Meeting Abstract
Mammalian vertebral patterns are highly conserved: almost all mammals have five vertebral series and total precaudal counts of 29 or 30. Movements of Hox gene expression domains cause homeotic movements of precaudal series boundaries, while different rates of somitogenesis cause different caudal counts. Whales exhibit a rarer phenomenon without a known developmental cause: the apparent absence of a vertebral series. Reduction of the hind limbs in early cetaceans was accompanied by disarticulation of sacral vertebrae and of the innominate and sacrum, but vertebral counts were largely conserved. This conservation suggests that homologs of the four sacral vertebrae were retained, even if they are not currently recognized. We analyzed vertebral anatomy and vertebral ossification patterns in seven odontocete cetacean groups to search for morphological discontinuities that could signal the location of sacral homologs within the lumbar series. The morphology of adult vertebrae was documented using geometric morphometric analysis of 2D images. The first two principal components of this analysis were used in iterative piecewise regressions to identify the column location with the highest probability (minimal residual sum of squares) of a morphological discontinuity. We also documented fetal neural arch osssification size and density using AMIRA software analysis of CT scans. These analyses identified the four vertebrae immediately anterior to the precaudal / caudal boundary as sacral homologs in physeterids and ziphiids, indicating that they retain ancestral counts and column regionalization. In contrast, lumbar and caudal counts of delphinoids have been coordinately increased. We find that sacral homolog count has also increased proportionally, with the result that the location of the first sacral homolog occurs far anterior to the precaudal / caudal boundary.
