Conservation of rib skeleton regionalization in the homoplastic evolution of the snake-like body form in squamates

Squamates have independently evolved an elongate, limb-reduced body form
numerous times. This transition has been proposed to involve either
changes to regulatory gene expression or downstream modification of target
enhancers to produce a homogeneous, deregionalized axial skeleton.
Analysis of vertebral morphology has suggested that regionalization is
maintained in snake-like body forms, but morphological variation in the
other primary component of the axial skeleton, the dorsal ribs, as not
been previously examined. We quantified rib morphology along the
anterior-posterior axis in limbed and limbless squamates to test different
regionalization models. We find that the relative position of regional
boundaries remains consistent across taxa of differing body types,
including in the homoplastic evolution of snake-like body forms. The
consistent retention of regional boundaries in this primaxial domain is
uncorrelated with more plastic abaxial region markers. Rather than loss of
regions, rib shape at the anterior and posterior of the axis converges on
those in the middle, resulting in axial regions being distinguishable by
allometric shape changes rather than by discrete morphologies. This
complexity challenges notions of deregionalization, revealing a nuanced
evolutionary history shaped by shared functions.