A new early Miocene Mesotheriidae (Notoungulata) from the Mariño Formation (Argentina): Taxonomic and biostratigraphic implications

Abstract

Old and new specimens from the early Miocene middle member of the Mariño Formation (Mendoza, Argentina) allow for a reinterpretation of a controversial Mesotheriidae taxon. Earlier known remains come from the Divisadero Largo locality and were, depending on the specimen, either previously described as Trachytherus? mendocensis or referred to a coeval taxon from Chile, Altitypotherium chucalensis. Based on new specimens, here described in detail for the first time, we propose that all mesotheriid material from Divisadero Largo, as well as new material from the Potrerillos locality belong to a new mesotheriid taxon. Recognition of the same taxon from these two localities establishes a sound biostratigraphic correlation. The combination of morphological characters leads to the proposal of a new generic name, Rusconitherium, for Trachytherus? mendocensis, establishing Rusconitherium mendocense New genus and New Combination. This nomenclatural act is due to the fact that T.? mendocensis, formerly thought as a Trachytheriinae from the Paleogene Divisadero Largo Formation, actually comes from the Mariño Formation and belongs to the same taxon as the new material. The missing holotype of T.? mendocensis prompts us to propose a neotype and a new diagnosis for this species, which better characterize the Santacrucian mesotheriids from Mendoza. A phylogenetic analysis does not support a close relationship between this species and Altitypotherium (or any other mesotheriine genus) counter to a previous hypothesis. An unnamed new taxon from the Chinches Formation (San Juan Province) could belong to Rusconitherium, but its full description is pending, as well as a thorough revision of most Mesotheriinae members. Sediments at Potrerillos reveal two paleoenvironmental scenarios: dune fields, influenced by volcanic events, and capillary water that provided humid conditions encouraging vegetation and fauna.