Turtles all the way down: loggerheads at the root of the chelonian tree.

The recent discovery of the oldest and most primitive stem turtle, Odontochelys semitestacea (Li et al. 2009), from the lower Upper Triassic of China reopens the debate regarding the origin of the turtle shell, the paleoecology of basal turtles (terrestrial or aquatic), and the placement of turtles among amniotes. The debate surrounding the origin of the shell dates back to Cuvier (1800–1805) with two competing hypotheses. The de novo hypothesis, argues that the carapace (dorsal shell) formed from the outgrowth of intramembranous bone from the periost of the ribs and neurals, and this view is largely supported by developmental data on current turtles (Gilbert et al. 2001; Cebra-Thomas et al. 2005). The composite origin hypothesis argues that the shell formed largely by the fusion of overlying osteoderms with the underlying ribs, and this perspective has some histological support (Scheyer et al. 2008).