Study discovers why turtles evolved to hide in their shells

The ability to quickly snatch prey, not the need for protection from predators, was probably the reason turtles developed the ability to retract their heads into their shells, according to new research published online last Thursday by the journal Scientific Reports.

As part of their study, co-author Jérémy Anquetin, a palaeontologist at the Jurassica Museum in Switzerland, and his colleagues analyzed the cervical bones of a 15-million-year old turtle fossil and found evidence of partial neck retraction linked to the quick capture of underwater prey.

According to the New York Times, Anquetin’s team studied a type of Late Jurassic period turtle known as Platychelys oberndorferi, and were able to determine from its skeleton and shell that it was part of the pleurodira group of turtles – that is, turtles that bend their muscles horizontally to pull their necks to the side and tuck it next to their shoulder during the retraction process.

However, these turtles, which are typically found in Africa, Australia, and South America, tend to pull their necks back horizontally, while the Platychelys oberndorferi did so vertically, much like another group of turtles – cryptodires, which include tortoises, box turtles, and sea turtles.

Anquetin suggested to the Guardian that the neck retraction mechanism likely developed more than once during the course of turtle evolution, as the early pleurodire specimen his team studied developed a mechanism similar to that which developed independently much later in cryptodires. But, he added, Platychelys oberndorferi was unable to fully tuck its head into its shell.

Mechanisms similar to modern-day snapping turtles

Since the creature could not completely fold its neck into its shell, it would have been somewhat exposed, suggesting that defense was not the main reason for the mechanism’s development. So what was the purpose for this morphology? To solve the puzzle, the researchers decided to study other features of the creature, and found that it resembled a mata mata or snapping turtle.

The mata mata and snapping turtle are distant relatives, the Times noted, and both are ambush predators that hunt by hiding among plants found on the bottom of swamps, ponds, and shallow lakes, then striking once their prey wanders close enough. “We can expect that our turtle was behaving the same way,” Anquetin told the newspaper.

Based on their findings, the study authors reported that the neck mechanisms discovered in their species and those belonging to modern cryptodires are a prime example of convergent evolution, meaning that they developed independently because of the evolutionary advantages they offered at their respective times, the Times said. They admit, however, that their findings do not explain why pleurodira turtles eventually evolved to retract their necks sideways.

“Most people believe that that particular way of retracting the head evolved for protection only,” Anquetin told the Guardian. But his team’s findings indicate that this is unlikely. While further research is needed to back up their hypothesis, he said that the study emphasized the importance of fossils. “Evolution is always more complex than what we thought at first,“ he added.

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Image credit: Patrick Roeschli

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