Cranial and postcranial remains of a new species of Prochelidella (Testudines: Pleurodira: Chelidae) from 'La Buitrera' (Cenomanian of Patagonia, Argentina), with comments on the monophyly of this extinct chelid genus from southern Gondwana

The genus Prochelidella is the oldest known panchelid taxon. It is widely distributed in several Cretaceous basins throughout Patagonia, Argentina. It was previously known from three named species. A fourth species presented herein, Prochelidella buitreraensis sp. nov., preserves both cranial and postcranial material from the Candeleros Formation (La Buitrera Area) of Rio Negro Province, Argentina. Features observed in the nuchal bone (i.e. wide, trapezoidal cervical vertebrae, shallow nuchal notch) allow us to assign this specimen to the genus Prochelidella. Several specific traits (i.e. a large open retropterygoideum foramen, and a strongly developed muscle attachment site on the basisphenoid) suggest that the form from the Candeleros Formation is a new species of the genus Prochelidella. A phylogenetic analysis that includes the three best-preserved species of Prochelidella recovers this taxon as monophyletic. The Cenomanian age of this new taxon allows us to compare the panchelid turtles from the Patagonian (Neuquen) and Australian (Surat) basins.