Crocodilia
Origins from pseudosuchians

Crocodilians are a type of archosaur, a clade containing the most recent common ancestor of crocodilians, birds, and all their descendants. Archosaurs are distinguished from other reptiles particularly by two sets of extra openings in the skull: the antorbital fenestra located in front of the animal's eye socket and the mandibular fenestra in the middle of the jaw. Archosaurs comprise two main groups: the Pseudosuchia (crocodilians and their relatives), and the Avemetatarsalia (dinosaurs, pterosaurs, and their relatives). The split between these two is assumed to have happened close to the Permian–Triassic extinction event (informally known as the Great Dying).

During the Early Triassic, pseudosuchians flourished and diversified. One form, the crocodylomorphs, would later give rise to modern crocodilians. While the most basal crocodylomorphs were large, the ones that gave rise to crocodilians were small, gracile, and leggy. This evolutionary grade, the so-called " sphenosuchians " first appeared around Carnian of the Late Triassic. They ate small, fast prey and survived into the Late Jurassic. As the Triassic ended, crocodylomorphs became the only surviving pseudosuchians.
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