Lumbar mobility in archaeocetes (Mammalia: Cetacea) and the evolution of aquatic locomotion in the earliest whales

Assessment of archaeocete vertebral function is vital for understanding the land-to-sea transition of cetaceans. Models of swimming evolution propose that some early cetaceans utilized undulation, which would have required significant lumbar flexibility compared to their artiodactyl ancestors. To assess archaeocete lumbar mobility, 14 linear and three angular measurements were collected from lumbar vertebrae of modern dorsostable and dorsomobile mammals. Principal component analyses successfully differentiate dorsostable mammals from dorsomobile mammals based on centrum length, neural spine length and angulation of transverse processes. Lumbar vertebrae of basal archaeocetes (pakicetids, ambulocetids and remingtonocetids) plot among dorsostable mammals, while those of more derived archaeocetes (protocetids and basilosaurids) plot among dorsomobile mammals. This supports the hypothesis that archaeocetes exhibited a trend of increasing lumbar mobility as they adapted to aquatic life. Notably, two modern cetaceans possess lumbar vertebrae that plot among dorsostable mammals. This is consistent with the observation that many modern cetaceans limit vertebral motion to the caudal vertebrae anterior to the fluke. In sum, these results suggest that the lumbar region of cetaceans underwent an increase in mobility during the transition from foot-powered swimming to undulation, followed by a subsequent decrease in mobility as cetaceans began to swim via caudal oscillation.