Article: Morphology and phylogenetic informativeness of early archosaur braincases

The braincases of the Triassic early archosaurs Vjushkovia triplicostata, 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 possible confidence levels that the data set contains significant hierarchical structure, interpreted as the result of phylogeny. The most parsimonious tree based only on braincase data agrees broadly with existing ideas of early archosaur relationships. However, it conflicts with recently published hypotheses in a number of details, most notably in the presence of a holophyletic Proterosuchia and a well-supported clade of Erythrosuchus + Shansisuchus. The use of Prolacerta as an outgroup does not perturb the parsimonious interpretation of relationship of the included early archosaurs. Topological constraints and additional analyses performed on subsets of the 11 taxa show that some of the hypothesized relationships based only on braincase data are not robust. Unremarkable consistency indices and weakly supported relationships suggest that braincase morphology does not represent an especially informative source of data for the reconstruction of earliest archosaur phylogeny, although this remains an area for further investigation.