| Turtles (Testudinata) are an ideal group for phylogenetic reconstruction, because they are morphologically diverse and because they possess an extensive fossil record. Current hypotheses of turtle relationships indicate a consensus regarding the arrangement of basal turtles. In particular, the terrestrial fossil turtle Proganochelys quenstedti is thought to be the only known representative of the phylogenetic stem of turtles, whereas all other known turtle fossils are placed within the aquatic crown. However, this stability is the result of numerous simplifying assumptions that were developed prior to computer assisted phylogenetic analyses.; A phylogenetic analysis was performed to test the primary assumptions that currently govern turtle relationships. The data matrix used consists of 139 osteological characters with 174 derived character states for 45 fossil and 22 living species of Testudinata. The results of this analysis are in conflict with the prevailing consensus regarding the basal split of turtles. In particular, the primitive turtles Proterochersis robusta, Keyentachelys aprix, Mongolochelys efremovi, Meiolania platyceps, and Kallokibotion bajazidi are removed from their current position as crown turtles and placed along the phylogenetic stem of this clade. The age of the turtle crown is adjusted from Late Triassic to Late Jurassic. This revised topology has important implications regarding the evolution of several character complexes, as it implies that the common ancestor of all living turtles possessed a partially braced brain case and a primitive trochlear mechanism. Furthermore, using forelimb measurements as a proxy, fossil species situated along the phylogenetic stem of turtles are demonstrably terrestrial, indicating that the origin of turtles occurred in a terrestrial environment and that the acquisition of aquatic habitat is a synapomorphy of the turtle crown.; To facilitate communication regarding relationships among turtles, a phylogenetic nomenclature of turtles is proposed. This is the first attempt to propose a coherent nomenclatural system to replace the traditional rank-based nomenclature. Novel issues and problems that emerged during this review include the unclear name/taxon association of traditional names, the creation of synonymy lists, difficulties associated with selecting a single criterion for choosing among multiple synonymous names, and identifying authorship for any given traditional name. |
