The origin and early evolution of turtles

A critical reexamination of turtle relationships continues to support a sis ter-group relationship of turtles with a clade of marine reptiles, Sauropte rygia, within crown-group Diapsida (Sauria). The high Homoplasy Index raise s concerns about the phylogenetic information content of various morphologi cal characters in broad-scale phylogenetic analyses. Such analyses may also suffer from inadequate statements of primary homology. Several such statem ents that have played an important role in the analysis of turtle relations hips (dermal armor, acromion, astragalo-calcaneal complex, hooked fifth met atarsal) are reviewed in detail. An evolutionary scenario for the origin of the turtle bauplan suggests an aquatic origin of turtles, which is support ed not only by their sauropterygian relationships, but also by paleobiogeog raphic and stratigraphic considerations. However, turtle relationships rema in labile, and further investigations of their relationships are required, involving molecular and physiological data.