Dinosaurs fascinate more people than any other reptile group. The achievement of great size and diversity and the long domination of the earth by dinosaurs form a large part of this fascination. As is true for the great gray apparition in Mozart's Don Giovanni or the Frankenstein monster, people love things that are terrible and wonderful at the same time. In fact, the name dinosaur comes from roots meaning "terrible lizard."

Unique evolutionary features are evident in the hands of some dinosaurs. The joint between the thumb and the palm was structured so that when the hand was closed, the thumb bent toward the palm, indicating that the hand was used for grasping and holding objects. In effect, the bipedal gait in early archosaurs set the hands free for other functions in both pterosaurs and dinosaurs. In the dinosaur pelvic girdle (hip girdle), the acetabulum (hip socket) either was open or was composed of cartilage in primitive forms, probably increasing the rate and range of leg motion from front to back. Also, one or more vertebrae were incorporated into the pelvic girdle, giving it more strength for vigorous locomotion.

Three main dinosaur clades are recognized by most paleontologists: the Theropoda, which were bipedal and mainly carnivorous; the Sauropodomorpha, which had long necks and were herbivorous; and the Ornithischia, which encompassed a diverse assemblage of beaked herbivores. The earliest members of all three groups are first known at about the same time in the late Triassic. During the early part of the late Triassic, dinosaurs were an important emerging group, but they actually were dominated by pseudosuchian ar-chosaurs and advanced synapsids (ancestral mammals with reptilian characteristics). It was only after the great extinction that took place at the end of the Triassic that dinosaurs became the dominant group of large terrestrial vertebrates. This dominance lasted until dinosaurs became extinct at the end of the Cretaceous.

Theropods had more primitive dinosaur characters than the other two groups. For instance, theropods retained their bladelike, serrated teeth; by this feature it is known that they were carnivores. All known theropods were bipedal, and many retained grasping hands. The most primitive known theropods were the herrerasaurids of the late Triassic of Argentina, Brazil, and North America. One specialization that these animals shared with later theropods was a joint in the lower jaw between the tooth-bearing and non-tooth-bearing portions. This innovation probably counteracted the stress associated with biting relatively large prey.

The two main clades of Theropoda are the Neotheropoda and the Coelurosauria. An early neotheropod, Coelophysis of the late Triassic, was gracile in build and had a kink in the upper jaw believed to be an adaption for holding on to small prey. Ceratosaurus of the late Jurassic was a robust form, with hornlike knobs on top of the front of its skull. Spinosaurus of the Cretaceous had a very long snout and large, conical (rather than bladelike) teeth; the teeth at the end of the snout were larger than the rest. It has been suggested that Spinosaurus ate large fish, because some fossil localities where spinosaurs were collected contained abundant fish remains. Allosaurus of the Jurassic, a huge bipedal carnivore with a compressed head, is featured in many museums and represents a significant branch of the theropod stem called carnosaurs.

The Coelurosauria, the other main branch of the thero-pod stem, differed from the neotheropods in numerous ways. The brain cavity was relatively larger, and the hands were more slender. The tail tended to be very stiff. Elements thought to be homologous to the feathers of birds, called protofeathers by some researchers, have been found in both primitive and advanced coelurosaurs, and it has been suggested that all primitive coelurosaurs may have had them. Several Jurassic and Cretaceous coelurosaurs, both small and large, were not ancestral to more advanced clades, but a clade called maniraptors became increasingly specialized.

Maniraptors are characterized by the development of a secondary palate and several changes in the structure of the brain case. They also had very slender hands and fewer tail vertebrae. Birds are a by-product of maniraptor evolution. There are several important maniraptor groups; among them, ornithomimosaurs, such as Struthiomimus, are not bird ancestors, but they show convergence with both modern ostriches and other flightless birds. Early examples had tiny teeth, but the teeth were lost in later forms. The fingers of the hands formed a hook-and-clamp structure that may have been used to grasp branches as the animals searched for food.

Tyrannosaurids are well known in the form of Tyran-nosaurus rex, popularly known as T. rex, as well as other large carnivores. It should be pointed out that tyran-nosaurids evolved independently of the narrow-faced Al-losaurus and its kin. Tyarannosaurids are characterized by massive, rather than narrow heads; nipping teeth (incisors) at the front of the jaws; thickened, rather than compressed lateral teeth on the sides of the jaws; and very minimal fore-limbs bearing only two claws. These animals were fast runners for their size and by means of their jaws alone could kill their prey and render it into portions small enough to be swallowed. Early Cretaceous tyrannnosaurids were only about 10 ft (3 m) long, but later ones such as Tyrannosaurus rex were 40-50 ft (12.2-15.2 m) long with a weight of up to seven tons or more.

Other maniraptors include such groups as the ovirap-torosaurs, of which Oviraptor is a well-known genus. Many species of oviraptosaurs are characterized by ornate crests and are thought to have brooded their nests in the manner of modern birds. Dromaeosaurids include the familiar genera Deinonychus, Velociraptor, and the larger Utahraptor. These animals had long, grasping forelimbs and a large, retractable, curved claw on the second digit of the foot. Dro-maeosaurids, like birds, had a backward-directed pubis and are thought by some researchers to be near the stem of bird evolution.

