dinosaurs

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.