called the "Polka Dot Ancestor" by Olivier Rieppel. The theory accounted for the evolution of fossil pareiasaurs from Bradysaurus to Anthodon, but not for how the ribs could have become attached to the bony dermal plates. More recent discoveries have painted a different scenario for the evolution of the turtle's shell. The stem-turtles Eunotosaurus of the Middle Permian, Pappochelys of the Middle Triassic, and Eorhynchochelys of the Late Triassic lacked carapaces and plastrons but had shortened torsos, expanded ribs, and lengthened dorsal vertebrae. Also in the Late Triassic, Odontochelys had a partial shell consisting of a complete bony plastron and an incomplete carapace. The development of a shell reached completion with the Late Triassic Proganochelys, with its fully developed carapace and plastron. Adaptations that lead to the evolution of the shell may have originally been for digging and a fossorial lifestyle. The oldest known members of the Pleurodira lineage are the Platychelyidae, from the Late Jurassic. The oldest known unambiguous cryptodire is Sinaspideretes, a close relative of softshell turtles, from the Late Jurassic of China. During the Late Cretaceous and Cenozoic, members of the pleurodire families Bothremydidae and Podocnemididae became widely distributed in the Northern Hemisphere due to their coastal habits. The oldest known soft-shelled turtles and sea turtles appeared during the Early Cretaceous. Tortoises originated in Asia during the Eocene. A late surviving group of stem-turtles, the Meiolaniidae, survived in Australasia into the Pleistocene and Holocene. External relationships The turtles' exact ancestry has been disputed. It was believed they were the only surviving branch of the ancient evolutionary grade Anapsida, which includes groups such as procolophonids and pareiasaurs. All anapsid skulls lack a temporal opening while all other living amniotes have temporal openings. It was later suggested that the anapsid-like turtle skulls may be due to backward evolution rather than to anapsid descent. Fossil evidence has shown that early stem-turtles possessed small temporal openings. Some early morphological phylogenetic studies have placed turtles closer to Lepidosauria (tuataras, lizards, and snakes) than to Archosauria (crocodilians and birds). By contrast, several molecular studies place turtles either within Archosauria, or, more commonly, as a sister group to extant archosaurs, though an analysis conducted by Tyler Lyson and colleagues (2012) recovered turtles as the sister group of lepidosaurs instead. Ylenia Chiari and colleagues (2012) analyzed 248 nuclear genes from 16 vertebrates and suggested that turtles share a more recent common ancestor with birds and