A Triassic stem turtle with an edentulous beak.

The early evolution of turtles continues to be a contentious issue in vertebrate palaeontology. Recent reports have suggested that they are diapsids, but the position of turtles within Diapsida is controversial and the sequence of acquisition of turtle synapomorphies remains unclear. Here we describe a Triassic turtle from China that has a mixture of derived characters and plesiomorphic features. To our knowledge, it represents the earliest known stem turtle with an edentulous beak and a rigid puboischiadic plate. The discovery of this new form reveals a complex early history of turtles.