Fossil ribs reveal new secrets about ichthyosaur swimming technique

Ichthyosaurs ruled the oceans for 150 million years in the Mesozoic era.

Their fish-shaped bodies appear to have been built for high speeds. In that case, being as light as possible is always an advantage.

“Ichthyosaur skeletons are in general quite light, but the ribs are not,” says Jørgen Krøglid.

The results from his research were recently published in the journal PeerJ.

“This was a bit surprising to us. Compact bones are typically found in animals that move slowly, such as manatees,” says Lene Liebe Delsett, a researcher at the Natural History Museum at the University of Oslo.

Delsett has taken part in excavating the ichthyosaur fossils on Svalbard and leads the research project Evolutionary Convergence in Historical Oceans: The case of whales and ichthyosaurs.

The question the researchers are asking is whether there are multiple evolutionary pathways to developing a fish-shaped body. The answer they get when comparing ichthyosaurs with living toothed whales is a clear yes.

From land to water – each in their own way

Both ichthyosaurs and whales appear to be perfectly adapted to life in water. It’s easy to imagine that they have always lived there. But they haven't.

Both descended from four-legged land animals that returned to the sea. Whales evolved from mammals, more specifically artiodactyls, and ichthyosaurs evolved from reptiles.

They have different evolutionary origins but resemble each other in both appearance and lifestyle. This makes them well suited to studying what is known as convergent evolution. This means that different groups of species develop similar characteristics completely independently of one another.

“Both went from being land animals to becoming fully aquatic. They feed in the water and they reproduce in the water, and they have evolved a very hydrodynamic body,” says Krøglid.

Since ichthyosaurs are extinct, we can’t see them in action. Researchers therefore have to use other methods.

That’s why Krøglid was allowed to cut into fossils and skeletons that are otherwise kept safely stored in the collections of the Natural History Museum. This is not something researchers do lightly.

“It’s not something we take lightly. We can’t just glue them back together. We need very good reasons when we ask for permission to cut up this material. We have to show that we can gain a lot of new knowledge about the physiology of these animals and how they functioned in the water,” says Delsett.

And fortunately, this time they did.

Ancient cell structure

The researchers cut ultra-thin slices of ribs from two ichthyosaur fossils and from the skeletons of two toothed whales: a porpoise and a beluga. These slices were studied using methods including CT scans and microscopes, right down to the cellular level.

It is almost incredible what they can see, especially in the fossilised bones.

“This gives us access to a whole new world. It’s no longer bone, it’s no longer cells, but I can see the cavities where the cells once were. We can see how the bones have grown from the inside, just as we can in skeletons from the whales. It’s so detailed that it’s like looking at the bones when they were alive,” Krøglid explains.

“Many people are surprised that we can see the same structures in bones that are 150 million years old as we can see in modern whales,” says Delsett.

That was when they got a bit of a surprise. Several of the ichthyosaur ribs were unexpectedly non-porous and therefore heavy.

“The ribs of whales are lighter. They have larger cavities,” says Krøglid.

“The ichthyosaurs also had more ribs, which also added weight. They must have had a heavier ribcage,” says Delsett.

Mammals vs. reptiles

To try to understand why, the researchers went back to the different origins of ichthyosaurs and whales. There they found a possible explanation.

Both have lungs, which create an unfavourable buoyancy after they have been at the surface to breathe and want to return to the depths. But their spines are different, and here whales have a big advantage.

“The spine of whales moves up and down, like in other mammals, and they have a horizontal tail,” says Krøglid. 

That makes it quite easy to tip the front of the body down and swim towards the depths. It was not that simple for ichthyosaurs.

“Ichthyosaurs descended from land-living reptiles, and their spine has a sideways movement, like in lizards. Their tail is vertical, like in sharks and fish,” says Krøglid.

Ichthyosaurs therefore needed a bit of help to achieve the right angle when diving. A little extra weight around their air-filled lungs may not be such a bad idea.

“I imagine that the slightly heavier ribs could counteract this buoyancy and help ichthyosaurs initiate their dive. A bit like divers use weight belts. Once they got that downward angle, they could just keep swimming,” says Krøglid.

Delsett has therefore gained new answers to her question: Yes, there are multiple pathways to a fish-shaped body.

“In the big picture, ichthyosaurs and toothed whales are a very good example of convergent evolution, first and foremost when it comes to body shape. But when we study microanatomy and cell structure, it’s obvious that there are differences,” she says.

Reference:

Krøglid et al. Rib microstructure in thunniform ichthyosaurs and toothed whales, PeerJ, 2026. DOI: 10.7717/peerj.21486

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Read the Norwegian version of this article on forskning.no

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