Evolution of the hydrodynamic performance in Mesozoic marine reptiles

: insights from flow simulations and morphospaces

Abstract

The end-Permian mass extinction set the stage for reptile dominance in the seas. During the
Mesozoic, various reptile groups colonized aquatic habitats becoming key components of their
ecosystems. Their locomotory ecomorphology and the factors underlying its evolution are, however,
still poorly understood. Mesozoic marine reptiles are exceptional models to test form-function
hypotheses related to locomotion in an evolutionary framework, as well as to explore patterns of
phenotypic diversification and convergence.
In this thesis, I combine full-body digital reconstructions of very complete fossils with state-ofthe-art computer flow simulations, an objective, physics-based methodology, to test how body shape
and size contributed to energy performance during steady locomotion in ichthyosaurs and plesiosaurs,
two of the most successful and longest-lived marine reptile lineages. Simulations show that the shift
from lizard-like to fish-shaped bodies in ichthyosaurs did not reduced their drag costs, contrary to what
classically had been suggested, with early ichthyosaurs having shapes that are as low-drag as modern
dolphins. Additionally, CFD analysis suggests that large flippers and very long necks in plesiosaurs
were potential sources of drag, but the evolution of large trunks compensated for these hydrodynamic
burdens. In both fossil groups as well as in modern cetaceans, body size appears as a dominant factor
influencing the mechanical costs of steady swimming, with larger sizes experiencing lower drag per
unit of body mass.
In addition to the computer flow analyses, disparity analysis of functionally informative
morphological characters was utilized to explore major anatomical transitions in locomotion in the
major clades of Mesozoic marine reptiles. Morphospace analysis shows that the diversification of
swimming modes occurred progressively throughout the Mesozoic and reached its maximum during the
Cretaceous, in contrast to trophic adaptations, which have been previously shown to peak in the Triassic.
Along with the CFD-based analysis, these results provide invaluable insight into the evolution of
locomotion in secondarily aquatic tetrapods and the dynamics of Mesozoic marine ecosystems.

Date of Award	23 Mar 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Michael J Benton (Supervisor), Benjamin C Moon (Supervisor), Thomas L Stubbs (Supervisor), Imran A Rahman (Supervisor), Colin Palmer (Supervisor) & Stephan Lautenschlager (Supervisor)