Anatomía de un nuevo ejemplar de tortuga (Pleurodira: Pelomedusoides) de la Formación Portezuelo (Turoniano - Coniaciano), costa norte de Lago Barreales, Neuquén, Argentina

Abstract

A new species of turtle (Pleurodira: Pelomedusoides) is introduced, recovered from the Portezuelo Formation (Turonian-Coniacian), Baal Site, on the northern coast of Lago Barreales, Neuquén, Argentina. The Baal Site represents a fluvial paleoenvironment and the biota recorded at the site is similar to that described for the Futalongko Site, also located in Lago Barreales, and for the type locality of the Portezuelo Formation, Sierra del Portezuelo. The main goal of this study is to carry out an anatomical description of the specimen MUC-Pv 2219 to expand the anatomical knowledge of the pleurodiran pelomedusoid turtles from the Cretaceous of Patagonia. MUC-Pv 2219 consists of a nearly complete skull, carapace, and plastron. The skull is articulated with the jaw and part of the hyoid apparatus. It has a length of 45 mm along the midline and a maximum width of 38 mm at the cavum tympani. The jaw is slightly displaced to the left, covering a large part of the ventral view of the skull. The skull exhibits several characteristics that relates it with the Pelomedusoides order within the pleurodiran group, including the presence of a quadratojugal bone and the absence of nasal bones. In lateral view, there is a slight elevation towards the posterior region of the skull. Additionally, the specimen shows an undetermined triangular bone in the jaw, located ventrally to the dentary. The carapace of MUC-Pv 2219 has a length of 210 mm, being suboval in shape, wider in the posterior part than in the anterior. The dorsal convexity reaches its highest point at the fourth pair of costals. The dermal plates are smooth, and the central series is complete, including the nuchal plate, which is wider at the base. The posterior margins show concave curvilinear sutures. The neural series consists of 7 neurals, the first of which is elongated and longer than wide, with an irregular hexagonal shape and shorter posterolateral edges than anterolateral edges. The second neural is quadrangular, with slightly curved lateral margins. The carapace exhibits an anomaly manifested as supernumerary peripheral plates (12 pairs). The plastron measures 183 mm and displays typical features of Pelomedusoides, such as the presence of rounded and equidimensional lateral mesoplastra, a long plastral bridge, and a wide posterior plastral lobe. A comparative analysis reveals that MUC-Pv 2219 constitutes a new taxon of Pelomedusoides, being more closely related to Elkanemys than to Portezueloemys. MUC-Pv 2219 differs from the known taxa in the Portezuelo Formation, such as Portezueloemys patagonica (Pelomedusoides) and Prochelidella portezuelae (Chelidae), thus increasing the diversity of identified turtles in this geological unit.