The early Mesozoic saurischian trackways Evazoum and Otozoum: implications for ‘prosauropod’ (basal sauropodomorph) gaits

Abstract

ABSTRACT Trackways of bipeds and quadrupeds attributed to Late Triassic and Early Jurassic basal sauropodomorphs (‘prosauropods’) fall into several ichnogenera. Four of these – Otozoum, Pseudotetrasauropus, Evazoum and Kalosauropus – are conceptually subsumed into the OPEK plexus, where the former two represent large facultative quadrupeds, and the latter two represent bipeds. In revaluating ~53 measurable Evazoum tracks from 13 Upper Triassic sites in the Northern Hemisphere, we found a consistently enlarged proximal pad on digit II, which together with digit I show highly variable registration. These suggest that the trackmakers tended to preferentially carry weight on digits III and IV. Furthermore, over a dozen Evazoum trackways also indicate notable variations in trackway width, consistent with previous inferences about the ‘prosauropodian’ gait. Comparisons to five Kalosauropus trackways from the Lower Jurassic of the Southern Hemisphere show that these two more closely related track types were likely made by basal sauropodomorphs (prosauropods) that were more fully adapted to obligatory bipedalism. The much larger Otozoum, reported from 16 trackways globally from both the Upper Triassic and Lower Jurassic, has trackway patterns that indicate changes in trackway width and shifts between bipedal and quadrupedal progression.