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International audience; 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.

International audience; El conocimiento de los mamíferos neógenos de Bolivia se ha centrado principalmente en unos pocos yacimientos del Mioceno temprano (Cerdas, Nazareno) y el Mioceno medio (Quebrada Honda). En la última década, se han incrementado los esfuerzos de exploración, dando como resultado un mejor conocimiento de la fauna de la localidad de Achiri (Prov. de Pacajes) del Mioceno tardío (~10,42–9,42 Ma; Formación Mauri). Dentro de esta fauna, los especímenes de caviomorfos, depositados en la Unidad de Paleontología del Museo Nacional de Historia Natural de La Paz, son abundantes y consisten en restos cráneo-mandibulares y post-craneanos, y dientes aislados. El estudio preliminar indica la presencia de Prodolichotis prisca, Cardiomys sp. (Cavioidea), Tetrastylus sp., Lagostomus sp. (Chinchilloidea) y un octodontoideo indet. Prodolichotis abarca el 80% de la diversidad, con cráneos excepcionalmente preservados (varios asociados a mandíbulas). Los otros taxones están representados por especímenes que, aunque escasos y fragmentarios, permiten una asignación genérica clara (excepto el octodontoideo por su mala preservación). Prodolichotis prisca es el dolicotino más antiguo, registrado en el Mioceno tardío–Plioceno temprano del noroeste de Argentina. Cardiomys, Tetrastylus y Lagostomus se encuentran ampliamente distribuidos en niveles del Neógeno tardío de Argentina. Además, Tetrastylus se registra en el Mioceno tardío de Brasil, Uruguay y Venezuela. Aunque es necesario un análisis profundo de la taxonomía de los roedores de Achiri, los mismos indican mayor afinidad con las faunas de edad Chasiquense-Huayqueriense de Argentina. El estudio de esta asociación permitirá aumentar el conocimiento biogeográfico del Neógeno tardío de América del Sur.

International audience; The Altiplano formed a broad Cenozoic inverted sedimentary basin (> 200 km wide) with up to 8 kmof fluvio-lacustrine filling in various sub-basins. This work is focused on the Corque basin which recordedevolution of the central Altiplano and Neotropical terrestrial mammalian assemblages during the lateNeogene. Indeed, this area is inferred to have experienced a rise from ∼1,500 to 4,000 m, between ∼10and 6 Ma. Paleoclimates rapidly changed from warm climate at 10 Ma, with a mean annual temperature(MAT) ˜20◦C, to the current highland climate (MAT ˜8-9◦C). These series yielded both rich land mammalassemblages essential for late Neogene biochronology and interbedded volcanic tuffs ranging a ∼10–2.8Ma interval, marked by drastic faunal changes by the Miocene-Pliocene transition. A multidisciplinary studyof late Neogene deposits from the Corque basin near the village of Pomata allows for describing thesedimentary evolution, refining chronostratigraphy through Ar/Ar datings of interbedded volcanic tuffs,and implementing the corresponding fossil record. The studied section encompasses the Totora, Pomata,and Mauri Formations, with 15 interbedded volcanic tuffs suitable for dating. The Totora Fm is mainlycomposed of continental fine-grained sediments yielding desiccation features indicating temporaryemersions and some palynomorphs in finest sediments. Paleoenvironments correspond to ephemeralbraided streams and lakes. In the channels, eastward paleocurrent directions indicate that sediments camefrom the Cordillera Occidental. The Pomata Fm begins few meters above an index volcanic tuff that cropsout over the whole basin and consists of mud or sand-supported mass flow deposits yielding numerousterrestrial mammal fossils. The most conspicuous taxa are the mesotheriid notoungulate Plesiotypotheriumachirense and new echimyid and caviid rodents. The upper surface of the Totora Fm, eroded, correspondsto an angular unconformity underlying the Mauri Fm. The latter starts with an index volcanic tuff (Toba 76;5.34 +/- 0.003 Ma) and corresponds to four thinning upward fluvial sequences, each one starting withcross-bedded conglomerates/coarse-grained sandstones, passing upwards into fine-grained argillaceoussandstones. In these southward-flowing deposits, we recently found a new nothrotheriid ground sloth,Aymaratherium jeani. The erosion of previous units and the change in transport direction may be related toa major phase of relief growth.