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We report systematic conclusions based on critical analyses of previous taxon diagnoses and systematic revisions of the Early-Middle Pleistocene mesotheriine notoungulate  Mesotherium cristatum , from Buenos Aires province, Argentina.  Mesotherium cristatum  is a key taxon, as it is the guide species of the Ensenadan Age (Early-Middle Pleistocene), it typifies the ‘ Mesotherium cristatum  Biozone’, and it is the last member of the Typotheria lineage. The taxonomic history and its successive revisions are especially complex due to the lack of recognition of the ontogenetic variation for this taxon, often considered as intergeneric variations ( Typotherium ,  Typotheridion ) or also as inter-specific variations (a total of seven different species have been named). In fact, the taxonomic complexity was increased due to misidentification with specimens or taxa originally defined in the late Pliocene (i.e. Monte Hermoso, Montehermosan SALMA). We conclude that the variation among the numerous purported Early-Middle Pleistocene mesotheriine taxa is consistent with intraspecific and ontogenetic variation in a single species, here recognised as  M. cristatum . This puts an end to the supposed sympatry of two other species ( Mesotherium maendrum  and  Typotherium pachygnatum ) with  M. cristatum  and other species that have scarcely been considered ( Typotherium angustirostrum  and  Bravardia pampaea ). Accordingly,  Mesotherium  is a monotypic genus.

historical biology

Taxonomic history and intraspecific analysis of  Mesotherium cristatum  (Mammalia, Notoungulata, Mesotheriidae) from the Early-Middle Pleistocene of Buenos Aires Province, Argentina