Extant Turtles Evolutionary Research

The focus of this essay is to describe the evolutionary history of turtles and why the clade has historically been difficult to place phylogenetically. Turtles are reptiles characterised by their bone/cartilaginous shell development and are of the order testudines which includes both living and extinct species. Turtles are one of the oldest reptile groups with the earliest members dating 150 million years ago. (Hutchinson, 1996 and Walter, 2007). The order testudines is a monophyletic clade which contains 260 species of turtles within 13 families. Turtles are a paraphyletic group because their last common ancestor is shared with tortoises. Extant turtles are divided into two clades, the cryptodira and the pleurodira. The pleurodira contains 3 families of turtles that have side necks and the cryptodiras contain the remaining families that can fold their necks dorso-ventrally. Limb morphology and habitat preference of extant turtles provides evidence to support the hypothesis that turtles first evolved in a terrestrial environment and then later moved in an aquatic environment (Joyce and Gauthier, 2004). Observations made on evolution of turtle skeleton and other external characteristics like the shell also support this hypothesis (Gaffney, 1990 and Lapparent de Broin, 2001). Another hypothesis was made that suggests the turtle shell is less functionally constrained in a terrestrial habitat than it is in an aquatic habitat (Claude at al, 2003).

Depending on how they retract their necks, turtles are divided into two groups, either the cryptodira which retract their neck backwards while contacting it under their spine or the pleurodira which contract their necks to the side (Tennant,