Found among the Sauropodomorpha are the largest land animals that ever lived; some reached a length of about 100 ft (30.5 m). All sauropods had long necks and small heads. Some of the primitive sauropodomorphs, known as prosauropods (such as Plateosaurus), spent some time on all fours, though they still could grasp objects with the hand. At the end of the Triassic, prosauropods were common large land herbivores. Long necks gave sauropodomorphs access to tree leaves, and their large body size enabled them to have a larger digestive system to process vegetation and served as a defense against small predators.

Sauropodomorphs called the Neosauropoda became a diverse group in the Jurassic and Cretaceous. Neosauropods

An adult Diplodocus was a 89.6-ft (27-m) long, lightly built sauropod, characteristic of the diplodocids. (Illustration by Barbara Duperron)

were characterized by columnar legs, wide hips, and specialized teeth. The two main neosauropod clades are the Diplodocoidea and the Macronaria. Diplodocoideans had thin, pencil-shaped teeth that supposedly were used for harvesting leaves and needles from the branches of trees. The skull was long and sloping, and the nostrils were positioned behind the eyes. The tails of diplodocoideans were very long and whiplike. Familiar examples are Apatosaurus (long called Brontosaurus) and Diplodocus. Shorter-necked forms include Amargasaurus of the early Cretaceous of Argentina.

In the Macronaria ("large nostrils") clade are sauropods with enormous nostrils that are larger than the eye sockets. Quite a few members of this group had spatula-shaped teeth as well. Brachiosaurs, including the well-known genus Bra-chiosaurus, make up a clade that had forelimbs much longer than the hind limbs and a very long neck, a condition that brings to mind modern giraffes. The combination of long front legs and a long neck allowed these animals to feed at the very top of trees. Sauroposeidon of the Cretaceous of Oklahoma is estimated to have been 80 ft (24.4) long, about 40 ft (12.2) of which consisted of the neck. Titanosaurs were giant macronarian sauropodomorphs that form a clade apart from the brachiosaurs. It has been discovered that advanced forms in this group developed pencil-like teeth similar to those in the diplodocids.

The Ornithischia are the third and last of the three main dinosaur clades. These animals were mainly armored, beaked, four-footed herbivores, but some of the primitive examples were at least partially bipedal. The three major clades presently recognized within the Ornithischia are the Thyreophora (armored dinosaurs), Ornithopoda (beaked, billed, and crested dinosaurs), and Marginocephalia (helmeted and horned dinosaurs). In the diversification of the thyreophorans, one finds that the modification of armored scutes into other defensive structures is a central tendency. The relatively unarmored Scutellosaurus of the early Jurassic, thought to be a basal member of the group, had a row of bony dermal plates from the neck to the tail. It is thought that this feature might have protected it against the small predators of the time but probably not against the large predators that appeared later in the Jurassic. Indeed, later forms modified this dermal armor into plates, spikes, and tail armor. Stegosaurs of the late Jurassic were true armored dinosaurs, in that they had compressed plates, conical spines, or armor of intermediate shape along the middle of the back and tails that were modified as spiked clubs. Some stegosaurs had shoulder spikes, and others had masses of knobs on the throat. Ankylosaurs had an armadillo-like dermal armor fused to the head and protecting most of the body. One group of ankylosaurs had club tails.

Ornithopods may be distinguished from other Ornithis-chia in that the tooth row in the front portion of the upper jaw (premaxilla) is more depressed than those in the remaining upper jaw. Evolutionary tendencies in this group included an increase in size and changes in the joints of the jaw and teeth that led to a grinding mode of chewing. Iguanodon, an early ornithopod first discovered from the early Cretaceous of England, was at one time thought to be a giant lizard, because it had leaflike serrated teeth like the modern iguana lizard. The duckbill dinosaurs of the late Cretaceous, with terminally expanded snouts, have been exceptionally well studied, and some are known literally from the cradle (nest and eggs) to the grave. Each species of lambeosaurs had a unique crest shape and sound-producing tubes within the skull. These features provided both visual and vocal signals that indicate complex social behavior in these animals.

The last group of Ornithischia are the marginocephalians (helmeted and horned dinosaurs), represented by both bipedal and four-footed forms. The main character of this clade was a ridge or shelf of bone that overlapped the back of the skull. This group is divided into two clades, the pachycephalosaurs and the ceratopsians. Pachycephalosaurs were bipedal forms with a thick helmet of bone over the brain case; in the most derived forms, the helmet formed a thickened dome. Many scientists believe that the dome was used for butting clashes, as occur among bighorn sheep today. Others think that because the dome was provided with a large number of blood vessels, that it might have been important in temperature regulation.

Ceratopsians had large beaks at the end of the snout. A primitive ceratopsian, Psittacosaurus, of the early Cretaceous had this deep beak but was bipedal and lacked the frill of more advanced forms. Advanced forms, called neoceratopsians, had a shelf modified as a frill at the back of the skull where the jaw muscles were attached. Neoceratopsians had rows of teeth packed together in such a way as to provide a continuous cutting surface, but it is not known what kind of plants these animals fed on. The North American giant ceratopsians had greatly elaborated horns on the skull.

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