Detecting morphological gaps in teeth outlines of a Pachyrukhinae (Hegetotheriidae, Notoungulata) lineage: systematic and paleobiogeographical significance of the records from Northwestern Argentina

ARMELLA, MATÍAS A.; ERCOLI, MARCOS D.; BONINI, RICARDO A.; GARCÍA-LÓPEZ, DANIEL A.

COMPTES RENDUS PALEVOL

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Año: 2022

1631-0683

Pachyrukhinae (Hegetotheriidae, Notoungulata) is a monophyletic clade highly frequent in the Late Miocene-Pliocene outcrops of southern South America. In Argentina, two genera were recognized: Tremacyllus Ameghino, 1891 and Paedotherium Burmeister, 1888. The simplified euhypsodont dentition in these Neogene forms involves important 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, proposing 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 the quantitative framework provided by geometric morphometrics and multivariate statistics to discriminating intra- from interspecific variability by teeth outline morphology. We analyzed a large sample of 82 specimens, two hypotheses were tested: (1) there are morphological gaps within the analyzed sample; and (2) morphology follows a pattern of geographical variation among them, suitable for recognition of species. We found that morphological variability is organized into two clusters. Morphological gaps are associated with geographical patterns on 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 the intra- and interspecific variation and contribute to the knowledge of available techniques to assess morphological variation.