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Crurotarsans were a group of archosaurs that dominated the planet for 50 million years after the Permian-Triassic extinction event. They were defined by their unique ankle structure and included many advanced cousins of modern-day crocodiles. They were often confused with dinosaurs, but were a separate group. The Triassic was characterized by competition between crurotarsans, therapsids, and archosaurs, including the ancestors of dinosaurs. Crurotarsans produced the largest animals of the period, including predators up to 7m long.
Crurotarsans (“crossed ankles”) are one of two groups of archosaurs (a large clade of reptiles that includes birds, crocodiles and dinosaurs), the other being ornithodirans (birds and dinosaurs). The only living crurotarsans are crocodilians, but during the Early and Middle Triassic, between about 250 and 200 million years ago, crurotarsans were responsible for most reptile diversity. Crurotarsans have been around for nearly 250 million years in total, often dominating wetland ecosystems in the form of large crocodilians, especially after the extinction of the dinosaurs 65 million years ago. Crurotarsans are defined as any taxa more closely related to present-day crocodilians than present-day birds, and by their unique crocodile-like ankle structure.
Crurotarsans became the dominant vertebrate a few million years after the Permian-Triassic extinction event 251 million years ago, which was the largest mass extinction in the history of the planet. Next are the therapsids (ancestors of mammals), who had dominated for 25 million years before the mass extinction, and the pelycosaurs, primitive ancestors of the therapsids who in turn dominated for 40 million years. Crurotarsans would have ruled the planet for only 50 million years, after which all major species would have gone extinct at the end of the Triassic extinction, paving the way for the advent of the dinosaurs.
The crurotarsans included many advanced cousins of the modern-day crocodile, displaying a much greater diversity of forms and ecological roles. There were the rausuchians, crurotarsans with erect limbs, large (4-6 m) and predatory, carnivorous poposaurs, which resembled small dinosaurs, the small, agile, agile, spehnosuchians with erect limbs, and the armored, armored herbivores called aetosaurs . Their success was in the late Triassic, and that’s when the group was at its peak of diversity. Crurotarsans are often confused with dinosaurs, although they are an entirely separate group.
The Triassic, during which crurotarsans were dominant, was characterized by competition between the surviving therapsids (“mammal-like reptiles”, although they were completely unrelated to reptiles) and archosaurs of all types, including the ancestors of the dinosaurs and towards the end of the period, real dinosaurs. For much of the Early Triassic, groups of therapsids held firm, but were overwhelmed by the middle of the period. Meanwhile, ornithodyran archosaurs were evolving into pterosaurs (flying reptiles) and dinosaurs. Crurotarsans were successful enough to produce the largest animals of the period, including predators up to 7 m (23 ft) long.
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