Between 65 and 200 million years ago, the largest herbivores — in fact, the largest land animals ever to have lived — were the giant, long-necked, 4-legged sauropods. As a group, they survived for some 50 million years. They evolved in the Late Triassic or Early Jurassic, reached their peak in the Late Jurassic, and became extinct by the end of the Cretaceous. Even in the early stages of their evolution, most sauropods were huge — well over 50 ft/15.2 m long.
The body plan of all sauropods was structurally similar. There was a small head on top of an extra-long neck; a long, deep body to accommodate an enormous gut; thick, pillarlike legs with 5-toed, spreading feet; and a long, thick tail tapering to a whiplash.
Two special adaptations of the skeleton were evident, even in the earliest sauropods. First, great cavities were hollowed out of the vertebrae; this helped to lighten the load of the animal considerably, while still retaining the structural strength of its skeleton. This hollowing-out became more extreme as the sauropods evolved. Bone was developed only along the lines of stress (comparable to the steel struts in a crane jib).
The second special feature was that the massive hip girdle was firmly fused to the backbone by 4 (and in later types, .5) sacral vertebrae, and formed a solid support for the heavy body and tail.
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