Turtles are divided into two groups, according to how they retract their necks into their shells (something the ancestral Proganochelys could not do). The mechanism of neck retraction differs phylogenetically: the suborder Pleurodira retracts laterally to the side, anterior to shoulder girdles, while the suborder Cryptodira retracts straight back, between shoulder girdles. These motions are largely due to the morphology and arrangement of cervical vertebrae. Of all recent turtles, the cervical column consists of nine joints and eight vertebrae, which are individually independent. Since these vertebrae are not fused and are rounded, the neck is more flexible, being able to bend in the backwards and sideways directions. The primary function and evolutionary implication of neck retraction is thought to be for feeding rather than protection. Neck retraction and reciprocal extension allows the turtle to reach out further to capture prey while swimming. Neck expansion creates suction when the head is thrust forward and the oropharynx is expanded, and this morphology suggests the retraction function is for feeding purposes as the suction helps catch prey. The protection the shell provides the head when it is retracted is therefore not the main function of retraction, thus is an exaptation. As for the difference between the two methods of retraction, both Pleurodirans and Cryptodirans use the quick extension of the neck as a method of predation, so the difference in retraction mechanism is not due to a difference in ecological niche.
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