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Secrets of the deep

Speed read
  • Lizard-like fish called ichthyosaurs ruled the Earth’s oceans in the Mesozoic era
  • During its 160-million-year reign, evolution severely altered this creature’s body
  • Computational fluid dynamics help scientists understand how this change affected the animal’s drag

Evolution is an interesting natural mechanism. It can lead to subtle changes like lactose tolerance in humans, or to the complete revamping of an animal’s physical structure. For an example of the latter, scientists can look to the ichthyosaur (Greek for fish+ lizard).    

<strong>Fish + lizard.</strong> Icthyosaurs are large marine reptiles that were widely distributed in the oceans of the Mesozoic Era, 251 million to 65.5 million years ago. Courtesy Ballista. <a href='https://creativecommons.org/licenses/by-sa/3.0/deed.en'>(CC BY-SA 3.0)</a>Living in the Mesozoic era, the ichthyosaur’s reign lasted nearly 160 million years. During this time, the ocean-based creature – which isn’t considered a dinosaur– saw its body change drastically from an elongated lizard shape to a more traditional fish body.

For Susana Gutarra Diaz, a paleobiologist at the University of Bristol, that change is worth studying. Using computational fluid dynamics (CFD,) Gutarra and her partners worked to understand how the creature’s body shape affected drag as it moved through Mesozoic seas.

“We were trying to see how this shift in body proportions, from the very narrow lizard-like ones to the fish shape, influenced drag relative to the creature’s corporeal mass,” says Gutarra.

Computing the drag

<strong>3D flow.</strong> Computational simulation of flow over 3D models. Velocity plot (l) and pressure coefficient (r) for a primitive ichthyosaur (Chaohusaurus), a derived fish-shaped ichthyosaur (Ophthalmosaurus), and a modern bottlenose dolphin (Tursiops).Courtesy Susana Gutarra, University of Bristol.Gutarra’s mission was to determine how each body shape affected the amount of force that pushed back from the water as the lizard-fish swam. She pitted various species of ichthyosaurs against each other for the experiment, for example, the primitive Chahohusaurus geishanensisand the later Ophthalmosaurus icenicus. While the former is lizard-shaped with a slender tail, the latter looks more like a modern tuna.

“We were hypothesizing that the fish-shaped ichthyosaurs would be better for sustained swimming,” says Gutarra. “Using CFD was a way to make the most of the resources we have, which is very good fossil records.”

Gutarra used the ANSYS-Fluent model, which allowed for the program to be run on a traditional computer rather than the high-performance computing machines that are often used for complex CFD models.

“CFD is quite precise, and it allows you to play a lot with the models so you can remove the limbs, for example, to see how much the body contributes to drag,” says Gutarra. “You can do different variations of the morphologies in order to understand which is the important element that affects the drag.”

<strong>The shape of you.</strong> The researchers analyzed models of nine types of ichthyosaur, tracking the evolution of their body shape from elongated to something more akin to a modern tuna. Courtesy Susana Gutarra, University of Bristol.Following the study, Gutarra was surprised to conclude that the varying body shape of the Ichthyosaur did not significantly affect drag. For this work, she used static animal models which allowed comparison of many different body shapes. Gutarra points out that it would be ideal to recreate the ichthyosaurs actively moving in the water. However, this poses various challenges, one of which is the need for greater computational resources such as HPC.

“First of all, the computational demands are way higher when you have dynamic simulations, so you need a more powerful computer,” says Gutarra. “The other limitation is how do you assume the animal moves? We are working from fossils. Maybe we can have some assumptions about the shape, but in order to know how they move we need to know lots of things about their backbone flexibility, the way their muscles were configured and also probably, again, the morphology and flexibility of the propulsion elements.”

Working with long-dead animals is a tough business. Scientists can try to apply what they know about living creatures, but that can only take them so far. So, if the ichthyosaur’s morphology didn’t change to reduce drag, what did cause the change?

From speculation to rigor 

“The changes could also be related to metabolism,” says Gutarra. “Many works suggest that ichthyosaurs were endothermic creatures. If they could generate their own heat, then having this kind of shape, which has a smaller ratio of surface area to volume, helps to keep the constant temperature more easily than having a shape that has more surface area exposed relative to the volume.

<strong>The future of paleontology.</strong> CFD and other computational techniques allow scientists like Gutarra to apply rigorous hypothesis testing to what was once a more speculative field. Courtesy Susana Gutarra, University of Bristol.Although there remains a lot of work to be done here, Gutarra is excited about the future of her field. She states that using new tools like CFD and other computational techniques for rigorous hypothesis testing will help paleobiology and biomechanics in paleontology to move forward.

Even a few years ago, running a CFD model like this on a personal computer would have been an impossible task. But technological improvements mean computational methods are available to more researchers in more fields. One can only hope that access to supercomputing resources will be so widespread in the future.

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