The hydrodynamics of plesiosaurs

Abstract

When the dinosaurs were walking on the earth, there was a marine reptile called a plesiosaur that was swimming in the seas. Plesiosaurs are unique in the animal kingdom because they had four large flippers with which they used to swim, but exactly how they used them has been controversial for almost 200 years. Debate has included 1) whether plesiosaurs use all four flippers for propulsion, or use the fore flippers for propulsion and the hind flippers for manoeuvring, and 2) if plesiosaurs use all four flippers, what phase did they flap with? i.e. did both the fore and hind flippers flap up and down together, did one go up as the other goes down, or was it something in between? This thesis helps to answer these questions by providing the first quantitative experimental data ever collected on the swimming method of plesiosaurs. By using a combination of flume-tank experiments, numerical simulations, and mathematical models, I show that 1) the hind flippers of plesiosaurs would experience large increases in thrust and efficiency, and it is therefore likely that they indeed used all four flippers for propulsion, 2) that the performance of the hind flipper is increased when it weaves in between the primary vortices that have been shed from the fore flipper. This flipper/vortex interaction is dependant on the phase, flipper spacing, flow speed, and flapping frequency. Therefore there would not be one phase that is optimum for all plesiosaurs under all conditions, and the optimum phase at for each plesiosaur at each instance in time would vary depending on the swimming conditions, and 3) that the tandem-flipper arrangement of the plesiosaur reduces the unsteady loads experienced by the body and therefore provides a smoother movement than would be achieved by only two flippers.

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ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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