Summary

This chapter described the continuing evolution of predatory dinosaurs during the Middle and Late Jurassic Epochs and the adaptations that led to their continued success.

1. Theropods, the predatory dinosaurs, were, like sauropods, saurischians, with a unique dinosaurian hip structure that enabled mobility and the evolution of large body sizes.

2. Theropods ranged in size from small, chicken-sized taxa to gigantic carnivores that measured upward of 50 feet (15 m) long.

3. Two theropod clades predominated during the Middle and Late Jurassic Epochs. The clade Ceratosauria includes the most primitive theropods. The basal Tetanurae includes the least derived nonceratosaurian theropods.

4. Basal tetanurans that lived during the Middle and Late Jurassic are divided into two subgroups: Spinosauridae, including megalosaurs, and Avetheropoda, including carnosaurs and coelurosaurs.

5. The best known carnosaur from the Late Jurassic is Allosau-rus, which weighed up to three tons and measured about 40 feet (12 m) long.

6. The existence of large predators such as Allosaurus and the later tyrannosaurs, spinosaurs, and carcharodontosaurs is evidence that prey animals such as iguanodonts, hadrosaurs, sauropods, and horned dinosaurs probably lived in great numbers and required predators to adapt highly specialized weapons and techniques to hunt them down.

7. The senses of vision, smell, and hearing were generally more acute in theropods than in prey animals; this helped to make the theropods effective predators.

8. The speeds of theropods are best understood by modeling their anatomy and musculature based on knowledge of extant animals. The top speed of Tyrannosaurus was probably between 10 and 25 mph (6 to 16 km/hr).

9. Three killing tactics used by theropods were grapple-and-slash, grapple-and-bite, and pursuit-and-bite techniques.

10. Archaeopteryx is the earliest known bird and represents a transitional stage in the evolution of birds from other dinosaurs. Archaeopteryx is not likely directly related to modern birds.

Armored and Plated Dinosaurs: Ornithischian Innovations

Ornithischian dinosaurs made up the other of the two major clades of dinosaurs, and that clade's members are united by a generalized morphology of the pelvis known as a "bird-like," or ornithischian, hip. As explained earlier, the group of theropods that includes birds and their ancestors independently evolved a very similar hip structure, although well after the ornithischian dinosaurs.

Ornithischians included a variety of bipedal and quadrupedal ^

herbivores and were highly successful and specialized plant eaters; many of them grew to large size and lived in herds. The most common groups of ornithischian dinosaurs were the Stegosauria, or plated dinosaurs (Middle Jurassic to Early Cretaceous); the Ankylo-sauria, or armored dinosaurs (Middle Jurassic to Late Cretaceous); the Ornithopoda (Middle Jurassic to Late Cretaceous), including the Iguanodontia and the Hadrosauridae, or duck-billed dinosaurs (Late Cretaceous); the Pachycephalosauria, or bone-headed dinosaurs (Late Cretaceous); and the Ceratopsia, or horned dinosaurs (Late Jurassic to Late Cretaceous). Of these, the stegosaurs and orni-thopods predominated in the Middle and Late Jurassic Epochs, the subject of Time of the Giants, although rare ankylosaurs and basal ceratopsians were around at that time, too.

This chapter describes the rise and diversification of the stegosaurs and ankylosaurs, two of the most unusual clades of dinosaurs due to their specialized plate and armor structures, adaptations for

eating, and defensive weaponry. The remaining groups of diverse and populous ornithischians are fully described in Last of the Dinosaurs.

evolution of the ornithischians

Fossils of ornithischians are rare until the Middle Jurassic. Their earliest representatives lived among the first saurischians of the Late Triassic but make only the scarcest of appearances in the fossil record. These early ornithischians were seemingly overshadowed by larger and more numerous saurischian meat eaters and plant eaters. Somehow, however, the small, herbivorous ornithischians persisted. They survived into the Jurassic Period and became the seeds of many great lines of later innovative and successful plant-eating dinosaurs.

