Eunotosaurus africanus seemed to capture an early stage in turtle evolution. It had wide, flattened ribs, a shortened trunk, and a body that, to generations of paleontologists, looked unmistakably like a shell in the making.
For nearly two decades, the 260-million-year-old reptile sat near the base of the turtle family tree, offering one of the earliest clues to how turtles began evolving their shells. Then, a team of researchers looked inside its skull.
In a new study published in Current Biology, researchers used CT scanning to examine the internal anatomy of Eunotosaurus in new detail. The scans suggest the animal was not an early turtle at all, but a member of a separate extinct reptile lineage that independently evolved a turtle-like body.
The finding removes a source of confusion from the turtle family tree and helps bring fossil evidence in line with DNA studies showing that turtles are closely related to archosaurs, the reptile group that includes crocodiles, birds, dinosaurs, and pterosaurs.
Read More: Paleontologists Unearth 9-Foot-Long Turtle With Massive Spikes on its Shell
Why Eunotosaurus Changed the Story of Turtle Evolution
Eunotosaurus specimen BP-1-7852
(Image Courtesy of Dr. Jonah Choiniere/Evolutionary Studies Institute at the University of Witwatersrand)
The braincase contained primitive bones and structures that had disappeared from the reptile lineage before turtles appeared. Eunotosaurus was also missing skull and foot traits found in true early turtles and other modern reptile relatives.
Instead, the scans showed a close match with millerettids, a group of extinct stem reptiles from the Permian Period. Milleretta rubidgei, one member of that group, shared 12 anatomical features with Eunotosaurus, including broad, overlapping ribs and a shortened trunk.
Its turtle-like body shape was not inherited from a common turtle ancestor. It evolved in a separate lineage that faced a similar ecological challenge: burrowing. Digging can favor a wide, reinforced body regardless of which animal is doing the digging. Eunotosaurus was less a proto-turtle than an evolutionary look-alike, a reptile that arrived at a turtle-like solution without belonging to the turtle lineage.
Why Fossils and Genetics May Finally Agree on Turtle Origins
DNA evidence had already linked turtles with archosaurs, while fossil evidence often seemed to point elsewhere. Now, with Eunotosaurus removed from the turtle stem, those two lines of evidence line up much more closely.
The team’s analysis, one of the largest anatomical datasets yet assembled to investigate turtle origins, compared 962 traits across 226 fossil species and found strong support for turtles as early members of Archosauromorpha, the broader group from which archosaurs eventually arose.
Confirmed early turtles, including the Late Triassic Proganochelys, share specific skull features with early archosaur relatives. Eunotosaurus simply does not.
The long disagreement between genetic and fossil evidence may not have been a true contradiction after all. It may have just been the result of one fossil sitting in the wrong place on the turtle family tree.
A Possible New Timeline for Turtle Evolution
Rather than stretching back into the middle Permian with Eunotosaurus, the turtle lineage likely split from its archosaur relatives later, in the late Permian, before diversifying through the Triassic Period, around the aftermath of the largest mass extinction the planet has ever seen.
With Eunotosaurus out of the picture, the origin of the shell looks different, too. The earliest confirmed stem turtles appear in aquatic settings, including freshwater lakes and shallow marine environments. That does not rule out digging altogether, but it makes the old burrowing-first story less certain. The shell may have been shaped by more than one pressure, including life in and around water.
For nearly 20 years, Eunotosaurus seemed to explain how turtle evolution began. Instead, it may have been obscuring the picture. With the fossil removed from the turtle lineage, scientists now have a clearer path for tracing the early history of turtles.
Read More: The 5 Biggest, Ancient Turtles That Ever Lived Were Among the Dinosaurs
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Current Biology: The phylogenetic origin of turtles
- This article references information from The Conversation: Turtles finally have a place in the tree of life: X‑ray study of South African fossils was a decider
