A fossil flightless ratite (Aves, Rheidae) from the late Pleistocene of northern Mesopotamia (Corrientes province, Argentina)

In this contribution we present and describe the first record of a fossil rheiform for the Late Pleistocene of the Toropí/Yupoí Formation (Marine Isotope Stage 3; ca. 65–50 ka.) in Corrientes Province, Argentina, which represents the second avian record for this unit. The specimen includes a distal fragment of a right tarsometatarsus preserving part of the diaphysis and an associated fragment of thoracic vertebra, recovered from fluvial deposits of the lowest levels of this study. The dimensions and morphology of the material allow it to be assigned to the flightless rheid Rhea sp. Considering the noticeable climatic oscillations recorded during the Marine Isotope Stage 3 (MIS 3), in which several Heinrich (colder) and Dansgaard-Oeschger (warmer) events existed, the presence of taxa related to temperate-cold environments recovered from the Lower Member (ca. 65 ka.) of the Toropí/Yupoí Formation (i.e. Chloephaga picta, Dolichotis patagonum) not recorded in the Upper Member suggests potential colder temperatures during the deposition of the Lower Member. This record increases the taxonomic diversity of fossil birds for the Pleistocene of Corrientes, which was previously limited to the anatid Chleophaga.