Abstract

A comparative analysis of the postcranial skeletal morphology of three species of pakicetid cetaceans provides new insights into their locomotor behavior. In gross morphology, they are similar to their smaller early artiodactyl relatives, lacking obvious signs of their transition to an aquatic niche. Features linking them are related to a cursorial adaptation centered in the reduction of joint mobility to the parasagittal plane, evident in both the elbow and the ankle. In addition to cursorial features of the limbs, the earliest whales and early artiodactyls both possess a long, stable lumbus and robust tails. The three pakicetid genera can be distinguished by size, proportion, and details of articular morphology. However, all pakicetid postcrania bear microstructural specializations commensurate with aquatic locomotion, and incompatible with cursoriality. The most striking modification is the presence of systemic increased bone density, likely used as skeletal ballast. When both postcranial morphology and microstructure are considered, it can be concluded that pakicetid cetaceans were highly adapted for an aquatic niche. As in several extant semiaquatic taxa, the pakicetid tail undoubtedly contributed to its locomotor repertoire, whether in propulsion or stabilization.

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