Insights on the evolution of synapsid locomotion based on tetrapod tracks from the lower Permian of Mallorca (Balearic Islands, western Mediterranean)

Abstract “Pelycosaur”-grade synapsids were a successful group of terrestrial tetrapods that lived during the Carboniferous and Permian, utilising a wide diversity of ecological niches. They are considered the trackmakers of the ichnogenera Dimetropus and possibly also Dromopus and Tambachichnium, found in upper Palaeozoic deposits of North America, North Africa and Europe. Here we describe a new morphotype from the lower Permian of Mallorca (Balearic Islands, western Mediterranean), identified as cf. Dimetropus isp., which was purportedly produced by caseid synapsids. The trackmaker identity is inferred based on the digit proportions, as caseids are the only pelycosaurs with species showing mesaxonic autopodia, and a relative depth pattern (corresponding to the functional prevalence of the autopodia of the trackmaker) showing an overall similarity to that of Dimetropus osageorum, which is also attributed to caseids. A detailed study of the expulsion rims and drag traces makes it possible to infer the mode of locomotion of the trackmaker, with a gait alternating the movement of manus and pedes of the two sides of the body and lateral undulation of the spine. Our results provide new information concerning the locomotion of early synapsids, which would undergo important functional modifications later in their evolutionary history such as a shift from abducted to adducted posture and lateral to sagittal bending of the axial skeleton.