Fossil ground sloth footprints: ichnotaxonomy and producers

Abstract This paper includes a comprehensive study on the ichnotaxonomy and potential producers of fossil ground sloth footprints from South and North America. We propose the new ichnofamily Megatherichnidae to include quadrupedal trackways characterized by elliptical or falciform footprints, often containing claw traces and a marginal ridge of lateral position. This ichnofamily is proposed to comprise footprints produced by ground sloths with a pedolateral posture. Megatherichnidae includes the previously proposed ichnogenera Megatherichnum and Falsatorichnum from the Late Miocene to Early Pliocene, as well as Neomegatherichnum and Mylodontidichnum essentially recorded from Upper Pleistocene deposits. A further new ichnogenus is proposed: Falciformichnus , represented in the Late Miocene and Early Pliocene of South America, and Late Pleistocene of North America. A new ichnospecies identified in Upper Pleistocene deposits from Argentina, Megatherichnum agilis , is established. The monotypic ichnogenera Venatoripes , Acugnaichnus and Iribarnichnum , previously attributed to fossil ground sloths, are considered nomina dubia . Key distinguishing features within Megatherichnidae are footprint morphology, dimensional and morphological differences between manus and pes prints, and pes print length. Certain ichnospecies are suggested to be associated with specific producers, including some sloths from the families Megatheriidae and Mylodontidae. However, comparison with the fossil record is sometimes difficult because most ground sloth species are based on cranial material. Furthermore, body mass estimates derived from footprint area indicate markedly higher values than those obtained from long bone measurements, highlighting the need for further research into the biomechanics of these extinct ground sloths.