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A partial mesotheriine skeleton (Notoungulata, Mesotheriidae) from the Late Miocene Casira locality, southern Bolivia, is fully described and reinterpreted taxonomically. This specimen (MNHN-Bol-V-003724) was partially described previously without a taxonomic discussion, as it was considered to be Plesiotypotherium sp. After comparison with other Bolivian, Argentinean and Chilean mesotheres, we recognize a new species of the genus Plesiotypotherium Villarroel, P. casirense sp. nov., characterized by: nasals slightly enlarged posteriorly; zygomatic arch with strong antero-distal angle; zygomatic plate starting at the level of P4 and reaching the level of the anterior part of M3; lacrimal bony process; large infraorbital foramen; posterior border of the mandible very convex; P4 without lingual groove and with narrow and projected parastyle; M3 with simple third lobe; larger and more robust skeleton than other Plesiotypotherium species. Most postcranial elements are described in detail for the first time for Plesiotypotherium. The incomplete skull GB-KKL 740 from the same locality is also referred to the new taxon. P. casirense is the sister group of P. achirense Villarroel in the phylogenetic analysis, but the genus Plesiotypotherium appears as paraphyletic. The revised partial skull GB-ACH 100 from Achiri, Bolivia, which was labelled as Plesiotypotherium sp., does not belong to this genus, and it appears phylogenetically closer to Pseudotypotherium and Mesotherium. Plesiotypotherium casirense sp. nov. increases the known diversity of the Mesotheriidae in both the Miocene of Bolivia and the South American faunal context. [ABSTRACT FROM PUBLISHER]

Cerro Patagua is a locality with fossiliferous outcrops of the Cerro Azul Formation in La Pampa Province (central Argentina). First fossil remains were known in 1999 and assigned to the Chasicoan Stage/Age (Late Miocene, Tortonian) after the presence of the Octodontoidea rodent Chasichimys bonaerense. Later references to vertebrate remains were scarce and mostly limited to some mentioned taxa in broader faunal contexts. Now, we present a detailed taxonomic study of the entire mammal assemblage from Cerro Patagua, which evidences that most taxa are typical of the Chasicoan Stage/Age. A peculiar enterolith-like structure, recovered from loess levels, is also described, and several taphonomic aspects observed on fossils are related to their level of provenance. In addition, we offer stratigraphical interpretations of the Cerro Azul Formation and a geomorphological analysis of the Cerro Patagua area, and provide a detrital zircon dating of 12.3 ± 1.8 Ma. This result reflects the time interval 14.1–10.5 Ma, a temporal range that includes the Serravalian Stage (late Middle Miocene) and could indicate that the oldest synorogenic deposits could have begun to accumulate at the end of this stage. Even considering the minimum age of this interval (Tortonian Stage), this dating is the oldest obtained for the Cerro Azul Formation, compared with previous published data from Arroyo Chasicó locality (9.43–9.07 Ma). This assignment could explain that taxa from Cerro Patagua show affinities with Late Miocene mammal assemblages from central Argentina, as well as with older faunas from Patagonia and Bolivia. • The oldest exposed level of Cerro Azul Formation (central Argentina) is described. • SynOrogenic deposits at Cerro Patagua begun to accumulate around 12 Ma. • Taxonomic evaluation indicates a Chasicoan Stage/Age for the Cerro Patagua assemblage. [ABSTRACT FROM AUTHOR]