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Pachyrukhinae (Hegetotheriidae, Notoungulata) is a highly frequent clade in the Late Miocene-Pliocene outcrops of southern South America. In Argentina, two genera have been recognized for this span: Tremacyllus Ameghino, 1891 and Paedotherium Burmeister, 1888. The simplified ­euhypsodont dentition of these Neogene forms creates significant difficulties when cheek teeth are described for systematic purposes. Tremacyllus has been scarcely studied in comparison with Paedotherium, and taxonomic analyses have interpreted diagnostic features as intraspecific variations and proposed the monospecific status of the genus. Given the discussion regarding the validity of Tremacyllus species and the fact that dental elements are the most abundant remains in the fossil record, we employed a quantitative framework provided by geometric morphometrics and multivariate statistics to discriminating intra- from interspecific variability by tooth outline. We analyzed a large sample of 82 specimens and two hypotheses were tested: 1) there are morphological gaps within the analyzed sample; and 2) morphology follows a pattern of geographical variation within the sample, suitable for recognition of species. We found that morphological variability is organized into two clusters. Morphological gaps are associated with geographical patterns in the P4 and upper premolars datasets. Based on the classification of the type specimens and supported cluster structure, we recognize Tremacyllus incipiens Rovereto, 1914 as a valid taxon, endemic from western outcrops of Northwestern Argentina. Segregation between northern and southern morphologies agrees with two different palaeo-phytogeographic provinces. This approach proved to be very effective to address intra- and interspecific variation and contribute to the knowledge of available techniques to assess morphological variation.

This study describes two new skulls referred to Hippidion principale (Lund) and vaRíous elements of the appendicular skeleton referred to H. devillei from two classic localities from Buenos Aires province: the Salado and Quequén Grande Rivers. For taxonomic identification of the appendicular remains was performed a multivariate analysis that included the metacarpals and phalanges described and several remains recorded in various locations in Argentina and Brazil. For the determination of the skulls was made a comparative study with other skulls from different localities. These data increasing the record of Hippidion in South America and provide new evidence about the chronological distribution of two species of Hippidion. One radiometric data was obtained by the method of AMS dating to the skull from Salado River (MHM-P54) about a 14C age 14120 ± 50 BP. In general, bearing sediments at both localities are referable to the Lujanian Age and the Biozone of Equus (Amerhippus) neogeus. En este estudio se describen dos nuevos cráneos referidos a Hippidion principale (Lund) y varios elementos del esqueleto apendicular referidos a H. devillei procedentes de dos localidades clásicas de la provincia de Buenos Aires, las márgenes de los ríos Salado y Quequén Grande. Para la identificación taxonómica de los restos apendiculares, se realizó un análisis multivariante en el que se incluyeron los metacarpianos y falanges descritos y los registrados en varias localidades de Argentina y Brasil. Para la determinación de los cráneos se realizó un estudio comparativo con otros cráneos de diversas localidades. Estos hallazgos incrementan el registro de Hippidion en América del Sur y aportan nuevas evidencias sobre la cronología de sus especies en Argentina. Se realizó una datación por AMS para el ejemplar de Río Salado (MHM-P54) que ha suministrado una edad 14C 14120 ± 50 BP. En general, los sedimentos portadores en ambas localidades son referibles a la Edad Lujanense, Biozona de Equus (Amerhippus) neogeus.