A revision of Nematheriinae sloths (Mammalia, Xenarthra, Scelidotheriidae) from the Early–Middle Miocene Santa Cruz Formation, southern Patagonia

Geological and paleontological surveys in the Santa Cruz Formation (SCF; Early–Middle Miocene; Southern Argentine Patagonia), begun in the 19th century, have uncovered a rich extinct fauna that has provided a remarkable overview of the paleobiology of several South American endemic mammalian lineages. Initially driven by the Ameghino brothers in the late 1800s and continued by subsequent scholars into the early 1900s, these early efforts resulted in an overproliferation of taxa based on weak diagnostic features. This is certainly the case with the findings assignable to Nematheriinae sloths, an early diverging group of Scelidotheriidae, whose current taxonomy recognizes several species distributed among three genera. New well-preserved remains recovered with precise geographic provenance and under strict stratigraphic control, together with analysis of older collections, consideration of intraspecific variation, and assessment of the validity of the early literature on Santacrucian nematheriines allow revision of the taxonomic status of the numerous species erected for this group. As there is no positive evidence for supporting the validity of other nematheriine genera, the remains are assigned to a single genus, Nematherium. Further, all specimens from the SCF, except for the holotype of Analcitherium antarticum Ameghino, are referrable to Nematherium angulatum Ameghino, for which a neotype is designated given that the original type is lost. We provide a reappraisal of the diagnostic features and an assessment of the intraspecific variation of Nematherium angulatum, its differences with Nematherium birdi Simpson and N. antarticum, and comments on other mylodontoid craniodental remains from Argentine and Chilean Patagonia.