Ichthyosaurs ruled the oceans for 150 million years in the Mesozoic era. Their fish-shaped bodies are built for high speeds. In that case, it is normally an advantage to be as light as possible.

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

The results from his master’s thesis were recently published as a peer-reviewed scientific paper 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 postdoctoral researcher Lene Liebe Delsett at the Natural History Museum at the University of Oslo.

Delsett has taken part in excavating the ichthyosaur fossils on Svalbard and leads the project “ECHO – Evolutionary Convergence in Historical Oceans: The case of whales and ichthyosaurs.” The question the researchers are asking is whether there are multiple pathways to a fish-shaped body. The answer they are getting when they compare ichthyosaurs with living toothed whales is a clear yes.

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 this is not the case Both descend from four-legged land animals that returned to the sea. Whales from mammals, more specifically artiodactyls, and ichthyosaurs from some yet unknown reptiles.

This difference in origin, combined with their similarity in appearance and way of life, makes them ideal for comparison in order to understand the phenomenon of and for looking for signs of what is called convergent evolution: when different groups of species independently evolve similar traits.

“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. Other methods have to be used. 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 at Økern in Oslo. That’s not something researchers do lightly.

“It’s not something we take lightly. We can’t just glue them back together,” says Delsett.

“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,” she says.

And fortunately, this time they did.

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, among other things, CT scans and microscopes, all the way down to the cellular level.

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

“That 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,” says Krøglid.

“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.

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’s not that simple for ichthyosaurs.

“Ichthyosaurs descend 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.

Therefore, ichthyosaurs need a bit of help to get the right angle when they dive. A little extra weight around the air-filled lungs may not be such a bad idea.

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

Delsett has thus 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.