Two upper Permian trackways attributed to the ichnogenus Sukhonopus, which were originally interpreted as produced by pareiasaurians, have been studied. Based on the statistical analysis of dimensions of the longer trackway representing 15.5 locomotor cycles, the Sukhonopus gait was defined as a lateral-sequence walk shifted by ipsilateral limb synchronization towards the slow pace. This conclusion gained additional support through comparison of dimensions of the two trackways with the glenoacetabular-to-pes length ratio measured on the skeletons of the pareiasaur Deltavjatia. With the sprawling limb posture typical to lower tetrapods, including pareiasaurians, the pace can be only performed with body rocking from side to side, according to the principles of passive-dynamic walking. A natural-sized (
185.cm long) model of the Sukhonopus producer was built. This model reproduces the passive-dynamic rocking pace, being driven manually by children pulling it forward by the rope and simultaneously pushing it from side to side. The passive-dynamic rocking pace of pareiasaurs, proved to be feasible by the model, requires functional differentiation of the hindlimbs and the forelimbs. The former were more appropriate for propulsion, while the latter were more efficient in generation of the transverse forces required to pump the rocking motion. It is hypothesized that this walking mechanism was associated with archaic deficiency of nervous and muscular supply of the limbs. Possibly, it was also employed by some other Paleozoic tetrapods. The ichnological evidence of this locomotion technique is the inward turn of the forelimb footprints, and the histological evidence is the extensive development in the long limb elements of spongy bone tissue in place of the superficial compact bone layer and marrow cavity.Â