A new phylogenetic hypothesis of Tanystropheidae (Diapsida, Archosauromorpha) and other "protorosaurs", and its implications for the early evolution of stem archosaurs.

The historical clade "Protorosauria" represents an important group of archosauromorph reptiles that had a wide geographic distribution between the Late Permian and Late Triassic. "Protorosaurs" are characterized by their long necks, which are epitomized in the genus Tanystropheus and in Dinocephalosaurus orientalis. Recent phylogenetic analyses have indicated that "Protorosauria" is a polyphyletic clade, but the exact relationships of the various "protorosaur" taxa within the archosauromorph lineage is currently uncertain. Several taxa, although represented by relatively complete material, have previously not been assessed phylogenetically. We present a new phylogenetic hypothesis that comprises a wide range of archosauromorphs, including the most exhaustive sample of "protorosaurs" to date and several "protorosaur" taxa from the eastern Tethys margin that have not been included in any previous analysis. The polyphyly of "Protorosauria" is confirmed and therefore we suggest the usage of this term should be abandoned. Tanystropheidae is recovered as a monophyletic group and the Chinese taxa Dinocephalosaurus orientalis and Pectodens zhenyuensis form a new archosauromorph clade, Dinocephalosauridae, which is closely related to Tanystropheidae. The well-known crocopod and former "protorosaur" Prolacerta broomi is considerably less closely related to Archosauriformes than was previously considered.