SYSTEMATIC AND BIOSTRATIGRAPHIC CONSIDERATIONS ON THOATHERIOPSIS MENDOCENSIS SORIA, 2001 (LITOPTERNA, PROTEROTHERIIDAE). The revision of the Family Proterotheriidae has led to the study of the specimen MLP 81-XI-28-1 of Thoatheriopsis mendocensis Soria, found in the so-called Anfiteatro de Cacheuta (Mendoza Province, Argentina). We expanded the diagnosis, incorporating dental characters that were not considered previously. Its origin is uncertain, but it could correspond to the finding mentioned by Rolleri in an unpublished report, in the “Mariño beds”, without totally discarding the possibility of being Minoprio the collector of the fossil. We conclude that the specimen MLP 81-XI-28-1 comes from the Mariño Formation, though it cannot be established whether it comes from the Areniscas Entrecruzadas Member or the Estratos de Mariño Member, both outcropping at Anfiteatro de Cacheuta. It constitutes in any case a new faunal element of this formation and adds to the mammals found at the Areniscas Entrecruzadas in Divisadero Largo area (Mendoza; early Miocene). The phylogenetic analysis suggests that T. mendocensis, together with the clade that contains the Huayquerian Neobrachytherium intermedium (Moreno y Mercerat), the Santacrucian Thoaherium minusculum Ameghino, and the Montehermosan Epitherium laternarium Ameghino and Eoauchenia primitiva Ameghino, is closer to Diadiaphorus majusculus Ameghino (Santacrucian Age) than to Lophogonodon paranensis Ameghino (Huayquerian Age). These results suggest that some lineages of Proterotheriidae recorded between the middle and late Miocene had an origin in an early diversification (Santacrucian), and they are unequally recorded both geographically and stratigraphically. A future analysis with a broader sampling of taxa and characters may confirm this hypothesis.