The Triassic amphibian Thoosuchus yakovlevi and the relationships of the Trematosauroidea (Temnospondyli: Stereospondyli)

A skull of the basal trematosauroid temnospondyl Thoosuchus yakovlevi from the Early Triassic of Russia is described. Characters showing phylogenetic affinities with the Trematosauroidea include the presence of a postorbital-prepineal growth zone, the well-developed sensory sulci, the ventrally knife-edged cultriform process of the parasphenoid, and the posteriorly expanded parasphenoid body. A preliminary phylogenetic analysis of trematosauroid relationships confirms that Benthosuchus is a basal trematosaurian rather than a basal mastodonsauroid, and that Thoosuchus is the sister group to all other trematosauroids. Relationships within the Trematosauroidea are poorly established with as yet little evidence for subdivision of the group as previously proposed. In addition, the hypothesis that the Metoposauroidea is nested within the Trematosauroidea is supported. However, the Trematosauridae sensu stricto appears to be paraphyletic. The basal stereospondyl dichotomy between the Mastodonsauroidea and its relatives (the Capitosauria), and the Trematosauroidea and its relatives (the Trematosauria), is supported, and now seems well established.