasaurus, were the largest animals that ever lived on Earth. The other evolutionary offshoot of the ancestral thecodonts was the ornithischians. The hip structure of the ornithischians differed from that of the saurischians in that their forward-extending pubis bone was turned toward the back. Because this bone structure also occurs in modern birds, the ornithischians are called bird-hipped dinosaurs. (The ornithischians are not ancestors of birds, they just have a similar hip structure.) The legs, feet, and anklebones of ornithischians were similar to those of the saurischians.

A peculiar adaptation of ornithischians was the beaklike covering of the front of the mouth, which is characteristic of the ceratopsians and duck-billed dinosaurs. Ornithischians also had a complex network of bony rods along their spine which supported the spine. All ornithischians were herbivores. The group contains some of the more distinctive-looking dinosaurs, including Stegosaurus, which had a series of large, triangular, horn-covered bony plates along its back and tail; the heavily armored Ankylosaurus; and the ceratopsians, including Triceratops, with its huge bony hood and horns.

The First True Dinosaurs

The first groups of true dinosaurs, the coelurosaurs, appeared about 210 million years ago in the late Triassic period. These dinosaurs were carnivorous theropods. They were agile and lightly built; most species were smaller than an adult human. Some of the best fossil specimens of the coelurosaur known as Coelophysis were discovered in the United States in New Mexico. Other coelurosaurs were the smaller Ornitholestes, a heavily jawed predator with nostrils that faced upward on its skull, and Coelosaurus perhaps the best-known coelorosaur. All coelorosaur feet had three toes pointing forward and a fourth facing back. The fifth toe was greatly reduced. This pattern persisted throughout the history of all the dinosaurs and is one way to document change within the group.

From the remains of dinosaurs that appear in the fossil record, paleontologists can infer what those animals looked like, how they moved, and what and how they ate. But paleontologists today are also asking: What dinosaur behavior can we infer from fossils? The Maiasaura ("Good Mother" dinosaur) eggs and young found in Montana in 1978 are an excellent example of one way paleontologists can hypothesize dinosaur behavior. In this case, the young dinosaurs at the site were too big to fit in the fossil eggs in nests that were found nearby. Paleontologists think that the parent dinosaurs brought food to the baby dinosaurs and protected them from predators. Also, since many nests were discovered together in a small area, the scientists think that these dinosaurs many have lived together in some sort of herd.

The thinking is that if the baby dinosaurs were too big to fit in the eggs, but were still in the nest they probably remained in nest for some time after hatching. They would have to had food brought to them by the parents until they were large enough to forage for food on their own. Many birds care for their young this way by bringing them food until the fledglings are old enough to feed and fly on their own. This is another piece of evidence that links birds and dinosaurs.

At the end of the Cretaceous era (around 64 million years ago), something happened that caused the remaining dinosaurs to die out. No one knows for sure what the event was. Many scientists believe an asteroid hit Earth, causing harsh atmospheric conditions that led to the dinosaurs' extinction. However, very few species of dinosaurs remained by the end of the Cretaceous. Scientists will probably never know what actually happened to the dinosaurs. Most think they are still here as birds. This is not difficult to imagine when you look closely at the characteristics of birds. It is comforting to imagine that these fantastic and lively creatures are still among us, only much smaller.

Recent discoveries of many species of feathered dinosaurs in China have really supported this idea. Most scientists know agree that birds are very closely related to dinosaurs, if not actually dinosaurs themselves. The so-called "missing links" between dinosaurs and birds have been found.

Leslie Hutchinson

Bibliography

Carroll, R. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company, 1988.

Czerkas, S., and E. Olsen. Dinosaurs: Past and Present, vols. I and II. Los Angeles: Natural History Museum of Los Angeles County, 1987.

Farlow, J. O., and M. K. Brett-Surman, eds. The Complete Dinosaur. Indiana University Press, 1997.

Horner, John R. Digging Dinosaurs. New York: Workman Publishing, 1988.

Norman, D. Encyclopedia of Dinosaurs. New York: Crescent Books, 1985.

Sloan, Christopher. Feathered Dinosaurs. Washington, D.C.: National Geographic, 2000.

dinosaur Any of a large number of reptiles that lived during the Mesozoic era, between 225 and 65 million years ago. They appeared during the Triassic period, survived the Jurassic, and became extinct at the end of the Cretaceous. There were two orders: Saurischia (‘lizard hips’), included the bipedal carnivores and the giant herbivores; the Ornithiscia (‘bird hips’) were smaller herbivores. Their posture, with limbs vertically beneath the body, distinguish them from other reptiles. Many theories are advanced to account for their extinction. It is possible that, as the climate changed, they were incapable of swift adaptation. A more catastrophic theory is that they died because of the devastating atmospheric effects from the impact of a large meteor. See also apatosaur; Diplodocus; tyrannosaurus

http://www.enchantedlearning.com/subjects/dinosaurs; http://www.nhm.ac.uk; http://www.nmnh.si.edu/paleo/dino

dinosaur An extinct terrestrial reptile belonging to a group that constituted the dominant land animals of the Jurassic and Cretaceous periods, 190–65 million years ago. There were two orders. The Ornithischia were typically quadrupedal herbivores, many with heavily armoured bodies, and included Stegosaurus, Triceratops, and Iguanodon. They were all characterized by birdlike pelvic girdles. The Saurischia included many bipedal carnivorous forms, such as Tyrannosaurus (the largest known carnivore), and some quadrupedal herbivorous forms, such as Apatosaurus (Brontosaurus) and Diplodocus. They all had lizard-like pelvic girdles. Many of the herbivorous dinosaurs were amphibious or semiaquatic.

dinosaur a fossil reptile of the Mesozoic era, often reaching an enormous size; in extended usage, a person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances.

Dinosaurs were all extinct by the end of the Cretaceous period (65 million years ago), the most popular theory being that the extinctions were in fact the result of the impact of a large meteorite.

The word is recorded from the mid 19th century, and comes from modern Latin dinosaurus from Greek deinos ‘terrible’ + sauros ‘lizard’. It was coined by the English anatomist and palaeontologist Richard Owen (1804–92).

dinosaurs Literally the name means ‘terrible lizards’, but in fact the dinosaurs were not lizards. They were diapsid reptiles whose closest living relatives are the crocodilians and birds. Dinosaurs first appeared in the Middle Jurassic and produced an astonishing array of different types and sizes before becoming extinct at the end of the Cretaceous. The two groups of dinosaurs, Saurischia and Ornithischia, are not usually thought to be more closely related to each other than to other archosaurs, so the concept of ‘dinosaur’ is a heterogeneous one.

di·no·saur / ˈdīnəˌsôr/ • n. 1. a fossil reptile of the Mesozoic era, often reaching an enormous size. 2. a person or thing that is outdated or has become obsolete because of failure to adapt to changing circumstances. DERIVATIVES: di·no·sau·ri·an / ˌdīnəˈsôrēən/ adj. & n. ORIGIN: mid 19th cent.: from modern Latin dinosaurus, from Greek deinos ‘terrible’ + sauros ‘lizard.’

dinosaur •Nassau • hacksaw • heartsore •bedsore • Ensor • fretsaw • chainsaw •Esau, seesaw •jigsaw •ripsaw, whipsaw •eyesore • Warsaw • bowsaw •footsore • Luxor • plesiosaur •stegosaur • Arkansas • Chickasaw •dinosaur • brontosaur