Concurrent evidence from ichnology and anatomy: the scelidotheriine ground sloths (Xenarthra, Folivora) from the Pleistocene of Argentina

Abstract

In the fossil record of South America, both ground sloths and their tracks are well known. However, most tracks described were assignable to megatherians and mylodontids, while tracks assignable to scelidotheriines show a very scarce record. In this contribution, we present new records of fossil tracks assignable to scelidotheriines and review and expand the original description of the ichnotaxon Acugnaichnum dorregoensis. In addition, we explored the correlation between track features and limb morphology, finding that the tracks can help to better understand the posture and function of scelidotheriine limbs. Features of the handprint suggest that these ground sloths would not rest hands in the substrate by its palmar aspect, but on the knuckles or its lateral aspect, similar to the extant giant and lesser anteaters. Concerning the foot tracks, their features indicate that digits V and IV rested on the substrate in a different orientation than the digit III, whose large ungual phalanx provided additional support by resting of its lateral side, which is biomechanically coherent with the stability of the foot. In addition, some kind of soft tissue cushion would be present below the main arch of the foot. Evidence for locomotion style and bipedalism is discussed.