Returning to the Ocean: The Transitional Mechanics of Legged Whales

Abstract

The land-water transition in whales represents a remarkable event in the history of evolution. Evolving from small terrestrial artiodactyls to obligate aquatic species that came to spread across the globe in as little as 15 million years, whales are a true evolutionary success story. As perhaps the most successful secondarily aquatic tetrapod group, with a rich fossil record spanning the process of aquatic specialisation, ancient whales (archaeocetes) represent an invaluable resource. In this thesis, the evolutionary biomechanics of aquatic specialisation have been investigated, using computational modelling techniques to reconstruct changes in hindlimb musculature and hydrodynamics across the land-water transition. A range of transitional archaeocete species were investigated, with a particular focus on Pakicetus, Ambulocetus and Maiacetus. Validated equations for reconstructing segmental mass properties were produced using a dataset of extant mammals, which we applied to archaeocetes with sufficient preserved material. Muscle moment arms about the hip joint were modelled in both archaeocetes and modern semi-aquatic mammals, demonstrating a startling degree of convergence in unrelated taxa and providing unexpected conclusions regarding the swimming biomechanics of Maiacetus. Fluid simulation of whale hindlimbs was similarly revealing for both Ambulocetus and Maiacetus. Based on our results, we suggest that there was significant evolutionary convergence in hindlimb morphology and biomechanics between different groups of secondarily aquatic mammals, which could be extended to identify swimming modes in other extinct groups. We suggest that Pakicetus was a more competent swimmer than previously suggested in the literature, and that the onset of lift-based propulsion as the primary swimming mode in archaeocetes occurred earlier than previously thought. This thesis aims to contribute to our, thus far, limited understanding of the evolution of locomotion in ancient whales, and demonstrates the utility of modelling approaches that should be extended further in future.

Date of Award	28 Feb 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Lee Margetts (Supervisor) & William Sellers (Supervisor)