Parham, J.F.: New perspectives on the origin(s) of marine turtles

Because they share a large number of morphological characters, living marine turtles (Chelonioidea) are thought to have evolved from a single ancestor. All chelonioids share paddle-like limbs, a reduced shell, and peculiar arterial perforations of the braincase. However, comparative studies of distantly related clades that inhabit marine or open waters shows that many of these characters are correlated to ecology and mode of locomotion. For example, besides chelonioids, the living Carettochelys is the only other turtle with a symmetrical underwater flight stroke and it shares many limb characters considered synapomorphies of marine turtles. Furthermore, the phylogenetic analyses of long-neglected Asian fossils shows that many of the other characters are actually primitive – lost by other living turtles, but retained in chelonioids. Taken together, these data cast doubt on the hypothesis of monophyly. An independent line of evidence for multiple origins is the first appearances of marine turtle clades in the fossil record. Marine sediments preserve the pulses of separate radiations as morphologically primitive turtles became more specialized and diverse. The timing of these appearances is congruent with a multiple origin hypothesis because some putative sister taxa are separated by about 50 million years. Finally, when marine-related characters are removed, the morphology of the primitive members of these groups corresponds to the morphology of coeval terrestrial lineages. The turtle bauplan is generally conservative, and the interaction of hydrodynamic and ontogenetic factors may have contributed to similar iterations of character complexes in marine turtles.