(MERI) Returning to the Ocean: The Transitional Mechanics of Legged Whales

Darwinian evolution, descent with modification, is elegantly supported by the primary evidence that maps change within recognisable lineages in the fossil record. A superb example of such documentable transition is the evolution of an aquatic lifestyle in cetaceans. We have an excellent fossil record showing that whales originated in the Eocene as small deer-like creatures such as Indohyus that rapidly evolved to semi-aquatic and much larger meat eating forms such as Pakicetus. In more derived forms we see some limb reduction in Ambulocetus and Rodhocetus leading to the almost complete lack of hindlimbs in Dorudon and modern aspect Cetacea. There is considerable debate about the locomotor habits of these early whale forms. Statistical analysis of fossil whale and modern semi-aquatic species morphology does not resolve the locomotor transition since the fossils do not fit well in the morphospace defined by the modern forms. Other evidence has been used to infer the degree of aquatic lifestyle: stable isotope analysis of diet indicating a range of aquatic food consumption; bone histology showing thickening of cortical bone associated with density increase in aquatic mammals; shape analysis of the bony labyrinth of the ear. However these techniques are all indirect measures of locomotor competency and do not provide an unambiguous signal. In addition there is considerable disagreement about whether the aquatic adaptations of hippopotamids and cetaceans are ancestral or independently derived which further hinders phylogenetic based reconstructions of the aquatic habits of legged whales.

Biomechanical analysis can improve the functional resolution of locomotor reconstructions in palaeontology. Functional morphological assessment allows the joint ranges of motion assessment in transitional forms, and bone microstructural histology has proved useful for more derived Cetacea. However, as highlighted in our previous work in this area, integrated computational engineering approaches offer perhaps the best way of maximising the information about locomotor habit contained within the fossil record. There are some specific challenges in reconstructing the locomotion of potentially semi-aquatic species. Firstly, because of the technical challenges of getting good biomechanical data from large animals underwater, there are very few comparative studies of suitable modern animals in a semi-aquatic habitat. Secondly, by their very nature, studies of locomotion at the water-land interface need to cope with not only the difficult job of understanding the mechanics of legged locomotion, they also have to deal with the added difficulties of a buoyant, high viscosity medium.

The aims of this project are to investigate and reconstruct the locomotor history of transitional whales to quantify the importance of semi-aquatic locomotion within their locomotor repertoires and to reconstruct their land, transitional and water-borne gaits. It will do this using a combined anatomical, engineering simulation, and robotic reconstruction approach to achieve the following objectives:

(1) Digitise legged whale fossils (Indohyus, Pakicetus, Ambulocetus, Rodhocetus, Dorudon) and construct 3D forward dynamic simulations using the skeletal morphology and comparative anatomy of extant relatives (living cetaceans and hippopotami).
(2) Produce simulations that include basic hydrodynamics so the basic costs of wading, paddling, and the effects of buoyancy can be predicted.
(3) Create physical representations of the key morphological elements identified in and test these using physical simulations controlled using an industrial robot arm and tested at appropriate Reynolds numbers using a flume.
(4) Produce validated simulations of terrestrial and aquatic locomotion in legged mammals and use this information to predict the locomotor capabilities of the transitional whale forms.