Whale swimming evolution recorded in vertebral microanatomy from the Eocene to present

Among modern aquatic amniote clades that evolved pelagic locomotion, whales are a prime example, but timing and mode of evolutionary changes in their swimming are poorly understood. Here, we use 3D vertebral microanatomy derived from CT scanning to investigate swimming patterns in extinct whales. All vertebral centra from a single specimen of the extant common dolphin Delphinus delphis were used to provide a baseline for the correlation of microanatomical parameters and spinal movements in our dataset of fossil whales. We find that bone anisotropy, and to some extent bone compactness, mirrors the amount of compression and movement in vertebral regions. We use these parameters to infer regional movements and thus swimming mode for several lineages of stem cetaceans (Natchitochia jonesi, Pachycetus wardi, Zygorhiza kochii) and crown cetaceans (Maiabalaena nesbittae, an undescribed Oligocene odontocete and a modern minke whale). Natchitochia jonesi could walk on land, whereas Pachycetus wardi and Zygorhiza kochii were obligately aquatic, but show more spinal flexibility than modern taxa. The early Oligocene mysticete and odontocete differ in microstructural properties and might have experienced a different trajectory towards their extant bauplans. Overall, microanatomical data confirm increasing pelagic specializations in cetacean evolution that include a stiffer torso, but our data also highlight modular patterns of compactness within vertebral regions. Our approach is promising for future use of three-dimensional vertebral microanatomical data to understand locomotory patterns.