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Dinosaurs: Evolution and Diversification

As currently known, all dinosaurs belong to one of two major groups, the saurischians and the ornithischians. From the onset of their long evolutionary history, ornithischians and saurischians already show the features characteristic of these groups. Ornithischians, like the Edmontosaurus, Triceratops, and Stegosaurus pictured above, comprise a series of plant-eating dinosaurs such as the armored stegosaurs and ankylosaurs, the duckbills and their kin, and the horned and dome-headed dinosaurs. Saurischians include the meat-eating theropods (the renowned Tyrannosaurus and Velociraptor among them) and the plant-eating sauropodomorphs (such as the gigantic Apatosaurus).

Among the ornithischians (the main dinosaurian group that includes all non-saurischian forms), the 228-million-year-old small Pisanosaurus of western Argentina is usually accepted as the group's earliest known member. The fact that Pisanosaurus walked on two legs and ate plants suggests that the common ancestor of all ornithischians was also a bipedal herbivore. This early dinosaur's anatomy further suggests that all ornithischians evolved from an ancestor whose pubic bone pointed backward.

Familiar among ornithischians are the thyreophorans-the group that includes the stegosaurs and ankylosaurs. All members of this group evolved from an ancestor having armor comprising bony scutes and plates that covered most of the entire surface of the body. The earliest known thyreophorans are primitive forms dating to the Early Jurassic, although the divergence of the group is likely to be much older. The best-known thyreophorans, however, are the stegosaurs and ankylosaurs of the Late Jurassic through Late Cretaceous times. Stegosaurs evolved small, narrow skulls, hindlimbs longer than forelimbs, two characteristic rows of vertical dorsal plates, and sharp spikes that made their tails formidable weapons (a skeleton of the 150-million-year-old Stegosaurus is housed at the Dinosaur Institute). Armor in the ankylosaurs evolved into a dense shield that covered much of the body, the skull incorporated extra bones that gave it a highly compact constitution, and, in the most specialized forms such as the 65-million-year-old Ankylosaurus, the tail developed into a stiffened structure terminating in an immense club.

Together, the dome-headed dinosaurs, or pachycephalosaurs, and horned dinosaurs, or ceratopsians, make up the ornithischianThe ornithischian family tree. group called marginocephalians. These dinosaurs are united by the presence of a bony shield that projects from the rear margin of the skull. Ceratopsians are further united by the presence of a wedge-like rostral bone covering the tip of the snout. Among the largest ceratopsians is the renowned three-horned Triceratops (fossils of 65-million-year-old pachycephalosaurs and Triceratops are housed in the Dinosaur Institute's collections).

The ornithischian group called ornithopods seems to be more closely related to the marginocephalians than to the thyreophorans. Ornithopods evolved sophisticated dental batteries that must have been highly efficient in grinding plant matter. The earliest records of this group date back to the Early Jurassic, continuing on through the end of the Mesozoic. Primitive ornithopods like the 190-million-year-old Heterodontosaurus were small and agile bipeds with prominent fangs in their lower jaws. Among the more advanced ornithopod groups, iguanodontids were generally larger and although bipedal, could have also walked on all fours. One well-known iguanodontid is the 150 million-year-old Camptosaurus (a mount of which can be seen at the museum). Later iguanodontids called hadrosaurs had a cosmopolitan distribution. These so-called "duckbilled" dinosaurs became particularly abundant in the latest Cretaceous ecosystems of Asia and North America. Some duck-bills, such as the lambeosaurines, developed extravagant headgears (as in the helmet-like crest of Corythosaurus, a specimen of which is also exhibited at this museum) that may have played a role in communication.

