What, Exactly, Is a Turtle?

For decades, paleontologists, herpetologists, and geneticists have all struggled with a surprisingly tricky question: what, precisely, is a turtle?

You might think the answer is easy: turtles are reptiles with a bony carapace formed from their ribs and spine, which they can pull their head and legs into whenever they hobble into trouble. But while fossil evidence makes it relatively easy to trace the origins of other reptile families through the great tree of life, turtles have remained a stubborn enigma. Their anatomy—from their shells to their skulls—is so strange and singular that for decades, nobody was sure where they belonged.

“It's an incredibly great thing that turtles aren't an extinct reptile lineage because then they would just be this weird mystery that nobody really cared about,” says Brandon Peecook, a National Geographic Explorer and paleontologist previously from the Idaho Museum of Natural History and now at the Denmark Natural History Museum. “But because they're alive, it's so important to figure out what they are.”

Now, turtles are coming out of their shell, thanks in part to a study released earlier this summer in the journal Current Biology. In it, Peecook and his colleagues united fossil evidence with pre-existing genetic clues to lock down turtles’ closest relatives: crocodiles and birds, not lizards and snakes.

(Fossilized egg from prehistoric giant turtle reveals baby inside)

That means they are most closely related to the large reptile family known as archosaurs, which also includes dinosaurs and pterosaurs, rather than lepidosaurs, which include lizards, snakes, and the tuatara. The finding helps solve a longstanding fossil mystery and helps researchers better understand how turtles evolved.

“I was fist pumping when I saw this,” says James Parham, a paleontologist at California State Fullerton who was not involved in the study.

Parham specializes in turtle evolution and even coined the name “Archelosauria” for the combined family of turtles and archosaurs back in 2015, but wasn’t sure he’d live to see the fossil evidence and genetic data line up. “Archosaurs are always the sexiest group, and turtles were always kind of weird and out there. And now it’s like ‘Ah, we’re right next to you, buddy.’”

The Genetic Clues

Since the late 1800s, one of the oldest known turtles was Proganochelys, a 210-million-year-old species from Germany that already had recognizable turtle anatomy, says Xavier Jenkins, a paleontologist at the American Museum of Natural History in New York and lead author of the study. But figuring out who its ancestors were was tricky.

Turtle shells—formed out of ribs and vertebrae that have thickened into a surrounding box—are unlike anything else in the fossil record. And while most other reptile groups have two holes at the back of their skull, turtles have none—a trait that they share only with much older reptile groups from the Permian, some 260 million years ago. The obvious answer, many paleontologists concluded, was that turtles’ ancestors were likely much more primitive than those of other reptiles. But nobody could agree on which group of extinct reptiles those ancestors might be, leading some researchers to tentatively place them closer to the lizard and snake side of the reptile family tree.

(The ‘Blessed Lizard’ That Inherited the Earth After Its Worst Apocalypse)

Meanwhile, during the 20th century, herpetologists studying living turtles kept noticing details in their soft tissues—like the anatomy of their hearts and windshield-wiper third eyelids—that looked like those seen in archosaurs, or “ruling reptiles,” which include crocodilians, pterosaurs, and dinosaurs like birds. A few paleontologists picked up on this and suggested that turtles might have evolved a seemingly more primitive anatomy, Jenkins says, but their suggestions didn’t make much of a dent. 

That changed in the 1990s, as geneticists studying the genomes of turtles and other reptiles increasingly came to similar conclusions.

In paleontological circles, “a lot of people kind of laughed about it,” says Tyler Lyson, a National Geographic Explorer and paleontologist at the Denver Museum of Nature and Science, who was not involved in the research. After all, none of the fossil anatomical data from early turtles seemed to put the family anywhere near crocodiles and birds. However, as more genetic studies came out finding the same thing, he adds, paleontologists began to take it more seriously.

But while the genetic evidence showing that turtles were related to archosaurs was unequivocal, only fossils could clarify how they were related, and the available fossil evidence seemed to point in a totally different direction. As far as anyone could tell at the time, none of the early turtle remains, such as Proganochelys, resembled early archosaurs. Neither did proto-turtles, such as the 220-million-year-old Odotonchelys, found in 2008, and the 240-million-year-old Pappochelys, discovered in 2015.

In 2010, Lyson and a team of researchers identified a possible turtle ancestor in the broad-ribbed early reptile Eunotosaurus. But while its broad, flat ribs and skull looked turtle-like, it dated back 260 million years, which didn’t align well with the timing of archosaur evolution. And it didn’t seem to be related to archosaurs, either.

“It just didn’t click,” says Peecook. “It just didn’t quite make sense.”

The Fossil Evidence

So in 2021, Jenkins, Peecook, and their colleagues decided to try and tackle the problem. The team carefully re-examined all available early turtle fossils, some newly discovered and others that had been in museum collections for decades. They scanned them under a synchrotron X-ray to look at details of the braincases and inner ears—features of bony anatomy that are extremely slow to evolve. They did the same for a huge number of other reptiles, extinct and modern, including other suggested turtle relatives such as knobby-faced Permian reptiles called Pareiosaurs, as well as early reptile groups nobody seriously considered as turtles.

(CSI: Cretaceous. A reptile body farm is shedding light on how dinosaurs died.)

As they gathered more detailed data, Jenkins says, they soon found that earlier paleontologists, expecting turtles to be primitive reptiles, had misidentified important anatomical features. When they scanned the 210-million-year-old Proganochelys, as well as other Triassic proto-turtles, they found that their braincases closely resembled those of archosaurs.

In every test the team ran on their new fossil data, the outcome was unequivocal: turtles were, in fact, closely related to archosaurs. At the same time, they found that the supposed turtle ancestor, Eunotosaurus, was a red herring and not related to the turtle family at all.

The study helps pin down the likely timing of turtles’ appearance on the evolutionary tree, Peecook and Jenkins argue. They say the family probably split off from the ancestors of crocodiles and dinosaurs around or shortly before the general frenzy of reptile evolution that followed the devastating Permian mass extinction 252 million years ago, also known as “The Great Dying.” The new data could also help the researchers unearth the turtle family’s earliest ancestors.

“We’re eventually going to find animals that pretty much look like a little monitor lizard, but they’ll have some detail in their brain,” Peecook says. “And we’ll know that it’s an early turtle.”

('Ridiculously rare' case of cancer found in fossilized turtle leg)

Lyson praised the research, noting that it did a good job of finally uniting fossil data that had long seemed at odds with the story told by genetics.

“That’s a real contribution and a real step forward,” he says. Though he disagrees with the study’s conclusion that Eunotosaurus is not a turtle ancestor.

For Peecook, the research brings a touch of magic to a common wildlife photo: turtles sunbathing on a log or a riverbank, near crocodiles and stalking herons or other waterfowl. All of them, it turns out, are descended from the same common ancestor, and all of them have taken on radically different shapes over the last 200 million years.

“Humans have a global internet. We’ve been to the moon. We have a space station,” Peecook says, “And until just recently, we still didn't really know what turtles are.”

Now, finally, we do.