PEDAL DIGITAL AND PHALANGEAL PROPORTIONS OF THE “DINOCROC” POPOSAURUS GRACILIS

The Triassic suchian Poposaurus gracilis showed morphological convergence with bipedal dinosaurs in an erect limb carriage and a short forelimb/hindlimb length ratio, suggesting that, like non-avian theropods and many ornithischians, Poposaurus was an erect, striding biped. We investigated whether convergence between dinosaurs and Poposaurus extended to relative lengths of the pedal digits, and of the pedal phalanges.

We used two data sets in our analyses, employing both bivariate comparisons and principal components analyses. One data set used our own measurements as well as data from the literature of phalanges I1, II1-II2, III1-III3, and IV1-IV3 (more distal phalanges, including unguals, were not used in order to increase sample sizes of feet) of Poposaurus, non-ornithodiran archosaurs, basal dinosauromorphs, basal sauropodomorphs, non-avian theropods, bipedal ornithischians, and ground birds. A second data set used only direct measurements from the specimens of pedal digital and phalangeal lengths (including unguals) of digits II-IV, and selected phalangeal widths, of Poposaurus, non-avian theropods, and bipedal ornithischians. Data were supplemented by photographs and published figures of specimens for which measurements were not available.

Poposaurus is more like non-avian theropods and basal ornithopods than non-ornithodiran archosaurs and most ground birds in the phalangeal proportions of digits II-IV. It is similar to many dinosaurs as well as crocodylians in the relative lengths of digits II and III, more like dinosaurs (many saurischians and some ornithischians) than non-ornithodirans in the relative length of digit IV, and more like non-ornithodirans, prosauropods, and some ornithischians than theropods in the length of digit I relative to the other pedal digits. Poposaurus is more like non-avian theropods and basal euornithopods (hypsilophodontids) than like other ornithischians in the relative widths of digits II-IV. All told, the pes of Poposaurus is in most features most like that of an unspecialized small theropod, prosauropod, or hypsilophodontid. Its footprint would be similar to larger ichnospecies of Anchisauripus, but possibly with a distinct impression of digit I as well as digits II-IV.