The small Permian reptile Eunotosaurus africanus was once thought to be an early turtle ancestor. © Gabriel Ugeuto
An ancient reptile once thought to be an early turtle ancestor actually belonged to an entirely different group, according to new research.
The findings are reshaping scientists’ understanding of where turtles fit on the reptile family tree and how their iconic shell may have evolved.
New research is helping to settle where turtles sit in the tree of life.
While the relationships between most vertebrate groups are well understood, the evolutionary origins of turtles have long been debated. DNA studies have identified turtles as close relatives of archosaurs, the group that includes dinosaurs, birds and crocodiles, but fossils suggest they descend from much earlier reptiles.
A new study, published in the journal Current Biology, has compared both early shelled turtles and their shell-less predecessors to better understand how turtles evolved. It found strong support for the relationship between turtles and archosaurs, as genetic evidence had indicated.
Dr Serjoscha Evers is a turtle expert from the Bavarian State Collections of Natural History and coauthor of the study.
“Our results show that turtles do indeed belong to the group of archosaurs and occupy a very basal position in the family tree,” he says. “At the same time, we can better constrain the timing of their origin 255 million years ago toward the end of the Permian.”
Eunotosaurus had wide, flattened ribs that formed broad plates, similar to an early turtle shell. © Jonah Choiniere
One of the main reasons for the uncertainty surrounding the position of turtles in the tree of life was due to a small reptile, Eunotosaurus africanus. This animal lived around 260 million years ago during the Middle Permian.
Eunotosaurus had wide, flattened ribs that formed broad plates, similar to an early turtle shell. However, as it lacked the openings modern turtles have on the side of their skull, researchers suggested it might be a proto-turtle that bridged the gap between primitive reptiles and turtles.
If Eunotosaurus was a step in the evolution of turtles, its other traits suggest these animals originated from an older-branching group of reptiles. Its occurrence in the Middle Permian would also push the origin of reptiles further back in time than is currently suggested by molecular data.
To investigate this, the international team of palaeontologists described a new specimen of Eunotosaurus discovered in 2016. This revealed crucial details of the animal’s skull that told a different story. Instead of belonging to the turtle lineage, the researchers now place Eunotosaurus among an extinct group of early reptiles called the millerettids.
It turns out that the broad ribcage that linked Eunotosaurus to turtles is actually found in other millerettids, so likely evolved independently in both groups.
Dr Mike Day, one of our curators of fossil reptiles and a co-author of the study, was part of the field team that discovered the specimen in 2016.
“The resemblance of Eunotosaurus to a theoretical turtle ancestor was noticed when it was first described in 1892, based on a specimen still in the collection I look after today,” Mike explains. “Opinions on this relationship changed as more fossils were discovered but, despite this, the idea that Eunotosaurus was a stem turtle has been increasingly debated.”
“Our new data shows that Eunotosaurus was not an ancestor of turtles after all but rather that the similarities are the result of convergent evolution.”
The broadened ribcage of Eunotosaurus, along with other features such as its robust claws, suggests it was likely a burrowing reptile. © Gabriel Ugeuto
As well as clarifying where Eunotosaurus belongs in the tree of life, the detailed fossil analysis also offers new insights into early turtles and the evolution of their shell. The research team used high-resolution CT scanning to examine fossils in unprecedented detail.
Dr Xavier Jenkins, from the American Museum of Natural History, was the study’s lead author.
“By peering inside the skull of Eunotosaurus and undisputed fossil turtles with this cutting-edge technology, we could see never-before-seen aspects of their anatomy,” says Xavier.
“The combination of modern CT technology, the expertise of an interdisciplinary team and our collaboration with scientists from South Africa was crucial to solving the problem of turtle origins.”
The broadened ribcage of Eunotosaurus, along with other features such as its robust claws, are similar to those seen in digging animals like armadillos. This suggests Eunotosaurus was probably a burrowing reptile.
Analysis of early turtle relatives with partially developed shells, meanwhile, seem to show they had adaptations for aquatic environments. The researchers speculate that the turtle shell may have evolved in water from ancestors that were originally adapted for burrowing.
While there’s still some way to go to resolve the origin of turtles once and for all, this research brings us one step closer to understanding the enigmatic evolution of these animals.
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