Several anatomical features unite the ornithischians. Aside from having a similar hip structure, all ornithischians had a toothless beak on the upper jaw and an unusual, scooplike predentary bone at the tip of the lower jaw. Except for the most basal taxa, all ornith-ischians had cheeks to help prevent food from falling out of the mouth as the food was chewed. When combined with leaf-shaped cheek teeth and a variety of highly specialized dental adaptations for grinding vegetation, these features of the mouth eventually elevated the ornithischians to the summit of plant-eating effectiveness in the dinosaur world. Unlike the saurischians, whose skulls were lightened by many enlarged fenestrae or "windows," and whose vertebrae were sculpted for strength and lightness, the ornithischians' skulls had smaller fenestrae (in some species, some or all of the fenestrae were lost entirely during the course of evolution); additionally, the skeletons of ornithischians were generally heavier and more robust in comparison to their body mass. The ornithischian skull was sometimes armored, and all ornithischians had a characteristic bony knob or protuberance in the area of the eyelid.

The hip region of the ornithischian backbone had five or more fused sacral vertebrae for added strength. The fifth toe of the ornithischian foot was greatly reduced, as in sauropods and thero-pods, but this had little effect on the ornithischians' ability to walk or run. Most ornithischians were good runners, with the exception, perhaps, of the wide-bodied ankylosaurs.

The earliest members of the Ornithischia are shrouded in mystery due to the scarcity of their fossil remains. Some are known only from teeth. The best-known basal ornithischians are Lesothosaurus (Early Jurassic, Lesotho); Pisanosaurus (Late Triassic, Argentina); Heterodontosaurus (Early Jurassic, South Africa); Stormbergia (Early Jurassic, South Africa and Lesotho); and Eocursor (Late Triassic, South Africa). Uniting these early dinosaurs with the ornithischians were their possession of the classic ornithischian pelvis; leaf-shaped dentition in the cheek areas of the jaws; a bony beak (albeit a very small one, restricted only to the tip of the jaw); and a predentary bone. All three were small animals, about three feet (0.9 m) long. Paleontologist Michael Benton has pointed out that tooth wear seen in the jaw of Lesothosaurus suggests that the animal used an up-and-down chopping motion but had not yet adapted the backward-and-forward-plus-sideways jaw mobility that characterizes the chewing motion seen in later ornithischians. The mere evidence of chewing of any sort, however, establishes that even the earliest ornithischians were developing adaptations for processing food in the mouth that differed greatly from the feeding strategies seen in the sauropodomorphs. Improvements to the ornithischian dental battery are seen in stegosaurs and ankylosaurs but were carried to their greatest lengths in other lines of later ornithischians, such as the duck-billed and horned dinosaurs.

Stegosaurs and ankylosaurs belong to a larger clade called Thyreophora, or "shield bearers." All members of this group were quadrupedal, had some form of body armor or plating, and were widely distributed geographically. The armor plating ranged from small bony nodes to large plates and spikes that were embedded in the skin and ran in rows along the back and sides; some even had armor on their faces, arms, and legs.

Thyreophorans shared a common ancestor. Some basal members of the clade were neither stegosaurs nor ankylosaurs. The three best-understood basal thyreophorans are Scuttellosaurus (Early Jurassic, Arizona); Emausaurus (Early Jurassic, Germany); and Scelidosaurus (Early Jurassic, England). All three were small to medium-sized herbivores that measured from 3.5 to 13.5 feet (1 to 4 m) long and were primarily quadrupedal.

Of these basal thyreophorans, spectacularly well-preserved specimens of Scelidosaurus with armor in situ (found intact in its original place of deposition) provide the best evidence of the placement of bony plating on the body of early ornithischians. The armor of Scelidosaurus consisted largely of small oval nodes, or scutes, made of bone. Some smaller plates were mere knobs, while the largest were round, with stout ridges. Plates were arranged in several rows along the spine and on the sides of the animal. The tail had four rows along its lateral and dorsal surfaces. The base of the neck was protected by triangular-shaped plates. Small plates along the spine were firmly attached to the skeleton through ligaments fastened to the vertebrae. Although many fossil scutes of various sizes have been associated with the remains of Scutellosaurus and Emausau--rus, the jumbled condition of the remains of those dinosaurs makes it difficult to ascertain the pattern of the armor; as in all later thy-reophorans, however, it likely was arranged in rows that paralleled the length of the body.

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