Meeting Abstract
Major evolutionary events in vertebrate evolution were accompanied by substantial changes in the axial skeleton including modifications in the Hox genes. These genes are critical regulators of axial differentiation into distinct vertebral regions. In contrast to other tetrapods, mammals and turtles are constrained in cervical count and, hence cervicodorsal regionalization. This raises the question if the numerical constraint is associated with a common Hox code not present in their non-constrained extinct relatives. On the basis of recent works that revealed a correlation between anterior Hox gene expression and vertebral shape, I investigated the cervical vertebral column of living and fossil mammals and turtles via 3D geometric morphometrics. The statistical assessment of shape changes between successive vertebrae enabled the establishment of the morphological subunit patterns in the neck of each taxon which is interpreted to reflect the Hox code. The results indicate that the modularity in the neck of the model organism mouse had already been established in the last common ancestor of mammals, but it differed from that of non-mammalian synapsids which display variable cervical counts. Thus, the constrained cervical count in mammals is likely to be associated with a common Hox code not present in early synapsids. During the evolution towards modern turtles the modularity of the neck diverged into a pleurodiran- and a cryptodiran-specific pattern which appears to be linked to their respective neck retraction mode. Despite the numerical constraint, the Hox code is likely to have been modified in turtles. The present study provides an important basis for future work investigating the factors that may restrict flexibility in axial patterning in tetrapods.
