New cranial fossils of the Jurassic turtle Neusticemys neuquina and phylogenetic relationships of the only thalassochelydian known from the eastern Pacific

Abstract

Neusticemys neuquina (Fernández and de la Fuente, 1988) is a turtle from the Upper Jurassic of the Neuquén Basin, Patagonia, Argentina. Here we describe in detail a new skull, lower jaw, and a vertebra, utilizing both traditional anatomical description and computed tomography (CT). New diagnostic cranial characters of Neusticemys neuquina are: a round depression on the ventral surface of the basisphenoid, a relatively larger oval foramen nervi trigemini, and reduced and steepened triturating surfaces on both the maxilla and dentary. The new morphological information presented in this study was included in a phylogenetic analysis, the primary result of which was recovery of Neusticemys neuquina within Thalassochelydia. Characters recognized as synapomorphies of this clade include: (1) anterolateral recess of the anterior surface of the quadrate positioned lateral to the processus trochlearis oticum, (2) presence of a fossa on the supraoccipital-opisthotic-exoccipital contact area, (3) foramina anterius caroticus cerebralis located close together but independently perforating the basisphenoid, and (4) the presence of the splenial in the mandible. Two contrasting dispersal scenarios could explain how this species of Thalassochelydia can be found outside of Europe. The presence of Neusticemys neuquina in the Neuquén Basin could be the consequence of an early dispersion event, for which we lack intermediate forms, or it could be the result of a later event once the clade was already established in Europe.