Piatnitzkysauridae

Piatnitzkysauridae is a family of comparatively small, basal megalosauroid theropods that lived during the Middle to Late Jurassic. It comprises only three to four genera: Piatnitzkysaurus and Condorraptor from the Cañadón Asfalto Formation of Argentina, Marshosaurus from the Morrison Formation of the United States, and possibly the Chinese Xuanhanosaurus, whose fragmentary remains leave its placement uncertain. These were among the first large theropods known from South America and are seen as evidence for a radiation of basal tetanurans in the Middle Jurassic. The group is usually recovered as the basalmost branch of Megalosauroidea and the sister group of Megalosauria, though it also shares some features with allosaurs, including a sigmoidal (S-shaped) humerus, and a few analyses instead place it within Allosauroidea. Piatnitzkysaurids can be distinguished by details of the maxilla, paradental plates, axis vertebra, dorsal neural spines, and the forward-canted condyles of the humerus.