The evolutionary position of turtles revised

Abstract

Consensus on the evolutionary position of turtles within the amniote phylogeny has eluded evolutionary biologists for more than a century. This phylogenetic problem has remained unsolved partly because turtles have such a unique morphology that only few characters can be used to link them with any other group of amniotes. Among the many alternative hypotheses that have been postulated to explain the origin and phylogenetic relationships of turtles, a general agreement among paleontologists emerged in favoring the placement of turtles as the only living survivors of the anapsid reptiles (those that lack temporal fenestrae in the skull). However, recent morphological and molecular studies have radically changed our view of amniote phylogenetic relationships, and evidence is accumulating that supports the diapsid affinities of turtles. Molecular studies favor archosaurs (crocodiles and birds) as the living sister group of turtles, whereas morphological studies support lepidosaurs (tuatara, lizards, and snakes) as the closest living relatives of turtles. Accepting these hypotheses implies that turtles cannot be viewed any longer as primitive reptiles, and that they might have lost the temporal holes in the skull secondarily rather than never having had them. Living turtles (order Testudines) are among the most striking and strangest tetrapods. Early on in their evolutionary history, these reptiles evolved distinctive shells as effective defense (Burke 1989). This innovation is probably the key to their evolutionary success and persistence, and has constrained the evolution of the rest of their morphology (Lee 1996). The general Baüplan of turtles has barely changed since the Triassic (200 MYA), yet their carapaces show a great variety of shapes that reflect adaptations to terrestrial, freshwater and marine niches (Pough et al. 1998). Based on the mechanism of retraction of their heads into the shell, extant turtles are classified into two main suborders, Cryptodira (turtles that bend their neck vertically) and Pleurodira (sidenecked turtles) (Shaffer et al. 1997). Cryptodires (approximately 200 species) have a worldwide distribution (though rare in Australia) whereas pleurodires (approximately 50 species) are currently found only in South America, Africa and Australia.