Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs

Mar 2019 Proceedings of the Royal Society B

Gutarra, S., Moon, B.C., Rahman, I.A., Palmer, C., Lautenschlager, S., Brimacombe, A.J. & Benton, M.J. 2019 Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs . Proceedings of the Royal Society B 286: 20182786 doi:10.1098/rspb.2018.2786

As part of her PhD, Susana Gutarra has been modelling ichthyosaur hydrodynamics, investigating the effects of body morphology on their swimming ability. This was earlier a project that I started then gave to MSc student Alison Brimacombe, and Susana added lots of fluid dynamical knowledge and finesse.

Here we used 3D models, based on exceptionally preserved ichthyosaur specimens from throughout their phylogeny, in computational fluid dynamics simulations to explore how the changing shape of ichthyosaurs affect the amount of drag and lift generated.

A simplified phylogeny of ichthyosaurs with example 3D models used in computational fluid dynamics simulations. From Gutarra et al. (2019).

We expected to see a clear trend where the later ichthyosaurs, traditionally considered to be better adapted to life in the oceans, had reduced the amount of drag generated, and so could put more of their effort into swimming forward.

Despite very different morphologies, there was only a very minor effect on the drag generated in computaional fluid dynamics models, and a lot of spread. The depth of the body seems to be the most important factor, rather the the shape as a whole. From Gutarra et al. (2019).

However, the story is more complex. While morphology does have some minor effect on drag, what becomes more important is the ability to create thrust to overcome this. We therefore looked at the total cost of transport – how much effort do you have to put into going forwards. This did seem to reduce through ichthyosaur phylogeny, while also being heavily affected by the size of things.

Cost of transport for a series of ichthyosaurs shows that a more dolphin- or tuna-like shape, and getting bigger, can help reduce the amount of effort needed for swimming. From Gutarra et al. (2019).

Our summary was that getting bigger and swimming in a more efficient manner, like dolphins or tuna, are your best bets for reducing effort.