Bayesian tip-dating and biogeographic approaches reveal multiple marine transitions in Pan-Chelonioidea and a North Atlantic origin for modern sea turtles (Chelonioidea)

Convergent evolution can undermine parsimony as a criterion for choosing among phylogenetic hypotheses, resulting in a challenge to find consensus among conflicting trees. This challenge is exemplified in the studies of sea turtle (Pan-Chelonioidea) evolution. Despite the wide interest in this iconic clade, rigorous analyses of character acquisition are currently limited by poorly resolved, largely parsimony-based phylogenies. In particular, there is uncertainty regarding the placement of the extinct family Protostegidae, which has been recovered in several different parts of the tree and even outside Pan-Chelonioidea as a separate radiation of marine turtles. The relationships among the stem of Cheloniidae are also far from a consensus, despite it having a relatively good fossil record. Here we investigate the evolution of sea turtles using a Bayesian tip-dating approach, a technique not previously applied to this group. We also conduct a biogeographic analysis using DEC and DECJ models. We find Protostegidae to be sister to the rest of Pan-Chelonioidea, implying convergent evolution of the sea turtle flipper and other marine adaptations. We find that Chelonioidea probably originated in the North Atlantic, and that the now global distribution of most sea turtle species suggests that the pelagic life phase of extant species is unique to crown Cheloniidae.