Data from: Evolution of neck vertebral shape and neck retraction at the transition to modern turtles: an integrated geometric morphometric approach

The unique ability of modern turtles to retract their head and neck into
the shell through a side-necked (pleurodiran) or hidden-necked
(cryptodiran) motion is thought to have evolved independently in crown
turtles. The anatomical changes that led to the vertebral shapes of modern
turtles, however, are still poorly understood. Here we present
comprehensive geometric morphometric analyses that trace turtle vertebral
evolution and reconstruct disparity across phylogeny. Disparity of
vertebral shape was high at the dawn of turtle evolution and decreased
after the modern groups evolved, reflecting a stabilization of morphotypes
that correspond to the two retraction modes. Stem turtles, which had a
very simple mode of retraction, the lateral head tuck, show increasing
flexibility of the neck through evolution towards a pleurodiran-like
morphotype. The latter was the precondition for evolving pleurodiran and
cryptodiran vertebrae. There is no correlation between the construction of
formed articulations in the cervical centra and neck mobility. An
increasing mobility between vertebrae, associated with changes in
vertebral shape, resulted in a more advanced ability to retract the neck.
In this regard, we hypothesize that the lateral tucking retraction of stem
turtles was not only the precondition for pleurodiran but also of
cryptodiran retraction. For the former, a kink in the middle third of the
neck needed to be acquired, whereas for the latter modification was
necessary between the eighth cervical vertebra and first thoracic
vertebra. Our paper highlights the utility of 3D shape data, analyzed in a
phylogenetic framework, to examine the magnitude and mode of evolutionary
modifications to vertebral morphology. By reconstructing and visualizing
ancestral anatomical shapes we provide insight into the anatomical
features underlying neck retraction mode, which is a salient component of
extant turtle classification.