Basal Sauropodomorphs Prosauropods

Despite their similarity in appearance to the Sauropoda (Sect. 10.4) which radiated much later, in the Upper Jurassic and Cretaceous periods, the sauro-podomorphs of the Upper Triassic were not related to them (Sect. 9.1). The apparent similarity must have resulted from parallel evolution which took place in similar environments at different periods of earth history. Plateosaurus (Figs. 65, 83e) is probably the best known of the prosauropods. Many well-preserved skeletons have been excavated from Upper Triassic sandstone beds throughout western Europe. The presence and accumulation of fossils can be explained by one of two hypotheses. According to the first, the animals travelled together in large herds from one feeding ground to another, and occasionally a whole herd perished as the result of some misfortune such as a landslide, falling over the edge of a cliff, or becoming trapped in a marsh. Alternatively, individual corpses might have been washed away and piled up together by periodic flash floods in a semi-arid environment.

Plateosaurus was much larger than its plateosaurid relative Massospondylus (Fig. 83f). It probably walked on all fours for most of the time, as already noted, feeding on the foliage of conifers, cycads and other trees and shrubs that flourished during the Late Triassic (Sect. 9.1). Massospondylus had especially large hands which would have been useful for grasping food plants. In contrast, the tiny Mussaurus (mouse lizard) from Patagonia was entirely quadrupedal, although it also belonged to the family Plateosauridae. It was originally believed not to have exceeded 20 cm in length, but the specimens from which the genus was described were very young, and palaeontologists now think that the adults could have reached 3 m.

Another family of basal sauropodomorphs, the Melanorosauridae, contains the largest of the prosauropods. One of these, Riojasaurus (Fig. 106) from South America, was up to 11 m long and much more heavily built than were any of the plateosaurs. Other large melanosaurids that lived in southern Africa at about the same time were Roccosaurus and Thotobolosaurus of the Upper Triassic, and Vulcanodon from the Lower Jurassic. These animals had to walk on four legs in order to support their weight, and their skeletons were modified accordingly. Vulcanodon from southern Africa showed a mixture of prosauropod and sauro-

■ Fig. 106. Riojasaurus (Melanosauridae; Upper Triassic-Lower Jurassic; length ca. 11 m). (Based on Palmer 1999)

pod characters and could have linked the two infraorders (Palmer 1999). Benton (2004) included it with the Sauropoda. The ancestry of the sauropods is still obscure, although the traditional assumption that they evolved from bipedal prosauropods was questioned as long ago as 1965 by Charig, Attridge and Crompton, to whose paper the reader is referred for a review of earlier opinions on the subject.

In general, members of the family Plateosauridae were mostly relatively agile bipedal and quadrupedal reptiles that could browse on tall trees with the aid of their long necks and hind legs. Their teeth were coarsely serrated and shredded plant fibres which were then pulped or stirred by the gastric mill. They probably lived in herds that afforded some protection from predatory theropod dinosaurs (Chap. 11) and defended themselves with the large claws on their hands. The melanorosaurids, in contrast, may have been relatively invulnerable to predators when full grown on account of their large size. They had small heads on long, flexible necks so that they could feed at ground level and also reach up into trees. The suggestion has been made that, as food plants became scarce on the ground when the world dried up at the end of the Trias-sic, long necks were advantageously selected (Brett-Surman 2000).

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