Osteology of the Middle Triassic stem-turtle <i>Pappochelys rosinae</i> and the early evolution of the turtle skeleton

<i>Pappochelys</i> extends the fossil record of Triassic stem-turtles back by some 20 million years, and provides important new insights into how and in what sequence the key features of turtles arose. It shares various derived features with the early Late Triassic stem-turtle <i>Odontochelys</i>, such as T-shaped ribs, a short trunk, and features of the girdles and limbs. At the same time, it is more plesiomorphic in retaining a fully diapsid skull and in lacking both neurals and a plastron with plate-like elements. <i>Pappochelys</i> provides important new evidence that the carapace and plastron of turtles are distinct moieties that evolved independently from each other. The skull of <i>Pappochelys</i> is more or less triangular in plan view. The marginal dentition comprises peg-like teeth with subthecodont implantation. Whereas the upper temporal fenestra is relatively small and rounded, the lower temporal opening is large and ventrally open. The slender vomers are fused. The vertebral column of <i>Pappochelys</i> comprises probably eight cervical, probably nine dorsal, two sacral, and more than 24 caudal vertebrae. The dorsal ribs have asymmetrical anterior and posterior flanges and their external surfaces are heavily ornamented. The gastralia form pairs of robust elements, one per vertebral segment. The scapula has a slender dorsal shaft and a rounded ‘acromial’ flange. The humerus has a slightly dorsally raised proximal head and a gently sigmoidal curvature. The pubis closely resembles that of <i>Odontochelys</i> in overall shape and has a well-developed lateral process. We performed a phylogenetic analysis using a modified character-taxon matrix (272 characters, 50 taxa) from a recent study to assess the placement of <i>Pappochelys</i> among amniotes. <i>Pappochelys</i> was found to nest among Pantestudines with <i>Eunotosaurus</i> and the more derived <i>Odontochelys</i> and <i>Proganochelys</i>. The diapsid affinities of turtles are well supported but it is still unresolved whether turtles are saurians or only related to the latter.