Cranial morphology of Heckerochelys romani Sukhanov, 2006, a stem turtle from the Middle Jurassic of European Russia, with implications for the paleoecology of stem turtles

The detailed morphological description of the skull of the stem turtle Heckerochelys romani from the Middle Jurassic (Bathonian) of the Peski locality, Moscow Region, Russia, is provided herein. All cranial structures of Heckerochelys romani prove to be more advanced than in the Triassic and Early Jurassic stem turtles, but more primitive than in crown turtles; its morphology can be basic for both Cryptodira and Pleurodira. Only a few insignificant differences in skull morphology could be found between Heckerochelys romani and Eileanchelys waldmani, except for the size. Several cranial characters of Heckerochelys romani, such as architecture of the feeding apparatus and geometry of the cavum tympani, may indicate the aquatic habitat preference for this turtle, which is supported by taphonomic conditions.