Fossil blubber shows ichthyosaurs were warm blooded reptiles

The discovery of blubber in an ichthyosaur fossil suggests these ancient marine reptiles were probably warm blooded, as has long been suspected.

Icthyosaurs have a striking resemblance to modern dolphins. They evolved from land reptiles that colonised the seas 250 million years ago and thrived until around 90 million years ago, meaning they lived alongside the dinosaurs.

A 180-million-year-old fossil, found in Germany, is so extraordinarily well preserved that pieces of the skin removed for analysis were still flexible. It shows that at least some ichthyosaurs had smooth, scaleless skin underlain by blubber, making them even more like dolphins than we thought.

All modern sea animals with blubber either maintain a constant body temperature, like whales and seals, or keep their body well above the water temperature, like the leatherback turtle, says team member Johan Lindgren of Lund University in Sweden. “At the very minimum, it’s what you see in the leatherback turtle today,” he says.

It’s also long been suspected that ichthyosaurs had dark backs and light undersides like many of the animals that live in the open ocean today. This countershading makes them harder to see when viewed from below or above.

When the team looked at the skin of the fossil ichthyosaur under a microscope, they saw what appear to be the remnants of individual pigment cells. They are virtually identical to the pigment cells of modern reptiles, which have a distinctive branched structure. The variations in the distribution of these cells suggest the animal had countershading.

“There is more to the fossil record than we could ever imagine,” says Lindgren.

In 2014, his team reported that another fossil ichthyosaur belonging to the same genus, Stenopterygius, appeared to have a uniformly dark covering. But that fossil was smaller. So juvenile ichthyosaurs may have been dark all over, Lindgren thinks, with the countershading appearing as they matured, as occurs in many modern sea turtles.

The team’s discoveries relied in part on an array of new technologies for studying fossils. But the German fossil is also unusual in that it appears to have fossilised very quickly, preserving soft tissues before they rotted away. It won’t be the only one of its kind, Lindgren says. “I expect there are other specimens out there, for sure.”