Evolutionary trends in the vertebral morphology of extant Delphinidae

Vertebral morphology in cetaceans is linked to various functional abilities that promote ecological diversity and adaptive radiation. While morphometric studies have examined vertebral shape evolution, few have quantified evolutionary trends in a phylogenetic framework. Here, we used three-dimensional landmark configurations and phylogenetic comparative methods to investigate how the vertebral morphology of Delphinidae is influenced by phylogenetic constraints, ecological adaptation, and allometric effects and how these influences vary along the vertebral column.

Phylogenetic ordination methods revealed that species with particular habitat requirements differ greatly from their closest relatives, exhibiting biomechanically advantageous vertebral shapes. A comparison of the orientations of these ordination methods, phylogenetic ANOVAs and phylogenetic signal tests revealed that vertebral morphology is affected by overlapping allometric, ecologic, and phylogenetic signals, with their relative importance differing across regions, phylogenetic levels, and dimensions of shape. In the anterior thorax, the posterior thorax, and the synclinal point, diversification was associated primarily with size and habitat, resulting in low phylogenetic signals. Conversely, the mid-torso and tail stock retain strong phylogenetic signals, reflecting subfamily level conservatism. Notably, in the Tm region, the ecological demands for fast swimming remain highly relevant to vertebral morphology, emphasising the functional significance of this region.

Vertebral morphology in Delphinidae may reflect a complex interplay of ecological, allometric and phylogenetic influences, with distinct regions evolving under different combinations of selective and historical constraints. These region-specific patterns highlight the modularity of the vertebral column and provide new insights into the adaptive radiation of oceanic dolphins. Further studies, including evolutionary modelling and considering intraspecific variation, will be essential to fully understand macroevolutionary trends in vertebral morphology and their implications for axial locomotion.