Crurotarsans

Crurotarsans are a clade of extinct archosaurian reptiles that were dominant during the Mesozoic Era. The name "crurotarsan" refers to a characteristic ankle structure, the crurotarsus, which is a highly mobile joint allowing for a range of foot movements. This ankle morphology is a key defining feature of the group, though it has been modified and lost in some later descendants.

This diverse group includes a wide array of extinct reptiles, most famously the rauisuchians, a group of large, predatory archosaurs that were apex predators of their time. Phytosaurs, which superficially resembled modern crocodiles with their elongated snouts and aquatic lifestyles, are also considered crurotarsans. Other notable crurotarsan groups include aetosaurs, which were heavily armored herbivorous animals, and ornithosuchids, which possessed a unique combination of crocodile-like and dinosaur-like features.

Crurotarsans first appeared in the Late Permian period and diversified significantly throughout the Triassic. They occupied a wide range of ecological niches, from terrestrial predators to semi-aquatic herbivores. While they were incredibly successful for much of the Mesozoic, their diversity declined towards the end of the Triassic, with many groups going extinct during the Triassic-Jurassic extinction event. However, some lineages, particularly those that evolved into the ancestors of modern crocodilians, survived and persisted into the Cenozoic and modern times. The study of crurotarsans provides valuable insights into the evolutionary history of archosaurs and the ecological dynamics of Mesozoic ecosystems.