The Vertebral morphology in dolphins (Delphinidae): A 3D approach

ABSTRACT

For almost 90 years, the cetacean vertebral column has puzzled scientists. This is the first 3D geometric morphometric study on the vertebral column of such a numerous group of small odontocetes (24 species). It uses a functional subdivision of the cetacean vertebral column and three landmark configurations to describe and compare vertebral morphology in Delphinidae, relating particular morphologies with the biomechanical requirements of each species. To this end, I assess the effect of size, and that of size and subfamily on vertebral morphology. I also analyzed the statistical differences in shape between species. Phylomorphospaces were created to assess similarities or differences in shape between closely related species with similar/dissimilar habitats. The allometric effect was low in all regions, and there were subfamily-specific allometric effects. Differences between species were greater in the mid-column but this was only partially confirmed statistically, presumably due to low n for some species. The percentage of variance explained by the first two PCs was higher than 58% in all regions, with the torso and the tail stock showing the greatest percentages of explained variance. The results suggest that the common ancestor of dolphins would have be a non-fast-swimming oceanic species. Coastal habitats seem to have evolved secondarily by means of a reduction in vertebral count, and vertebral morphology associated with greater flexibility (i. e., longer centra, smaller faces). On the contrary, an increased total count and disk-shaped vertebrae were observed to varying degrees in non-coastal species, with the most extreme modifications being found in species with particular habitat specializations. My results support the hypothesis that diversification in vertebral morphology in association to particular habitats was a key factor in delphinid explosive radiation, and provides descriptive basis for analysis of the phylogenetic constrains in vertebral morphology needed to elucidate dolphin diversification and the factors behind it.