A strange nothrotheriid ground sloth (Xenarthra, Megatherioidea) from Pomata-Ayte (Miocene-Pliocene transition, Bolivia)

PUJOS, F.; DE IULIIS, G.; ADNET, S.; ANDRADE FLORES, R.; BILLET, G.; MAMANI QUISPE, B.; MARIVAUX, L.; PRÁMPARO, M.; ANTOINE, P.-O.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

IPA

The locality of Pomata-Ayte (Oruro department) was first reported by Hoffstetter and colleagues at the end of the 1970s and partially exploited by St-André in the 1990s. New collects have allowed recovering a diverse mammalian assemblage dated between 5.97 ± 0.4 My and 5.42 ± 0.6 My (Montehermosan SALMA, Mio-Pliocene transition). Among the taxa are two native ungulates, the litoptern Macrauchenia sp. and the toxodontid Posnanskytherium cf. viscachanense, a caviomorph rodent, a large phorusrhacoid bird, and six xenarthrans, including three armored cingulates (the pampatheriine Plaina sp., a glyptodontid, and a dasypodid) and three ground sloths (the mylodontid Simomylodonuccasamamensis, the large megatheriid Megatherium (Megatherium) altiplanicum, and a small nothrotheriid). In the Pomata Ayte section, this Montehermosan fauna is intercalated between a new early late Miocene fauna with numerous mesotheriid notoungulates and caviomorph rodents (K/Ar dating in progress) and a late Pleistocene fauna (?Lujanian SALMA) with Macrauchenia patachonica, a gomphotheriid, a machairodontine felid, Megatherium sp., and Glyptodon sp. One of the most interesting Montehermosan mammals is the new nothrotheriid. This small to medium-sized sloth is represented by a complete mandible and several postcranials including humeri of three distinct individuals. Mandibular dentition is strongly hypsodont (Hypsodonty Index= 1.14). It has a small tricuspid caniniform (a character unique among sloths) and three molariforms quadrate and mesiodistally compressed with two transverse lophids (as in megatheriines). There is neither diastema nor apicobasal sulci on m1-m2 (although present on m3). The humerus is quite robust, with an entepicondylar foramen, and a rather massive deltopectoral shelf. The ulna is rectilinear in anterior view and the proximal articular facets differ markedly from those of other nothrotheriids. The calcaneus is massive and comparable in shape to that of Planops and Thalassocnus. A morphology-based phylogenetic analysis suggests that within the Nothrotheriidae clade, this new taxon is a stem Nothrotheriini, being more advanced over Pronothrotherium and Mionothropus from the late Miocene of Patagonia and Amazonia.