An unusual Amphisauropus trackway and its implication for understanding seymouriamorph locomotion

Vertebrate ichnology has proved to be a valuable tool for reconstructing tetrapod locomotion. Grounding on 1, the discovery of a new, exceptionally preserved Amphisauropus trackway (Permian Orobic Basin, Northern Italy), showing continuous tail and pedal scratch impressions; and 2, a comparative analysis of track and bony records, we provide a first synapomorphy-based attribution of Amphisauropus tracks to their most plausible producers, seymouriamorph reptiliomorphs, and propose a detailed reconstruction of the locomotion biomechanics in this group. The markedly sinuous and continuous tail impression with a relatively constant width, the curve arrangement of pedal scratches and the inferred position of head, trunk and tail during the locomotion suggest large lateral movements, while vertical movements were nearly absent, consistently with a primitive sprawling gait. We also document the functional prevalence of the hindlimbs and provide a detailed description of femur biomechanics during step cycle. Trackway parameters suggest that the speed was quite slow: a comparison with a large database of Amphisauropus trackways from Italy and Germany proves that this peculiar preservation was gait-and substrate-related, rather than trackmaker-related. In all, this study sheds light on the locomotion of Seymouriamorpha and aimed to contribute to a full integration of track and skeletal data in exploring locomotory biomechanics of extinct tetrapods.