Functional Morphology of the Vertebral Column in Remingtonocetus (Mammalia, Cetacea) and the Evolution of Aquatic Locomotion in Early Archaeocetes.

Abstract

Knowledge of early cetacean evolution has grown greatly in recent decades due to the discovery of dozens of species of archaeocetes that bridge the gap between aquatic cetaceans and their terrestrial ancestors. However, many of the details of how this transition occurred have yet to be elucidated. Assessment of vertebral function in archaeocetes is crucial for understanding the ecologies of these taxa and reconstructing the evolution of aquatic locomotion in the earliest whales. This dissertation documents the vertebral morphology of an early archaeocete (Remingtonocetus domandaensis) and develops quantitative methods for assessing vertebral function in fossil forms.

Remingtonocetus domandaensis is known from the middle Eocene Domanda Formation of Pakistan and is one of six species in the archaeocete family Remingtonocetidae. A newly described, well-preserved specimen (GSP-UM 3552) demonstrates that this taxon had a long neck that was stabilized by robust cervical musculature and imbricating transverse processes. Its lumbar vertebrae suggest that this animal swam by powerful movements of the hind limbs rather than dorsoventral undulation of the vertebral column.

This interpretation is supported by two independent quantitative assessments of lumbar mobility in early cetaceans. Multivariate analyses of lumbar measurements in modern mammals demonstrate that the lumbar vertebrae of Remingtonocetus domandaensis are more similar to those from stable lumbar regions, while vertebrae of protocetid and basilosaurid archaeocetes compare closely with those from mobile lumbar regions. Virtual rigid-body modeling simulations of the L4-L5 joints of R. domandaensis and the protocetid Maiacetus inuus demonstrate that M. inuus possessed a greater range of motion in flexion and extension than R. domandaensis, regardless of soft tissue parameters. These findings indicate that early protocetids had more mobile lumbar spines than their remingtonocetid contemporaries and suggest that the evolution of tail-powered swimming in early cetaceans was preceded by an increase in lumbar mobility.

By assessing the locomotor capabilities of an early cetacean in detail and providing two quantitative methods for elucidating vertebral function in fossil taxa, these studies can serve as a robust starting point for confidently assessing the evolution of swimming mode in later cetaceans and other aquatic mammals.