Young individuals, deciduous dentition, and rooted teeth of Late Miocene Mesotheriidae (Notoungulata) from central Argentina: new data and interpretations on a long debate

Abstract

Unpublished material of Typotheriopsis chasicoensis (Notoungulata, Mesotheriidae) from the Late Miocene Arroyo Chasicó locality (Buenos Aires Province, Argentina; Cerro Azul Formation) reveals ontogenetic changes in the cheek teeth (lingual grooves and proportions) among young individuals. Additionally, the specimen MD-CH-07-04 bears a rooted upper premolar. This sample sheds new light on the long-standing controversy regarding the deciduous/permanent mesotheriine premolars, but also raises new questions. No specimen considered as young (bearing an ‘extra’ premolar) has proved to be rooted through X-ray or CT-scan images. Among the compared mesotheriines from La Pampa Province, two specimens (Mesotheriinae indet. from Telén and Laguna Chillhué) show revealing features: one presents a rooted p4 and the other bears a permanent premolar (p4) positioned below and lingually to the rootless dp4, similarly to the holotype of ‘Pseudotypotherium carlesi’ (Huayquerías of Mendoza Province). Interpreting the rooted teeth is difficult, but their deciduous condition is herein discarded: in MD-CH-07-04, due to the occlusal morphology of both the rooted P4 and the accompanying molars compared with young individuals with P2; in the rooted p4 from Telén, its morphology is similar to that of other compared p4 without roots, and the mandibular bone does not indicate a very juvenile condition. The specimen from Laguna Chillhué and the holotype of “P. carlesi” are the first mesotheriines described with both deciduous and corresponding permanent premolars altogether. A better interpretation of these scarce cases of rooted and deciduous premolars would be possible with further findings and the revision of other large samples from single localities.