Unpublished material of Typotheriopsis chasicoensis (Notoungulata, Mesotheriidae) from the Late Miocene Arroyo Chasicó locality (Buenos Aires Province, Argentina; Cerro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.