Cretaceous sea turtle soft tissues clarify ancestry of scale loss in chelonioids

Scale loss is a quintessential hydrodynamic adaptation in marine reptiles, and paralleled the pelagic specializations of Mesozoic ichthyosaurs, plesiosaurs, and metriorhynchid crocodylians, as well as the modern Leatherback Sea turtle (Dermochelyidae). By contrast, modern hard-shelled sea turtles (Cheloniidae) retain both scutes and scaly flippers, despite evolving from among partially scale-less antecedents after the earliest Eocene, ∼54 million years (Ma) ago. Here, we resolve the ambiguous ancestry of scale loss using the oldest known sea turtle (total-group Chelonioidea) soft tissues preserved in a mid-Cretaceous (middle-to-upper Cenomanian, ∼97 Ma) protostegid (basally divergent chelonioid) from Lebanon. This fossil combines scale-less flipper skin with a scuted carapace similar to other extinct chelonioids, but confirms lineage specific rather than ubiquitous scale loss in an ancestral states analysis. Scale-less skin is therefore an ancient sea turtle trait that was repeatedly modified from scaly ancestors within disparate chelonioid clades during their recurrent independent invasions of oceanic environments.