Within the saurischian group of dinosaurs (the main group that includes all non-ornithischian forms) are the sauropodomorphs, plant-eating dinosaurs that include a series of small-headed, long-armed dinosaurs usually called prosauropods and the colossal, long-necked sauropods, the largest terrestrial animals of all time. As in the case of other main dinosaur groups, the earliest known examples of both prosauropods and sauropods date back to the Late Triassic. With bodies significantly smaller than those of the sauropods, the prosauropods were able to walk on either two or four legs. The 200-million-year-old Plateosaurus is a well-known example of these dinosaurs. By Early Jurassic times, some 125 million years ago, sauropods were already diversifying globally into various subgroups, each distinguished by its own set of features. By the Late Jurassic, groups like the diplodocids (including the well-known Diplodocus, possessing a long "whiplash" tail that may have served as a weapon or to produce sound) and the brachiosaurids (including enormous forms with longer front than hind legs, such as Brachiosaurus) constituted the world's dominant herbivores. While the predominance of sauropods lessened in the Cretaceous as other kinds of plant-eating dinosaurs took over their niches, one sauropod group, the titanosaurs, impressively diversified and flourished, especially in South America and Europe.

Theropods constitute the second major saurischian group. The evolutionary history of these animals also dates back 230 million years, when dinosaurs first appear in the fossil record. As with other dinosaurian groups, the earliest theropods already exhibit features typical of this entire group - a movable joint at the center of the lower jaw, powerfully clawed hands, and sharp, knifelike teeth. All theropods descended from a bipedal dinosaur that had feet with three main forwardly pointing toes, of which the central one was the longest. Herrerasaurus, a 230 million-year old dinosaur from western Argentina, is generally accepted as one of the earliest theropods, although some researchers regard it as a more primitive saurischian outside the group.

Theropods break down into numerous subgroups, two main groups being the ceratosaurs and the tetanurans. Ceratosaurs, the most primitive of these assamblages, range in size from very small, long-necked forms such as 190-million-year-old Coelophysis of southwestern North America (several specimens of which are housed at the Dinosaur Institute), to the much larger and more advanced Ceratosaurus, from this continent's Late Jurassic. Some ceratosaurs are distinguished by skull ornamentation, such as the spectacular double crests of Dilophosaurus and the nasal horn of Ceratosaurus.

Most known theropods are tetanurans, a large group whose members all have hands with three or fewer fingers. Tetanurans subdivide into three basic groups, the spinosaurs, carnosaurs, and coelurosaurs. The Late Jurassic to Late Cretaceous spinosaurs, known to date from all continents except Antarctica, include apparently fish-eating forms like the huge 190-million-year-old, sail-backed Spinosaurus of Egypt and 127-million year old, long-snouted Baryonyx of England and Niger.
Carnosaurs first appeared in the fossil record during the Middle Jurassic, some 175-161 million years ago. These large-bodied theropods include such predaceous giants as Allosaurus (a skeleton of which is housed at the Dinosaur Institute), from the Late Jurassic of North America, and the even larger Carcharodontosaurus and Giganotosaurus, from the Late Cretaceous of Africa and South America, respectively.

Coelurosaurs comprise the most diverse theropods, this group including small forms like Juravenator, from the Late Jurassic of Germany, to the gigantic Tyrannosaurus (a number of specimens of which are in the Dinosaur Institute's collections), from Late Cretaceous North America. Included among the coelurosaurs are a number of atypically non-carnivorous theropods. Examples of these include the ornithomimosaurs, which superficially resembled ostriches, the therizinosaurs, rather bizarre forms having unusually short tails and long forearms, and the alvarezsaurids, with stout forelimbs resembling those of anteaters. Therizinosaurs and alvarezsaurids are usually classify within the maniraptorans, a coelurosaur group identified by a suite of features, most notably a wrist bone called the semilunate carpal that allowed the hand to be folded back against the body, a condition that, together with an elongated forelimb afforded enhanced reach. Some maniraptoran dinosaurs were remarkably birdlike-in fact, birds are considered to be a subgroup of maniraptorans. Some birdlike maniraptorans include the oviraptorids, odd-looking toothless forms such as Oviraptor. However, the more birdlike were the dromaeosaurids, sickle-clawed (and comparatively large-brained) hunters like Velociraptor, hailing from Late Cretaceous Mongolia. Throughout their long history, theropods developed dozens of highly specialized features, but perhaps none more striking than feathers, which, in time, helped them to become warm-blooded and airborne. Indeed, one coelurosaurian group - the above-mentioned maniraptorans - includes the only dinosaurs living today, the birds.