The patterns of phylogenetic differentiation in the closely related sea turtle families Toxochelyidae and Cheloniidae followed strikingly different courses. In the Toxochelyidae there is a gradual increase in specialization toward a pelagic mode of life with time. The phyletic picture is thus that of a shrub. In the Cheloniidae, by contrast, the geologically oldest representatives of the family are the most advanced, pelagic species, and the Recent species, except for Eretmochelys imbricata, are among the least specialized members of the family. There is good evidence that the living species are the only cheloniids that have their hind limbs modified as rudders, and the very close similarity between the limb skeletons of these species suggests that they are more closely related to each other than any of them are to any earlier form. A similar case can be made for the early Caenozoic cheloniids. The phylogenetic pattern, therefore, suggests repeated episodes of pelagic specialization from a central, unspecialized stock, a pattern referred to as iterative evolution.