New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation

BUTLER, R.J.; SULLIVAN, C.; EZCURRA, M.D.; LIU, J.; LECUONA, A.; SOOKIAS, R.B.

Berlin

Congreso; Annual Meeting of the Society of Vertebrate Paleontology; 2014

The origin and early evolutionary radiation of archosaurs and their close relatives (Archosauriformes) during the Triassic was a critical event that led to the dinosaurdominated ecosystems of the Jurassic and Cretaceous. The timing and dynamics of this evolutionary radiation are currently obscured by the poorly constrained phylogenetic positions of several key early archosauriform taxa, including several species from the Middle Triassic of Argentina (Gracilisuchus stipanicicorum) and China (Turfanosuchus dabanensis, Yonghesuchus sangbiensis). Morphological phylogenetic analyses of early archosaurs have identified these species as unstable 'wildcards' that reduce phylogenetic resolution. We present new anatomical data for the type specimens of Gracilisuchus, Turfanosuchus, and Yonghesuchus, and incorporate these data into a revised phylogenetic analysis. Our results indicate that these three previously enigmatic taxa form a wellsupported clade of Middle Triassic archosaurs that we refer to as Gracilisuchidae, which is placed basally among suchian archosaurs. The approximately contemporaneous and morphologically similar Gracilisuchus and Yonghesuchus may be sister taxa within Gracilisuchidae. Our results provide increased resolution of the interrelationships of early archosaurs, with increased levels of phylogenetic support for several key early pseudosuchian clades. Moreover, they falsify previous hypotheses suggesting that Turfanosuchus and Yonghesuchus are not members of the archosaur crown group. The recognition of Gracilisuchidae provides further support for a rapid phylogenetic diversification of crown archosaurs by the Middle Triassic. The disjunct distribution of the gracilisuchid clade in China and Argentina supports the hypothesis that early archosaurs were distributed over much or all of Pangea, although they may have initially been relatively rare members of faunal assemblages.