Bipedalism evolved more than twice among archosaurs, and it is a characteristic of basal dinosaurs and a prerequisite for avian flight. Nevertheless, the reasons for the evolution of bipedalism among archosaurs have barely been investigated. Comparative analysis using phylogenetically independent contrasts showed a significant correlation between bipedality (relative length of forelimb) and cursoriality (relative length of metatarsal III) among Triassic archosaurs. This result indicates that, among Triassic archosaurs, bipeds could run faster than quadrupeds. Bipedalism is probably an adaptation for cursoriality among archosaurs, which may explain why bipedalism evolved convergently in the crocodilian and bird lineages. This result also indicates that the means of acquiring cursoriality may differ between archosaurs and mammals.

AppendixResults of correlation tests between cursoriality index and body size, and between cursoriality index and bipedality index after the effect of body size was removed, among Triassic archosaurs. The default settings of the correlation tests were (1) a branch length of one set for all branches and (2) reconstructed phylogenetic relationships among Triassic archosaurs (Fig. 2). Additional sensitivity analyses with arbitrary branch length (Grafen, Nee, and Pagel), temporal duration as branch lengths, or polytomic relationship (Fig. 2B) did not alter the result significantly. The analysis was also conducted with restricted taxa, which are crurotarsans, ornithodirans, and ornithodirans without sauropodomorphs except Eoraptor.rev kubo 11015 suppl. appendix.docrev kubo 11015 suppl. appendix.pdf