Résumé
Mesotheriid taxonomy for Monte Hermoso, Buenos Aires province, Argentina is extremely complex, with much of the controversy centered on the Late Miocene–Pliocene mesotheriine notoungulate type species Pseudotypotherium pulchrum, which is a subjective junior synonym of P. exiguum. Confusion around P. exiguum intraspecific variation is exacerbated by specimens of the Pleistocene taxon Mesotherium cristatum originally defined in Toscas del Río de La Plata. As a result, remains of Monte Hermoso mesotheres have been referred to six species and four genera (Typotherium, Pseudotypotherium, Xenotherium, and Typotheriodon) during the past 100 + years. The current consensus is that Pseudotypotherium is the correct genus for Monte Hermoso mesotheres, but detailed analyses of variation, especially ontogenetic, have been lacking. We rectify that deficiency in this work through bibliographic and firsthand study of type materials and comparison of molar size (PCA of linear dimensions) and i1 ontogenetic variation (via CT) using a broad ontogenetic series. We conclude that the variation observed in Monte Hermoso mesotheres is consistent with ontogenetic and individual variation within a single species. As previously suggested, Pseudotypotherium (=Typotherium) exiguum is the valid senior synonym for Monte Hermoso mesotheres. We provide a detailed diagnosis and description of P. exiguum, including a rather complete ontogenetic series, the first for any species of Pseudotypotherium.