Résumé
Miocene vertebrate localities are uncommon in central South America. In Bolivia, the best known mammalian faunas ofthis period come from Quebrada Honda (late middle Miocene, Tarija Department) and Cerdas (early middle Miocene, PotosíDepartment). The Achiri locality (La Paz Department) was reported first by Hoffstetter in 1972. Subsequently, campaignswere conducted in this locality by several paleontologists, including Villarroel, Anaya, Saint-André, and by our team over thelast decade. Recently, we have obtained two precise absolute dates (40Ar/39Ar) on feldspar contained in ashes intercalatedbetween fossiliferous levels and confirmed a late Miocene age (10.35±0.07 Ma and 10.42±0.09 Ma, late Mayoan–earlyChasicoan South American Land Mammal Ages) as suggested by Marshall and colleagues in 1983. Almost all the specimenscome from the Cerros Virgen Pata and Jiska/Jacha Pisakeri localities, the latter located 3–4 km southeast of Achiri village.In the past, discoveries of numerous spectacular specimens have allowed the identification of new mammalian speciessuch as the notoungulates Plesiotypotherium achirense and Hoffstetterius imperator, the sparassodontan Borhyaenidiumaltiplanicum, and the xenarthrans Trachycalyptoides achirense and Xyophorus villarroeli. Xyophorus was erected by Ameghinoin 1887 on the basis of a dentary fragment from the lower Miocene Santa Cruz Formation (Argentina). This taxon is generallyconsidered to be a nothrotheriid sloth (although has never been formally included in a phylogenetic analysis based onosteological characters). Six species are recognized in Argentina. This genus is also recorded in Achiri through the endemicspecies X. villarroeli, and also in Cerdas and Quebrada Honda through X. cf. bondesioi. Unfortunately, all the specimensreferred to Xyophorus are extremely fragmentary. Here we present a partial skull (MNHN-Bol-V 12690, National Museumof Natural History, La Paz, Bolivia) discovered in Achiri, belonging to an adult, and referred as “Xyophorus” villarroeli. Itconsists of a right posterolateral portion of the skull, including parts of squamosal, parietal, basioccipital, exoccipital, andits complete ear region with ectotympanic, entotympanic, and petrosal. Preliminary observations of this new specimenreveal the presence of at least seven autapomorphies, including a very rugose external surface of ectotympanic, a clearcontact between styliform process of ectotympanic and pterygoid, and a reduced or absent subarcuate fossa. Thismegatherioid sloth shares several synapomorphies with nothrotheriids, including a dorsoventrally elongated ectotympanicand an ovate stylohyal fossa. It exhibits also transitional features between basal megatherioids and nothrotheriids, like aventral portion of the ectotympanic that is expanded transversely in ventral view (more than Hapalops and less thanNothrotheriidae) and deeper in lateral view than that of Hapalops, although similar in proportions to Pronothrotherium andMionothropus. This specimen thus suggests that “Xyophorus” villarroeli could be an early-diverging nothrothere, withaffinities to Hapalops and also early Nothrotheriidae, and probably distinct from Xyophorus of more austral localities. Acomprehensive phylogenetic analysis of the Megatherioidea including this form from Achiri, Aymaratherium from the earlyPliocene of Pomata-Ayte, and Lakukullus and Hiskatherium from Quebrada Honda, should allow for a better understandingof the relationships among Patagonian and Andean Megatherioidea.