Skull morphology of the oldest turtles: a preliminary description of Proganochelys quenstedti

The skull of the late Triassic turtle Proganochelys quenstedti from Trossingen, West Germany, shows a number of generalized amniote features not found in any other turtles. Proganochelys retains the supratemporal bone, lacrimal bone, lacrimal duct, movable basipterygoid articulation, and a middle ear that lacks the expansion and bony walls found in other turtles. With a chelonian shell Proganochelys is clearly identified as a turtle and is hypothesized as the sister taxon to all other turtles which are in turn hypothesized as a monophyletic group, the Casichelydia, characterized by advanced features of the skull.