Locomotor kinematics of the manus and pes in dinocephalian therapsids reconstructed from three-dimensional morphology of footprints from Gansfontein, South Africa

Abstract

The Gansfontein paleosurface from the Late PermianAbrahamskraal Formation (Beaufort Group, TapinocephalusAssemblage Zone) near Fraserburg, South Africa, preserves several vertebratetrackways. Among the best-preserved trackways is a series of several footprintsthat have been attributed to a single dinocephalian therapsid walking acrossthe surface. This attribution is based on both the large size of the footprints(25 cm in breadth), and the reduction of digit I in the manus and pes. However,a curious feature of this trackway is that, in contrast to the straight digitsindicated by skeletal specimens of dinocephalians, the digit impressions from theprints are curved so that their tips point medially. To test hypotheses aboutthe locomotor kinematics of the dinocephalian manus and pes that produced thesecurved-digit prints, we constructed contour maps that depicted how the depth ofan impression varied within individual prints. In our initial analysis, we constructedthe maps by pouring milk into the prints in successive increments of 2 mm inheight, and tracing the perimeters of filled areas on translucent paper fixedto the paleosurface by tape. We have also used surface scanners to generatedepth profiles with finer scale resolution. These analyses indicated that thedistal tips of digit impressions are among the shallowest portions of theprints, a pattern consistent with outward rotation of the hand and foot duringstance. In addition, heel impressions are lacking in the prints, indicatingthat the animal that made them may have used digitigrade foot posture. Spacing betweenleft and right prints is small, less than the breadth of individual prints.However, the presence of foot rotation during stance suggests thatdinocephalians used sprawling, rather than parasagittal limb posture. Closespacing of footprints likely resulted from a combination of lateral bending ofthe body and significant cranio-caudal limb excursion, rather than increasedadduction of the limbs under the body.