In some turtles, the ability to retract their neck inside their shell may have evolved in order to enable the head to project forward quickly to capture prey according to a paper published in Scientific Reports. The authors suggest that protection of the head by complete withdrawal within the shell was a secondary benefit to this evolutionary process.
It has been suggested that turtles developed mechanisms allowing the neck and head to retract within the shell for protective reasons. In the two lineages of modern turtles, different mechanisms have developed: pleurodires pull their heads in sideways, while cryptodires draw their heads straight back. The two mechanisms are thought to have evolved independently after the Late Jurassic period (approximately 161-145 million years ago).
Jeremy Anquetin and colleagues describe the sixth and eighth cervical vertebrae of Platychelys oberndorferi, an early pleurodiran turtle found during the Late Jurassic in the area that is now defined as Germany and Switzerland. The authors found that P. oberndorferi was able to bend its neck in the vertical plane using a similar mechanism to modern cryptodires, and partially withdraw its head within the shell. They suggest that since the head was only partially retracted, this mechanism primarily evolved to enable the fast forward projection of the head, therefore improving the ability to capture darting prey. The authors note that this theory needs to be tested further by exploring the mechanisms that control the head during feeding in modern turtles.