Crurotarsi

The Crurotarsi ("cross-ankles") are a group of archosaurs, whose name was erected as a node-based clade by Paul Sereno in 1991 to supplant the old term Pseudosuchia. Crurotarsi are by definition the sister group of the Avemetatarsalia (all forms closer to birds than crocodiles).

Description
Crurotarsi is one of the two primary daughter clades of the Archosauria. The skull is often massively built, especially in contrast to ornithodires; the snout narrow and sometimes tending to be elongate, the neck is short and strong, and the limb posture ranging from typically reptilian sprawling to dinosaur or mammal-like erect (although this is achieved in a different way to dinosaurs and mammals). The body is often protected by two or more rows of armored plates. Many crurotarsans reached large size: approximately around three meters or more in length.

Evolution
Crurotarsans appeared during the late Olenekian (early Triassic); by the Ladinian (late Middle Triassic) they dominated the terrestrial carnivore niches. Their heyday was the Late Triassic, during which time their ranks included erect-limbed rauisuchians, the crocodile-like phytosaurs, herbivorous armoured aetosaurs, the large predatory poposaurs, the small agile crocodilians Sphenosuchia, and a few other assorted groups.

At the end Triassic extinction, all of the large crurotarsans died out. The cause of their extinctions is yet unknown, but it is believed that it may have been an asteroid impact, sudden climate change, or a planetary cataclysm. Futhermore, this allowed the dinosaurs to succeed them as the dominant terrestrial carnivores and herbivores. Only the Sphenosuchia and the Protosuchia (Crocodylomorpha) survived.

As the Mesozoic progressed, the Protosuchia gave rise to more typically crocodile-like forms. While dinosaurs were the dominant animals on land, the crocodiles flourished in rivers, swamps, and the oceans, with far greater diversity than they have today.

With the end Cretaceous extinction the dinosaurs became extinct, with the exception of the birds, while the crurotarsan crocodilians continued with little change.

Today, the crocodiles, alligators, and gavials continue as the surviving representatives of this lineage.