The turtle plastron
(= lower shell) has its origins in the dermal bones of related reptiles. The interclavicle, renamed entoplastron (= gular) in turtles, is incorporated (Fig. 1) in Odontochelys (Li et al. 2008). The clavicles, renamed the epiplastra in turtles, are also incorporated. More can be learned with Proganochelys, a basal turtle, likewise from the Late Triassic.
modify these patterns somewhat. Pleurodire turtles add an extra scute, the intergular).
Earlier we looked at the origin of turtles in Stephanospondylus, an early splinter from the pareiasaur lineage derived from millerettids. Turtle relationships, and many others, are clarified in the large reptile tree.
Li C, Wu X-C, Rieppel O, Wang L-T and Zhao L-J 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456: 497-501.