Passive-Dynamic Walkers of the Late Paleozoic

Abstract

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 22 propulsion, while the latter were more efficient in generation of the transverse forces required 23 to pump the rocking motion. It is hypothesized that this walking mechanism was associated 24 with archaic deficiency of nervous and muscular supply of the limbs. Possibly, it was also 25 2 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.

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“…This could be a typical walking technique of sprawling quadrupeds in mammalian lineage (Gambaryan & Kuznetsov, 2013; Kuznetsov, 1999). The same mechanism was later presumed for pareiasaurians based on trackway analysis and the presence of Tf‐type articulations anterior to the sacrum, otherwise unknown beyond mammals (Kuznetsov, 2020).…”

“…In this case, prezygapophyses do not form outgrowths, and their articular facets are located directly on the neural arches of the vertebrae and oriented along the circumference of the vertebral center—tangentially or almost horizontally (Slijper, 1946; Virchow, 1907). This structural type is unknown among non‐mammalian tetrapods beyond rare exceptions such as horizontal zygapophyseal articulations in a few presacral joints of pareiasaurians (Kuznetsov, 2020).…”

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“…This also suggests that the ‘upright sprawling’ gait of echidnas, along with other aspects of their unusual morphology (e.g. pes), may be a derived trait and not the ancestral condition as indicated by some mechanical models [19].…”

“…As extant lizards with abducted limbs use extensive lateral (side-to-side) bending of the backbone, and extant mammals use sagittal (up-down) bending during ‘asymmetric’ running gaits, it has long been assumed that the synapsid–mammal locomotor transition is also typified as a shift from lateral–sagittal vertebral motions. Of note, the monotreme echidna employs an ‘upright sprawling’ gait that involves rocking of the trunk from side-to-side [18], a behaviour also proposed for pareiasaurs and other Palaeozoic tetrapods [19]. However, recent quantitative analysis of vertebral shape across amniotes revealed distinct vertebral anatomies in early synapsids compared to modern-day reptiles and mammals, and that the synapsid spine was ancestrally stiff rather than laterally compliant [20].…”