Giant ichthyosaurs evolved stealthy flippers to sneak up on prey

Large ichthyosaurs silently stalked their prey from the shadows.

More than 180 million years ago, Temnodontosaurus lurked beneath the ocean’s surface. In the darkness of Jurassic seas, any sound could give away its position to prey – so the marine reptile evolved to move more quietly.

These adaptations have been revealed by an exceptionally preserved fossil of an ichthyosaur flipper uncovered in southern Germany. It still preserves much of its soft tissue, including structures similar to the winglets that help aeroplanes reduce drag, as well as serrated edges that helped to dissipate sound.

Dr Johan Lindgren, the lead author of new research into the fossil, says that the discovery changes what scientists understand about Temnodontosaurus and its relatives.

“These were more sophisticated animals than previously thought, and used elaborate features to sneak up on unwitting prey under the cover of darkness,” Johan says. “The flipper’s fleshy tip, for example, could have contributed to minimising drag and dampening the noise made by the ichthyosaur.”

“We would like to further our understanding of how this flipper worked by carrying out more simulations, and hopefully building a physical replica that we can test in the water.”

Dr Marc Jones, our Curator of Fossil Reptiles and Amphibians, says that the study is “impressive”.

“Ichthyosaurs have fascinated scientists for over 200 years, but this study shows that there’s still more we can learn about them,” adds Marc. “This is an amazing specimen, and the investigation has revealed new details about the behaviour of these extinct reptiles.”

“We have some smaller specimens in the Natural History Museum which show similar structures that the team should come and investigate.”

The findings of the study were published in the journal Nature.

What do we know about Temnodontosaurus?

The first complete skull of what is now known as Temnodontosaurus was found by Joseph Anning in the early 1800s, followed by his sister Mary Anning’s finding its skeleton. These ancient remains captured the attention of scientists and kicked off the formal study of these marine reptiles.

Temnodontosaurus are the largest ichthyosaurs found in the UK, with the biggest measuring over 10 metres long. Their eyes are equally massive, reaching over 25 centimetres in diameter. This is almost as big as those of giant and colossal squid.

It’s believed that the ichthyosaurs had such big eyes to capture as much light as possible. This would have allowed the marine reptiles to see in dark conditions as they hunted at night or in the depths of the oceans.

But while the eyes have left behind a hardened structure known as the scleral ring, it’s rare for other soft tissue to survive from these marine reptiles. In some ichthyosaur specimens, the faint outline of the body is left behind in the rock, although these fossils generally come from smaller animals.

Body outline from larger individuals are typically non-existent, meaning little is known about soft tissues from these top predators.

What does the flipper reveal?

The new flipper from Germany is just the kind of fossil that ichthyosaur experts have been waiting for.

Soft tissues are preserved along one side of the metre-long flipper where it’s believed to have been pressed into the sediments of the seafloor shortly after the animal’s death. This provided an oxygen poor environment that preserved the flesh’s structure for tens of millions of years.

While parts of the Temnodontosaurus flipper are missing, its shape suggests that it extended out to a fleshy tip beyond the end of the skeleton. This characteristic hasn’t previously been seen in any living or extinct marine vertebrate, and it’s thought the tip could flex to help the animal move more efficiently.

“The way that water moves over the surface of the flipper causes rotating vortices to form, which cause drag,” Johan explains. “By flexing the fin this tip would act like a winglet, changing how the water flows and reducing this impact.”

“This would also enhance Temnodontosaurus’ lift-to-drag ratio, meaning that it didn’t have to swim with its tail as much. Less movement means less noise.”

Another unique structure of the flipper has been named ‘chondroderm’ by the scientists. It is a type of cartilage reinforced with calcium for strength, which helps to strengthen the trailing edge of the flipper.

This helped produce a wing-like shape that allowed the ichthyosaur to glide beneath the waves, as well as producing a serrated edge that reduced the volume of low frequency sound. Similar adaptations have been seen in the wings of owls so they can fly silently in search of prey.

Johan says that it’s possible that similar structures might be found in other large ichthyosaurs, and could inspire engineers to produce quieter propellers and hydrofoils that reduce underwater noise pollution. Even at 180 million years old, Temnodontosaurus is still cutting edge.