MORPHOLOGY AND PHYLOGENETIC INFORMATIVENESS OF EARLY ARCHOSAUR BRAINCASES

The braincases of the Triassic early archosaurs Vjushkovia triplicosta ta, Fugusuchus hejiapensis, Xilousuchus sapingensis, and Shansisuchus shansisuchus are described in detail for the first time. A preliminary analysis investigating the phylogenetic informativeness of braincase morphology in the earliest archosaurs incorporates 11 archosauromorph taxa and 17 informative characters. A further seven uninformative and eight problematical braincase characters are discussed. Parsimony and character compatibility permutation tests suggest at the highest possi ble confidence levels that the data set contains significant hierarchi cal structure, interpreted as the result of phylogeny. The most parsim onious tree based only on braincase data agrees broadly with existing ideas of early archosaur relationships. However, it conflicts with rec ently published hypotheses in a number of details, most notably in the presence of a holophyletic Proterosuchia and a well-supported clade o f Erythrosuchus + Shansisuchus. The use of Prolacerta as an outgroup d oes not perturb the parsimonious interpretation of relationship of the included early archosaurs. Topological constraints and additional ana lyses performed on subsets of the Il taxa show that some of the hypoth esized relationships based only on braincase data are not robust. Unre markable consistency indices and weakly supported relationships sugges t that braincase morphology does not represent an especially informati ve source of data for the reconstruction of earliest archosaur phyloge ny, although this remains an area for further investigation.