CENOZOIC DINOSAURS IN SOUTH AMERICA REVISITED

Of course there were Cenozoic dinosaurs (theropods) in South America, phorhusrhacids among others, but that is not the subject here. Why did anyone think there were Cenozoic (non­avian) theropods in South America? Because of a misinterpretation of Ameghino's belief that derived mammals lived along with dinosaurs in Late Cretaceous Argentina. But also because isolated 'theropod' teeth were found associated with derived (Eocene) mammal fossils. These turned out to be the teeth of sebecosuchian crocodylomorphs, probably Sebecus icaeorhinus. This is a small animal, skull length c. 45 cm, compared to large theropods. More recently discovered sebecosuchians were substantially larger, Barinasuchus arveloi had an (estimated) skull length of c. 88 cm, similar to that of Daspletosaurus (100 cm). These crocodylomorphs are generally believed to have been terrestrial animals, presumably preying on large mammals (as did the Australian megalania). Thus, although there were no large non-avian theropods in Cenozoic South America, there were crocodylomorphs that seem to have been ecological vicars of large theropods. The reconstruction of terrestrial trophic networks of large terrestrial tetrapods after the Cretaceous-Paleogene extinctions seems to have been slower than often supposed. At (or near) the Cretaceous-Paleogene boundary, large herbivores turned over from archosaurs to mammals, but turnover of large carnivores was slower and varied by continent. In South America, dinosaur-size crocodylomorphs lived as late as the Miocene, in Australia a giant goanna persisted into the Late Pleistocene. Thus modern terrestrial ecosystems do not, trophically, reflect those of even the Early Neogene in some southern continents. Sebecosuchians, at least in South America, seem to have been unaffected by the Cretaceous-Paleogene extinctions. These forms also 'resurrected', at least superficially, the bauplans of Early and Middle Triassic archosaurs, the erythrosuchians and rauisuchians.