Paleoartistic reconstruction of a pair of the reptile Eunotosaurus africanus For years, the ancient reptile Eunotosaurus africanus has played a lead role in one of evolution’s biggest mysteries: how turtle shells evolved.
“Turtles are evolutionary oddities,” said Xavier Jenkins, a postdoctoral fellow in the Museum’s Division of Paleontology. “Their shells are unlike anything else among vertebrates, and scientists have long debated how such a dramatic body plan evolved.”
With broad, T-shaped ribs and unusual body shape, fossils of the 260-million-year-old Eunotosaurus africanus from South Africa seemed to represent a link between early reptiles and modern turtles. Many scientists interpreted the reptile as a “proto-turtle,” an early turtle relative demonstrating a transitional stage in shell evolution.
But a new study published in Current Biology and led by Jenkins and a multinational team of scientists at the Museum, the University of Witwatersrand, Iziko Museums of South Africa, and others disputes that link. Instead of being a turtle ancestor, the scientists conclude that Eunotosaurus belonged to a completely different reptile group called millerettids.
Genetic studies consistently suggest that crown reptiles—the group containing all living reptiles—originated later in the Permian. Yet Eunotosaurus lived significantly earlier. Placing it on the turtle branch meant pushing reptile origins much deeper into the past than genetic data and the broader fossil record seemed to support.
The new study resolves that conflict by removing Eunotosaurus from the turtle family tree entirely. Using anatomical analysis and CT scans of fossil skulls, the researchers found evidence linking Eunotosaurus to millerettids, an extinct group of early reptiles.
The research team suggests that the turtle-like features of Eunotosaurus evolved independently, an example of convergent evolution, where unrelated animals evolve similar traits because they adapt to similar lifestyles. In this case, the broad ribs of Eunotosaurus might have been adapted for digging.
“Eunotosaurus is not a turtle ancestor at all but an unrelated reptile that independently evolved a broadened ribcage superficially resembling an early shell,” Jenkins said. “A simple modern analogy would be turtles and armadillos, which both have armored bodies, but evolved those features independently.”
The findings reshape how scientists think about turtle evolution. For years, Eunotosaurus supported a relatively straightforward narrative in which digging reptiles gradually evolved broader ribs that eventually became shells. That story now becomes much more nuanced, possibly with early turtles first adapting to digging underground and later moving into aquatic environments.