Cranial variation in Bairdemys turtles (podocnemididae: Miocene of the Caribbean region) and description of new material from Urumaco, Venezuela

Abstract

Synopsis Four new skulls referable to Bairdemys, from the Late Miocene Urumaco Formation in Venezuela, are described and compared with other specimens allocated to this genus. Several characters previously used to distinguish B. hartsteini (Cibao Formation, middle Miocene of Bayamon, Puerto Rico) from B. venezuelensis (Urumaco Formation) are present in the new Urumaco specimens, which show a mosaic of features from both species as originally diagnosed. Features of another Bairdemys skull from the Castillo Formation in the early Miocene of Lara State, Venezuela are unique, suggesting this is a new species, but poor preservation of known specimens prevents a formal diagnosis. In Bairdemys from Castillo the ventral surface of the secondary palate is proportionally larger in size, correlated with a smaller width of the internal nares and the distance between the foramina palatinae is relatively shorter. The ventral position of the quadrate in relation to the basisphenoid is far more posterior in the Castillo Bairdemys than in any other known specimen of this genus. The lower jaw of a Bairdemys specimen from Urumaco is described for the first time and compared to that of several Recent and fossil Pelomedusoides taxa. The specimen from Urumaco has a relatively large triturating surface, a large and wide dorsal opening of the fossa meckelii, a curved tomial ridge that slopes dorsally, a curved labial ridge that is less acute than in Podocnemis and no obvious accessory ridges comparable to ridges in Podocnemididae. Several shared features separate Bairdemys and Stereogenys from other fossil and Recent Podocnemididae.