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

## TL;DR

Fossil sea turtles from the Cretaceous show that scale loss is an ancient trait linked to oceanic adaptations, and it evolved multiple times in different sea turtle lineages.

## Contribution

The study uses the oldest known sea turtle soft tissues to clarify the evolutionary history of scale loss in chelonioids.

## Key findings

- Scale-less skin is an ancient trait in sea turtles, not a recent adaptation.
- Early sea turtles had scale-less flippers but scuted shells, showing partial scale loss.
- Scale loss evolved repeatedly in chelonioid lineages during oceanic invasions.

## Abstract

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.

•Scale loss is a convergent trait linked to aquatic specialization in reptiles•Some earliest sea turtles had both scale-less flippers and scuted shells•Scale retention in modern sea turtles is ancestral and possibly habitat related•Scale loss evolved repeatedly with oceanic lifestyles in sea turtles

Scale loss is a convergent trait linked to aquatic specialization in reptiles

Some earliest sea turtles had both scale-less flippers and scuted shells

Scale retention in modern sea turtles is ancestral and possibly habitat related

Scale loss evolved repeatedly with oceanic lifestyles in sea turtles

Paleobiology; Paleontology; Zoology