This chapter investigated the traits, lifestyles, and members of the sauropods that thrived during the Middle and Late Jurassic, the heyday of the largest herbivorous dinosaurs.
1. Sauropods were the tallest, heaviest, and longest animals to ever walk the Earth. They were members of the saurischian clade known as Sauropodomorpha, which also included the "Prosauropoda." "Prosauropods" and sauropods shared a common ancestor.
2. The earliest known sauropod is Antetonitrus (Late Triassic,
South Africa), dating from 220 million to 215 million years ag°.
3. The anatomical features of sauropods that made them a unique clade of dinosaurs revolved around two aspects of their lifestyle—a tendency toward gigantism and their vegetarian diet.
4. General anatomical features shared by all sauropods included long necks and tails, straight and strong limbs, four or more sacral vertebrae, and small skulls optimized for stripping leaves from plants.
5. Sauropods are divided into two main groups: Eusaurop-oda, somewhat primitive members from the Early to Late
Jurassic; and Neosauropoda, from the Middle Jurassic to Late Cretaceous.
6. Sauropods were medium- to high-browsing herbivores. Their teeth were able to pluck and strip leaves from trees. Digestion was aided by stomach stones for grinding plant material.
7. Metabolism is the combination of all biochemical processes that take place in an organism to keep it alive.
8. Thermoregulation in dinosaurs ranged from probable endo-thermy in birds and the theropods most closely related to them to homeothermy in sauropods and other gigantic species.
9. Homeothermy allowed sauropods and other kinds of large dinosaurs to maintain high body temperatures while still having lower, "cold-blooded" metabolic rates.
10. Mobility in sauropods was aided by sturdy limbs, having a center of gravity near the pelvic region, and the lightness of their vertebral column due to the pneumaticity of the vertebrae.
11. Sauropods laid moderately large, round eggs in circular pits, semicircular arcs, and parallel rows, a factor seemingly governed by the size of the mother.
Theropod Diversity: Giant Predatory Dinosaurs
Theropods—the clade of carnivorous dinosaurs (and a number of later taxa that converted to herbivory)—first appeared in the Late Triassic. Their origins and early diversification are recounted in Dawn of the Dinosaur Age. Like sauropods, theropods were sauris-chians, with a unique dinosaurian hip structure that encouraged mobility and the evolution of large body sizes. This is not to say that all theropods were giants, or that all giant dinosaurs were sauris-chians, however; the theropods also included some of the smallest known non-avian dinosaurs, as well as birds.
As with sauropods, the Middle and Late Jurassic were a time of increased diversity in theropod evolution. As sauropods grew to larger and larger sizes, dominating the world of herbivorous vertebrates, so, too, did theropods become increasingly specialized in their carnivorous adaptations, size, range, and weaponry. Although the largest of the theropods are generally associated with the Cretaceous Period, some theropods approaching the size of tyrannosaurs began to appear during the Late Jurassic. These included some supersized specimens of Allosaurus (Late Jurassic, western United States) and Sinraptor (Middle Jurassic, China), two enormous predators capable of attacking the largest plant-eating dinosaurs. At the other end of scale of theropod size, an equally important evolutionary trend took place in the Middle to Late Jurassic: adaptations in small theropods leading to the evolution of feathers and flight.
This chapter describes the continuing evolution of predatory dinosaurs during the Middle and Late Jurassic Epochs and adaptations that led to their continued success.
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