RESUMO
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.
RESUMO
Vertebral morphology has profound biomechanical implications and plays an important role in adaptation to different habitats and foraging strategies for cetaceans. Extant porpoise species (Phocoenidae) display analogous evolutionary patterns in both hemispheres associated with convergent evolution to coastal versus oceanic environments. We employed 3D geometric morphometrics to study vertebral morphology in five porpoise species with contrasting habitats: the coastal Indo-Pacific finless porpoise (Neophocaena phocaenoides); the mostly coastal harbor porpoise (Phocoena phocoena) and Burmeister's porpoise (Phocoena spinipinnis); and the oceanic spectacled porpoise (Phocoena dioptrica) and Dall's porpoise (Phocoenoides dalli). We evaluated the radiation of vertebral morphology, both in size and shape, using multivariate statistics. We supplemented data with samples of an early-radiating delphinoid species, the narwhal (Monodon monoceros); and an early-radiating delphinid species, the white-beaked dolphin (Lagenorhynchus albirostris). Principal component analyses were used to map shape variation onto phylogenies, and phylogenetic constraints were investigated through permutation tests. We established links between vertebral morphology and movement patterns through biomechanical inferences from morphological presentations. We evidenced divergence in size between species with contrasting habitats, with coastal species tending to decrease in size from their estimated ancestral state, and oceanic species tending to increase in size. Regarding vertebral shape, coastal species had longer centra and shorter neural processes, but longer transverse processes, while oceanic species tended to have disk-shaped vertebrae with longer neural processes. Within Phocoenidae, the absence of phylogenetic constraints in vertebral morphology suggests a high level of evolutionary lability. Overall, our results are in accordance with the hypothesis of speciation within the family from a coastal ancestor, through adaptation to particular habitats. Variation in vertebral morphology in this group of small odontocetes highlights the importance of environmental complexity and particular selective pressures for the speciation process through the development of adaptations that minimize energetic costs during locomotion and prey capture.
