Bipedal ground sloths? The footprint evidence from the Pleistocene of Argentina

SILVERIO F. FEOLA; TERESA MANERA; RICARDO NESTOR MELCHOR

Santa Marta

Simposio; IV Simposio Latinoamericano de Icnología (SLIC 2018); 2018

Universidad de Caldas

Pleistocene ground sloths have been essentially considered bipedal animals based on trackway and osteological evidence. Ground sloth trackways lacking manus impressions have been mostly interpreted as reflecting a bipedal posture or, more rarely,as result of overprinting of the pes on the manus. A late Miocene to early Pliocene ground sloth trackway included under Megatherichnum Casamiquela, 1974 had been recently interpreted as quadrupedal. In this ichnogenus, the manus is slightly smaller than the pes. We describe well-preserved quadrupedal ground sloth trackways from the late Pleistocene Pehuen Co Paleoichnological site, Buenos Aires province, Argentina.This site contains abundant and exquisitely preserved trackways of diverse mammals and birds. Some of the studied trackways exhibit different degree of overprinting of the pes on the manusand other trackways display distinctive manus-pes sets lacking overprinting (Fig.1).Most footprints are elliptical, lack digit impressions, exhibit a lateral marginal ridge,and a remarkableclaw impression of digit III (suggesting a pedolateral posture). The five analyzed trackways show a clear size difference between manus and pes. The manus is up to 73% smaller in length and 53% smaller in width than the associated pes. In some cases, the manus is subcircular and the pes is elliptical. When manus and pes are not overprinted they compose sets with an alternate arrangement. Partial overprinting of the pes on the manus is indicated by cross-cutting relationships and discontinuity of the marginal ridge.Pace angulation is 80-130°, as expected for a quadrupedal locomotion of a large animal. Two footprint size classes can be distinguished: the large population averages 48 cm by 35 cm for the manus prints and 70 cm by 38 cm for the pes prints; whereas the small population averages 23 cm by 18 cm for the manus prints and 53 cm by 23 cm for the pes prints. Considering the size and shape differences, the presence of the claw impression of digit III and the osteological record for the Pleistocene of the Pampean region, the likely producers are extinct ground sloths belonging to Megatheriidae (large population) and Mylodontidae (small population). Some of the likely candidates are Megatherium, Lestodon (large size), Glossotherium, Mylodon and Scelidotherium (small size). We suggest that, even if it is possible that Pleistocene ground sloths acquired a transient upright posture, they moved quadrupedally. Ground sloth trackways lacking manus prints are likely due to a complete overprinting by the pes. Finally, the comparison of late Miocene-early Pliocene and late Pleistocene ground sloth tracks suggests a progressive size reduction in the manus relative to the pes for younger tracks.