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Dinosaur Success in the Triassic: A Noncompetitive Ecological Model

The initial radiation of the dinosaurs in the Triassic period (about 200 million years ago) has been generally regarded as a result of successful competition with the previously dominant mammal-like reptiles. A detailed review of major terrestrial reptile faunas of the Permo-Triassic, including estimates of relative abundance, gives a different picture of the pattern of faunal replacements. Dinosaurs only appeared as dominant faunal elements in the latest Triassic after the disappearance of several groups of mammal-like reptiles, thecondontians (ancestors of dinosaurs and other archosaurs), and rhynchosaurs (medium-sized herbivores). The concepts of differential survival ("competitive") and opportunistic ecological replacement of higher taxonomic categories are contrasted (the latter involves chance radiation to fill adaptive zones that are already empty), and they are applied to the fossil record. There is no evidence that either thecodontians or dinosaur demonstrated their superiority over mammal-like reptiles in massive competitive take-overs. Thecodontians arose as medium-sized carnivores after the extinction of certain mammal-like reptiles (opportunism, latest Permian). Throughout most of the Triassic, the thecodontians shared carnivore adaptive zones with advanced mammal-like reptiles (cynodonts) until the later became extinct (random processes, early to late Triassic). Among herbivores, the dicynodont mammal-like reptiles were largely replaced by diademodontoid mammal-like reptiles and rhynchosaurs (differential survival, middle to late Triassic). These groups then became extinct and dinosaurs replaced them and radiated rapidly (opportunism, latest Triassic). The late Triassic extinctions may be linked with floral and climatic changes. Explanations of dinosaur success based on the competitive superiority of their thermoregulation or locomotory capability are unnecessary in this model.