Sixty-five years ago, at the height of the Cold War, the U.S. government released a short animated film about what Americans should do in the event of a nuclear attack. The film featured as its protagonist perhaps the most famous example of an animal with a tendency to “duck and cover”: the wary, protective turtle.
It may not be the giraffe's slender leaf-picker or the owl's elastic head-spinner, but the turtle's retractable neck is nothing to scoff at. When danger strikes, turtles use their flexible necks to retract their heads safely inside their shield-like shells. Evolutionary researchers have long assumed that turtles developed this trick as protection against predators, but new research suggests that its original purpose was likely not defense, but food.
Platychelys oberndorferi was a species of turtle that lived in central Europe roughly 150 million years ago, just a few million years after turtles evolved. It measured roughly 1 to 2 feet in length, and likely made its home in the wetlands and lakes of the warm Jurassic earth. However, the similarities to the majority of modern turtles end there, according to Jurassica Museum paleontologist Jérémy Anquetin, lead author of a study published yesterday in the journal Scientific Reports.
"This Jurassic turtle is actually very bizarre when you look at it," says Anquetin, referring to a shell made up of sharp knobs and a wide skull with eyes close to the nose. P. oberndorferi fossils are rare; Anquetin says he and his team came upon the fossil used in their study by chance, when a specimen from 1862 was returned to a museum in Basel, Switzerland, from New York.
Most fortuitiously, this fossil happened to have the delicate bones of the turtle's vertebrae preserved, giving the team an opportunity to compare its skeletal system to turtles of today.
Modern turtles are divided into two broad groups based on how they retract their necks: pleurodires, which turn their necks to pull their heads into their shells sideways, and cryptodires, which pull their heads straight back into their shells. Pleurodires appear to have evolved more than 200 million years ago, with cryptodires branching off around 165 million years ago.
Based on the shape of its skull and its number of vertebrae, P. oberndorferi has long been classified as a pleurodire. But when Anquetin and his collaborators modeled the extinct turtle's vertebrae, and found something puzzling. "They were shaped as they should be in a different group of turtles," Anquetin says, referring to the cryptodire-like vertebrae. "We didn't expect to find anything like that."
P. oberndorferi's vertebrae were shaped to allow the turtle to partially pull its head straight back into its shell—not to the side, like pleurodires. The partial part is key, says Anquetin. Partial retraction of the head provides no protective benefits, as a predator could easily reach and attack the turtle's curled, vulnerable head. So the idea that this kind of head retraction could have evolved for defense didn’t make sense.
"[This] had to evolve for a reason," Anquetin says. "We started to look for a reason."
A clue came in the form of two modern-day doppelgangers of P. oberndorferi—New Zealand's mata mata turtles and North America's snapping turtles, which both also have the spiked shells found on P. oberndorferi. These turtles evolved independently of each other on opposite sides of the world. And yet both use their neck retraction abilities not only for protection, but also for hunting. They represent an example of convergent evolution—two unrelated species evolving the same behavior or physical trait.
Turtles may be stereotyped as slow, herbivorous creatures, but these two species launch their necks violently forward to attack prey. (Similarly, the snapping turtle infamously lunges forward to capture small fish with the snap of its powerful jaws). To gain momentum and surprise their victims, the two species will launch their retracted heads from within their shells.
Anquetin suspects that P. oberndorferi is a case of the early evolution of this ability. Since partial retraction wouldn’t help this turtle protect itself, it makes much more sense that it developed the trait for hunting others. Being able to partially retract their necks like P. oberndorferi would let them strike at prey with more force.
This would make sense for the evolution of neck retraction in other modern cryptodires too, Anquetin adds.
Evolution often works to co-opt features that evolved for one purpose for a drastically different one, in a process known as exapation. For instance, says Tyler Lyson, vertebrate paleontology curator at the Denver Museum of Nature and Science, bird feathers were originally used to regulate body temperature, but later came to play a critical role in flying.
Similarly, the added benefit of turtle neck retraction for defense then likely evolved from this partial point, Anquetin says. Because of the significant physical changes required to allow a turtle to fully retract its neck, the process of evolving that ability likely took a very long time. "They cannot appear in one night," he says.
Brad Shaffer, an evolutionary biologist at the University of California at Los Angeles, thinks Anquetin and his collaborators are "pushing the envelope" a bit with their confidence in their hypothesis. In particular, Shaffer says he's unsure about the amount of extrapolation from a partial fossil required to reach a conclusion in this study.
"Given that there are only two neck vertebrae, they're doing an awful lot of reconstructing," says Shaffer, who was not involved in the new study. However, he adds that the theory will hopefully spark more discussion in the field of turtle research, and that it advances the debate among researchers on how rigid the boundary between pleurodires and crytodires turtles really is.
"Those early turtles—they were experimenting a bit," Shaffer says.
With such a limited pool of well-preserved early turtle fossils, Anquetin says he doesn't expect to be able to do many studies on this aspect of turtle evolution again. But he predicts that biologists will now be able to study the different feeding behaviors of modern turtles and see how their neck anatomies compare. Shaffer agrees that analyses of how turtles use neck retraction "across the turtle tree of life" will help paint a better picture of how this trait evolved.
"There must be another explanation than protection at the beginning," Anquetin says. "People will have to investigate now."