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, has not been previously examined.

gene expression predicts tetrapod-like axial regionalization in the skate, .

The axial skeleton of tetrapods is organized into distinct anteroposterior regions of the vertebral column (cervical, trunk, sacral, and caudal), and transitions between these regions are determined by colinear anterior expression boundaries of , , , and paralogy group genes within embryonic paraxial mesoderm. Fishes, conversely, exhibit little in the way of discrete axial regionalization, and this has led to scenarios of an origin of -mediated axial skeletal complexity with the evolutionary transition to land in tetrapods. Here, combining geometric morphometric analysis of vertebral column morphology with cell lineage tracing of gene expression boundaries in developing embryos, we recover evidence of at least five distinct regions in the vertebral skeleton of a cartilaginous fish, the little skate ().