The youngest species of Polysternon: A new bothremydid turtle from the uppermost Maastrichtian of the southern Pyrenees

Shell remains of a new species of Polysternon (Chelonii, Bothremydidae) have been recently discovered in the new locality of Barranc de Torrebilles (late Maastrichtian, southern Pyrenees). The specimens studied share features with Polysternon provinciale and ‘Foxemys’ mechinorum and reveal some exclusive characters such as: neural 1 with concave lateral margins; first marginal scutes trapezoidal; posterior margin of the second marginal very close to the suture between peripheral 1 and costal 1; anterior margin of the first vertebral rounded; and anterior lobe of the plastron emarginated. The shell morphology, taphonomy and sedimentology suggest that the proposed species, Polysternon isonae, was fully adapted to freshwater and inhabited fluvial meandering river environments. The Barranc de Torrebilles site is located in the uppermost part of the Maastrichtian succession in the Tremp Basin, only 35 m beneath the unit that contains the Cretaceous/Palaeogene boundary. Thus, P. isonae may be among the last representatives of this genus before its extinction at the end of the Cretaceous Period, according to the current knowledge of the fossil record.