Recognition of a new nothrotheriid genus (Mammalia, Folivora) from the early late Miocene of Achiri (Bolivia) and the taxonomic status of the genus Xyophorus

New remains of a relatively plesiomorphic nothrotheriid sloth have been recovered from upper Miocene-aged deposits near the village of Achiri in the Altiplano of Bolivia. The new specimens appear allied to other middle and late Miocene remains from Argentina and Bolivia that have been assigned to the pseudo-genus 'Xyophorus'. 'Xyophorus' has not previously been recognised as a distinct genus because of the paucity of material it encompasses. The new specimens, however, include a well-preserved squamosal with attached auditory region and an isolated astragalus. These elements, which are described in detail, provide a sufficient number of distinctive characters to place the previous fossils assigned to 'Xyophorus' into a new monotypic genus. Beyond exhibiting a suite of distinctive autapomorphies which justify its formal taxonomic designation, the new taxon not only shares several synapomorphies with more derived members of Nothrotheriidae but also retains a number of plesiomorphies characteristic of basal megatherioid taxa and shows a number of intermediate features. Although the new taxon is too incomplete to justify a full phylogenetic analysis, it appears to represent a basal member of Nothrotheriidae. Resolution of the taxonomic status of the genus Xyophorus awaits a better understanding of the taxonomy of early Miocene-aged basal megatherioids.