The phylogenetic origin of turtles

The evolutionary origins of turtles remain poorly understood. Molecular systematics posit a sister relationship between turtles and archosaurs, but morphological studies return conflicting hypotheses of turtle relationships both to other living reptiles and to their closest fossil relatives. The middle Permian Eunotosaurus africanus has been widely regarded as the oldest stem turtle, which makes it instrumental in recent discussions of the origins of turtles and their highly transformed anatomy. Here, we use anatomical observations derived from X-ray computed microtomography (μCT) of Eunotosaurus and candidate stem turtles to evaluate phylogenetic hypotheses of turtle origins. Eunotosaurus has many plesiomorphies and lacks synapomorphies of crown reptiles, whereas Proganochelys, one of the oldest uncontentious stem turtles, shares similarities with archosauromorphs, especially in the endocranium. Our comprehensive phylogenetic analysis recovers Eunotosaurus as a millerettid stem reptile not related to turtles. This indicates that features shared by Eunotosaurus and turtles, notably including broadened dorsal ribs, evolved independently. These similarities are better explained by convergent evolution of fossorial ecologies rather than shared ancestry. Furthermore, we find strong support for the molecular phylogenetic hypothesis that turtles are sister to Archosauria among living reptiles, resolving long-standing conflicts between morphological and molecular datasets. By placing turtle origins among the earliest archosauromorphs in the late Permian, we provide new insights into the anatomical transformations involved in the origins of the distinct turtle body plan.