ancestral, this vestigial digit made a heroic comeback and became large again. The more likely explanation is that the common ancestor to both prosauropods and sauropods had this large digit V as a plesiomorphic trait. It then became reduced in the prosauropod lineage after it diverged from the sauropod lineage, which retained the digit during its different evolutionary journey.
The main problem with resolving evolutionary relationships between prosauropods and sauropods was the previous lack of sauropod fossils from the Late Triassic. The gap between the appearance of the first prosauropod (229 Ma) and the first sauro-pod (208 Ma) was a significant one, and meant that sauropods would have had a ghost lineage of nearly 20 million years. However, the identification of Blikano-saurus, Antetonitrus, and some remains of "Euskelosaurus"as Late Triassic sauropods and sauropod tracks, recently identified in Upper Triassic rocks from New Mexico, has helped to close this gap.
Predator-prey interactions probably contributed to the coincidence of the largest land herbivores overlapping in time with the largest land carnivores that ever lived (Chapter 9). In modern terrestrial animals today, large size and herding behavior are still deterrents to predation. Trends toward gigantism that ecologically separated the sauropods from other potential large herbivore competitors, such as ornithopods (Chapter 11), stegosaurs and ankylosaurs (Chapter 12), and ceratopsians (Chapter 13), also cannot be discounted. Interestingly, prosauropods had almost no overlap with these latter herbivorous clades of dinosaurs, which along with sauropods also may have filled the various former niches of prosauropods. Only a few sauropods were relatively small compared to other herbivorous dinosaurs (Fig. 10.9).
Another hypothesis for sauropod gigantism is that sauropods became large so that their bodies could accommodate an alimentary canal long enough to digest massive amounts of food with low nutritional quality. Moreover, their increased dimensions, as a result of this adaptation, could have made more food sources easily accessible. Longer necks meant more reach, for example, whether in a horizontal or vertical plane. Yet another hypothesis is that the extremely long necks, tails, and legs of some sauropods served as "heat vents," meaning that these body parts conveyed the excess body heat generated by movement and digestion away from the torso. This is a controversial interpretation of sauropod physiology discussed earlier (Chapter 8). Of course, a synthesis viewpoint is that an interaction of numerous factors, such as changing climates, habitat alterations, and biological factors, would have provided for confluent situations that favored the selection of larger, longer, and heavier sauropods. Clearly this extreme sort of selection has not happened to terrestrial vertebrates since the Mesozoic, thus paleontologists can confidently say that the circumstances leading to this type of evolution must have been unique.
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