Intraspecific craniomandibular and dental analysis of Pseudotypotherium exiguum (Mesotheriidae, Notoungulata) from Monte Hermoso, late Neogene, Buenos Aires Province, Argentina

Abstract

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.

Redefining the Huayquerian Stage (Upper Miocene to Lower Pliocene) of the South American chronostratigraphic scale based on biostratigraphical analyses and geochronological dating

“…This biozone is characterized by the joint presence of three diagnostic taxa, Pseudotypotherium subinsigne (Rovereto, 1914), Macroeuphractus morenoi (Lydekker, 1894), and Lagostomus pretrichodactyla (Rovereto, 1914) (Figs 6N, 7T, 8D; and see diagnosis from Rovereto 1914, but see Fernández‐Monescillo et al . 2022; Scillato‐Yané 1982; Rasia 2016, respectively), which together constitute a fossil association present in the Late Miocene and Early Pliocene. The co‐occurrence of these three taxa is not found in older deposits of Chasicoan age nor in younger deposits of Montehermosan age.…”