Crurotarsans ("cross-ankles") are one of the two groups of archosaurs (a large clade of reptiles that includes birds, crocodilians, 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 reptilian diversity. Crurotarsans have existed for almost 250 million years in total, often dominating swamp 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 crocodiles than present-day birds, and by their unique croc-like ankle structure.
Crurotarsans became the dominant vertebrates a few million years after the Permian-Triassic extinction 251 million years ago, which was the greatest mass extinction in the planet's history. They followed the therapsids (ancestors of mammals), which had dominated for 25 million years prior to the mass extinction, and the pelycosaurs, primitive ancestors of therapsids which themselves dominated for 40 million years. The crurotarsans would only rule the planet for 50 million years, after which all the large species would die out in the end Triassic extinction, paving the way for the coming of the dinosaurs.
Crurotarsans included many advanced cousins of the modern-day crocodile, displaying a much greater diversity of form and ecological roles. There were the rausuchians, erect limbed, large (4-6 m) and predatory crurotarsans, carnivorous poposaurs, which resembled small dinosaurs, the small, lithe, agile, erect-limbed spehnosuchians, and the large-bodied, armored herbivores called aetosaurs. Their heydey was the late Triassic, and that's when the group was at its height of diversity. Crurotarsans are often confused with dinosaurs, though they're an entirely separate group.
The Triassic, during which crurotarsans were dominant, was characterized by competition between the surviving therapsids ("mammal-like reptiles," though they were completely unrelated to reptiles) and archosaurs of all types, including the ancestors of dinosaurs and towards the end of the period, true dinosaurs. For much of the early Triassic, therapsid groups held on, but were overwhelmed by the middle of the period. Meanwhile, ornithodiran 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.