New insights into the upper and lower deciduous dentition of Pyrotherium from the late Oligocene of South America

Abstract

The Order Pyrotheria is one of the most bizarre and least-known groups of South American native ungulates, and its biochron extends from the early? Eocene to the late Oligocene. The emblematic genus of this order is Pyrotherium, which is known by several maxillae, mandibles, and postcranial bones of adult individuals. In this paper, we describe the most complete juvenile mandible of Pyrotherium found to date, MPEF-PV 2581, which bears dp3–m1 and alveoli of di1–di2. The study of this specimen, re-study of previously published specimens (MACN-A 52-290 and MNHN-F-DES 104), and study of unpublished deciduous upper and lower cheek teeth from several collections allows us to designate MLP-PV 13-3 as the lectotype of P. romeroi, to establish dental age stages for Pyrotherium, to differentiate two juvenile stages (early and late), and to define differences between deciduous and permanent upper premolars. Our observations show that adult individuals of Pyrotherium have a narrower and posteriorly expanded mandible (due to the increase in the size of the tusk-like teeth) and a straighter mandibular incisura. We also observe that di1, di2, and dp2 are present in juveniles but not in adults, except for the i2. X-ray imaging suggests that delayed dental eruption is not present in the juvenile mandible, whereas cheek tooth wear suggests an anteroposterior eruption sequence in p3 through m3. This work adds new information about the dental age stages of Pyrotherium and proposes the following modified deciduous dental formula: dI ?/2, dC ?/0, dP 3/3.