A comparison of the external adductor tendon in
a) non-chelonian reptiles, b) cryptodiran turtles, and c) pleurodiran turtles.

We have good evidence that while these pulley mechanisms
were evolving, turtles divided into two lineages; two different
pulley mechanisms characterize the two main groups. Members
of both groups had relatively primitive neck bones that did
not allow them to retract their heads into the shells. Later
on, both groups independently evolved highly specialized
methods of neck retraction.

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