Altiplano bolivien, anatomie, genre nouveau, espèces nouvelles. RÉSUMÉDeux nouveaux Mylodontinae (Xenarthra, Tardigrada, Mylodontoidea) provenant de l'Altiplano bolivien sont décrits. L'un, Pleuro lestodon dalenzae n. sp., a été découvert à quelques mètres sous un tuf volcanique servant de repère stratigraphique, la Toba 76, daté à 5,4 Ma ; il pourrait être d'âge huayquérien (Miocène supérieur) ou à la limite Huayquérien-Montehermoséen. Simo mylodon uccasamamensis n. gen., n. sp., a été découvert dans plusieurs gisements d'une formation comprenant la Toba 76 à sa base et un tuf volcanique daté à 2,8 Ma à son sommet ; il est d'âge Montehermoséen-Chapadmalaléen (Pliocène inférieur et moyen). L'étude de ces taxons montre que ce sont des Mylodontidae plus proches de Glosso therium, Glosso theridium, Kiyumylodon et Paramylodon que de Lestodon ou Th inobadistes.

The oldest known mesotheriids are referred to the genus Trachytherus, a taxon traditionally included in its own subfamily 'Trachytheriinae'. The ontogeny, morphological variation and systematics of the genus Trachytherus are still poorly understood and the monophyly of this taxon is questionable. For the first time, a large collection of basal mesotheriids (belonging to the genus Trachytherus) yielded by the Bolivian Salla beds (Deseadan, late Oligocene) and previously referred to T. spegazzinianus was included in a large-scale analysis. This has led to the recognition of a unique new species in the locality of Salla-Luribay, distinct from T. spegazzinianus: Trachytherus alloxus sp. nov. Skulls of this species are described and cheekteeth variation range throughout ontogeny is discussed. These new data are incorporated in a parsimony analysis of basal mesotheriids. The most probable hypothesis of a clade T. spegazzinianus-mesotheriines is discussed. The distinction of the new Salla species from the Patagonian species T. spegazzinianus confirms the faunal differences observed between these Bolivian and Patagonian Deseadan localities. Moreover, the assignment to Trachytherus spegazzinianus of mesotheriid remains from the Bolivian Lacayani deposits complicates the problem of these faunal differences. Further systematic investigations on the Lacayani fauna and on the age of the Patagonian localities are needed.

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Tardigrade xenarthrans are today represented only by the two tree sloth genera Bradypus and Choloepus, which inhabit the Neotropical rainforests and are characterized by their slowness and suspensory locomotion. Sloths have been recognized in South America since the early Oligocene. This monophyletic group is represented by five clades traditionally recognized as families: Bradypodidae, Megalonychidae, Mylodontidae ( †), Megatheriidae ( †) and Nothrotheriidae ( †). A new nothrotheriid ground sloth represented by a dentary and several postcranial elements, Aymaratherium jeani gen. nov., sp. nov., from the early Pliocene locality of Pomata-Ayte (Bolivia) is reported. This small-to medium-sized species is characterized especially by its dentition and several postcranial features. It exhibits several convergences with the 'aquatic' nothrotheriid sloth Thalassocnus and the giant megatheriid ground sloth Megatherium (M.) americanum, and is interpreted as a selective feeder, with good pronation and supination movements. The tricuspid caniniform teeth of Aymaratherium may represent a transitional stage between the caniniform anterior teeth of basal megatherioids and basal nothrotheriids (1/1C-4/3M as in Hapalops or Mionothropus) and the molariform anterior teeth of megatheriids (5/4M, e.g. Megatherium). To highlight the phylogenetic position of this new taxon among nothrotheriid sloths, we performed a cladistic assessment of the available dental and postcranial evidence. Our results, derived from a TNT treatment of a data matrix largely based on a published phylogenetic data set, indicate that Aymaratherium is either sister taxon to Mionothropus or sister to the clade Nothrotheriini within Nothrotheriinae. They further support the monophyly of both the Nothrotheriinae and the Nothrotheriini, as suggested previously by several authors.

A new species of sparassocynid marsupial, Sparassocynus maimarai n. sp. from the late Miocene of Maimará Formation (Jujuy Province, Argentina) is described from a left mandibular fragment with a complete p2-m4 series. It differs from the remaining species of the genus S. bahiai (Montehermosan-late Miocene/early Pliocene-of Buenos Aires Province, Argentina) and S. derivatus (Chapadmalalan and Marplatan-Pliocene of Buenos Aires Province) by its smaller size, the relatively longer m1 with respect to the m4, the presence of a lingual cingulum extended between para- and metaconid on the m1-3, and its more robust entoconids. As part of this study the taxonomic status of Sparassocynus heterotopicus (Montehermosan, Umala, Bolivia; Pliocene) was reviewed concluding that this taxon should be referred to as 'Sparassocynus' heterotopicus and considered a Didelphoidea of uncertain affinities. Sparassocynus maimarai n. sp. is the oldest records of the genus, adding new information to evaluate the origins and early diversification of sparassocynids. Sparassocynus maimarai n. sp. was recovered with precise stratigraphic control, highlighting its potential biostratigraphic significance to the temporal correlations between Maimará Formation and other Mio-Pliocene stratigraphic units from the northwestern Argentina.

Sexual dimorphism (SD) is extremely common in species that have reproductive roles segregated into separate sexes, and it has been recognized in several mammalian lineages, both extant and extinct. Sexual dimorphism is low to moderate in living sloths, but it had a more important role for extinct sloth taxa. The presence of SD in extinct sloths was first suggested at the end of the 19th century and it is now commonly advocated as a possible explanation of high intraspecific variation in many extinct sloth species. In this paper, we report the presence of SD in Simomylodon uccasamamensis, a Late Miocene to Late Pliocene sloth from the Bolivian Altiplano. We present evidence of SD in the morphology of cranial and postcranial remains, representing the earliest unequivocal occurrence of size-based SD in an extinct sloth species. Differences between sexes are mainly observed in the morphology of the feeding apparatus and general body size. Comparisons with extant large mammals allow us to hypothesize different food selection between the two sexes, with probable divergent habitat use and concomitant niche separation. This, in turn, could have represented an important selective factor for adaptation to environmental changes experienced by the Bolivian Altiplano in Late Neogene times.

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