Scientists have found turtles share common roots with the ancestors of birds and crocodiles

Now an international team of researchers has re-examined the South African fossils using computed tomography - essentially a very detailed X-ray.

The work is published in the journal Current Biology. The authors say the fossil evidence now better matches genetics: turtles, birds and crocodiles are related through a common ancient ancestor.

Important: this doesn't mean that turtles evolved from birds or crocodiles. It's about an older evolutionary fork. These groups shared a common ancestor, and then their lineages took different paths.

Details

The protagonist of the study was Eunotosaurus africanus, a small ancient reptile about 30 centimetres long. It lived about 260 million years ago in what is now South Africa and Malawi. In the past, it was often thought of as a sort of "proto-turtle" because it had broadened ribs and a wide body, remotely resembling the beginnings of a carapace.

New work has cast doubt on this theory. The scientists studied almost all available Eunotosaurus specimens from South African collections and compared them to other ancient reptiles and early turtles. In total, the analyses included anatomical features from more than 200 fossil species.

Of particular importance were not the ribs, but the bones around the brain and inner ear. These details had previously been difficult to see because they are hidden inside the skull. Computed tomography allowed them to be seen without destroying the fossils.

That's where the researchers found the key differences. Eunotosaurus turned out to have very primitive skull features that real turtles and modern reptiles don't have. For example, it retained bones in the back of the skull that were lost in turtles and reptiles. Meanwhile, in undisputed ancient turtles such as Proganochelys quenstedti, the structure of the brain box more closely resembled the lineage associated with the ancestors of birds and crocodiles.

Simply put, externally Eunotosaurus may have appeared similar to an early turtle, but the internal structure of the skull says otherwise. Its resemblance to turtles could have been deceptive rather than a direct indication of kinship.

The authors believe that Eunotosaurus was not a direct ancestor of modern turtles. Instead, it probably belongs to a much older branch of reptiles that has no living representatives today. And true turtles, according to the new data, fit better next to archosaurs, a large lineage that includes crocodilians, birds, dinosaurs, and pterosaurs.

Why it matters

This study helps resolve a long-standing conflict between genetics and the fossil record. The DNA of modern turtles has long drawn them closer to birds and crocodiles, but the anatomy of ancient finds has looked more confusing. Now the authors show that when the bones are analysed in more detail, the contradiction becomes less.

For science, this is an important example of how new methods change old conclusions. Previously, researchers saw Eunotosaurus as a possible ancestor of turtles because of its wide ribs. But CT scans have allowed them to look inside the skull and compare features that were previously nearly inaccessible.

This is not a "definitive point" in the story of the origin of turtles. Their evolution remains a complex topic, and the group's origins are still described as a subject of scientific debate. But the new work provides a strong argument in favour of turtles being closer to the lineage of birds and crocodiles than previous anatomical evidence had made it seem.

Background

Turtles look like a special branch of life. Their shell is formed by more than just "armour" on top of the body: it is connected to the ribs, spine and other parts of the skeleton. Because of this, it was hard to tell from the bones which reptiles they were closer to.

Over the years, scientists have offered different versions. Some brought the turtles closer to very ancient reptiles, others to lizards and their relatives, and still others to a line of birds and crocodiles. Genetic studies more often supported the last version: turtles are part of a broader group close to archosaurs.

The problem was that the fossil turtles and their possible relatives looked too unusual. The shell and rearranged skeleton may have hidden signs of kinship. The new work shows that not only the external bones but also the hidden structures of the skull are particularly useful for such cases.

Source

Research: Xavier A. Jenkins et al, "The phylogenetic origin of turtles", Current Biology, 2026.