Sauropodomorpha consists of the great herbivorous saurischian quadrupeds Sauropoda, and an early offshoot, Prosauropoda. Prosauropods were primitive large dinosaurs, appearing in the Late Triassic at the dawn of Dinosauria, and surviving through the Early Jurassic. Initially thought to be the ancestors of Sauropoda, they are now considered to represent an early sau-rischian radiation, representing the world's first high-browsing herbivores.
Sauropoda were the largest land animals ever to walk the Earth, reaching 40 m from the tip of the snout to the tip of the tail. These obligate herbivorous quadrupeds were highly evolved, with many biomechanical adaptations for large size and weight, including four pillar-like limbs and a massive pelvis, a tendency to lighten bones not immediately involved in support functions, and a complex girder-like neck design, tipped by a small skull, to maximize leverage and lightness. Among the many striking features in the design of sauropod bones are the presence of pleurocoels, hollow spaces suggestive of avian-style air sacs. The likely presence of air sacs, as well as the inefficiency, in an animal of such great size, of mammalian-style bellows breathing, suggest that sauropods may have used avian-style unidirectional breathing.
The skulls of sauropods were relatively small, and the group showed a general tendency toward migrating the nostrils or nares to the top of the skull. Dentition varied, from simple leaf-shaped teeth to pencil-like teeth restricted to the front of the mouth. Sauropods in general lack the chewing adaptations present in ornithischians (particularly Genasauria), and gastroliths preserved in their abdominal cavities attest to grinding in the gizzard and the likely use of bacterial fermentation.
Sauropods appear to have been social animals, particularly as reflected in sauropod bonebeds and in trackways. The trackways clearly indicate that sauropods did not drag their tails. Sauropod gregariousness is also reflected in the recent discoveries of extremely large sauropod nesting grounds. Sauropod lifespans, once thought to be in the hundreds of years, are now thought to have been around 100 years, with extremely rapid growth of juveniles. The large number of eggs and babies associated with sauropod nesting grounds implies that these dinosaurs may have been r-strategists.
Sauropod defense was likely accomplished mainly by size, with perhaps an assist from the whip-like tail and the broad, trenchant claw on the forefoot.
The immense size of sauropods makes analogizing them with living terrestrial vertebrates extremely difficult. Although not likely possessed of a "warm-blooded" metabolism, at least as adults, they nonetheless must have consumed copious quantities of food, and must have required virtually incessant food consumption through the comparatively small mouth. In good health and with its full complement of individuals, it is not hard to imagine wholesale defoliation of a region by a herd. This in turn leads to at least the possibility that sauropods were often on the go, searching out new foliage to consume.
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