A nesting site and egg morphology of a Miocene turtle from Urumaco, Venezuela:: Evidence of marine adaptations in Pelomedusoides

Fossil eggshells from the Late Miocene Urumaco Formation of north-western Venezuela are reported. Stereomicroscopy and scanning electron microscopy were used to examine shell gross morphology and ultrastructure, respectively. Diagnostic turtle features in the eggshells include a distinct pattern of crystal aggregations within the shell units. The eggs had an elliptical shape with a maximum diameter of c. 56.5 mm and width of c. 43.5 mm, as measured in a specimen preserving the egg's outline. Scattered clutches of broken eggshells were found embedded in one horizon of a coarse, sandy sediment, with grains not well sorted containing foraminifera and fragments of bivalve shells. The sediments are rich in ichnofossils and the reddish colour of the sandstone indicates an oxidizing environment. These facts suggest that the eggs were deposited in a beach directly facing the sea or brackish waters, perhaps near a river delta or lagoon, as is typical of the Urumaco sequence. In the same stratigraphical layer and next to one of the egg clusters, a carapace assignable to the pelomedusoid Bairdemys venezuelensis was found, suggesting that this species was a colonial nester that laid its eggs in beaches and lived in a marine or nearshore marine environment.