Arizonasaurus (Arizona lizard) is a rauisuchian from Middle Triassic Arizona (obviously). It was not very large for a rauisuchian, only reaching about 3 meters long, but what immediately stands out about it is the large sail on its back. Two specimens have been found, one of them nearly complete.
Discovery and naming
Description
Classification
Biogeography
Paleoecology
Like Spinosaurus, Ctenosauriscus, Dimetrodon, Edaphosaurus, and Platyhystrix, Arizonasaurus had a sail on its back to control temperature, although it wasn't very closely related to any of these. So far, the purpose of these sails is unknown, although considering the warm environment of the Triassic, they were most likely used for thermoregulation, which was probably necessary especially for a creature as small as it that couldn't rely on gigantothermy.
Arizonasaurus likely lived alongside Coelophysis, and competed heavily with these early dinosaurs. It was one of the first rauisuchians to go extinct, likely because even larger rauisuchians like Saurosuchus proved to be too much competition.