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

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

COMPTES RENDUS PALEVOL

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Lugar: Paris; Año: 2022 vol. 21 p. 323 - 348

1631-0683

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 euhypsodontdentition of these Neogene forms creates significant difficulties when cheek teeth are described forsystematic purposes. Tremacyllus has been scarcely studied in comparison with Paedotherium, andtaxonomic analyses have interpreted diagnostic features as intraspecific variations and proposed themonospecific status of the genus. Given the discussion regarding the validity of Tremacyllus speciesand the fact that dental elements are the most abundant remains in the fossil record, we employed aquantitative framework provided by geometric morphometrics and multivariate statistics to discriminatingintra- from interspecific variability by tooth outline. We analyzed a large sample of 82 specimensand 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 gapsare associated with geographical patterns in the P4 and upper premolars datasets. Based on the classificationof the type specimens and supported cluster structure, we recognize Tremacyllus incipiensRovereto, 1914 as a valid taxon, endemic from western outcrops of Northwestern Argentina. Segregationbetween northern and southern morphologies agrees with two different palaeo-phytogeographicprovinces. This approach proved to be very effective to address intra- and interspecific variation andcontribute to the knowledge of available techniques to assess morphological variation.