Pterosaur Stance and Gait and the Interpretation of Trackways

Abstract

The tracks ascribed to pterosaurs from the Late Jurassic limestones at Crayssac, France, must be pterosaurian because the manus prints are so far outside those of the pes, the pes print is four times longer than wide, and the manus prints appear to preserve distinct traces of a posteromedially directed wing-finger. These tracks are different in important ways from previously described Pteraichnus trackways, which have been variably considered pterosaurian, crocodilian, or indeterminate. No Pteraichnus (sensu stricto: those not from Crayssac) tracks have diagnostic features of pterosaurs and in none can a complete phalangeal or digital formula be reconstructed; however, all published Pteraichnidae tracks fulfill the criteria of poor preservation, and some have some diagnostic features of crocodile tracks. Reconstructions of pterosaurs walking in pteraichnid tracks do not fit those tracks well, but crocodiles do.In contrast, the Crayssac tracks demonstrate the erect stance and parasagittal gait previously reconstructed for pterosaurs. They also demonstrate that the footfall pattern was not as in typical reptiles (LH-RF-RH-LF), but that the manus must have been raised before the next forward step of the ipselateral foot (LH-LF-RH-RF), suggesting that the quadrupedal pattern was secondary. The metatarsus in pterosaurs was set low at the beginning of a stride, as it is in crocodilians and basal dinosaurs. The diagnosis of the Ichnofamily Pteraichnidae comprises features of possible crocodilian trackmakers, but not of possible pterosaurian trackmakers. Trackways considered for attribution to pterosaurs should show (1) manus prints up to three interpedal widths from midline of body, and always lateral to pes prints, (2) pes prints four times longer than wide at the metatarso-phalangeal joint, and (3) penultimate phalanges longest among those of the pes.