Palaeontology: Ichthyosaur resemblance to whales more than skin deep

The skin of a fossil ichthyosaur with preserved inner and outer layers, underlying blubber, and pigmentation is described in a paper published online this week in Nature. These findings, which represent the first reported fossilized ichthyosaur blubber, suggest that the creatures were warm-blooded reptiles.

Ichthyosaurs are extinct marine reptiles that resemble modern toothed whales, such as dolphins. Their similar appearance suggests that ichthyosaurs and whales evolved similar strategies to adapt to marine life - an example of convergent evolution. Ichthyosaurs have long been suspected to be warm-blooded, but the limited preservation of their fossils has made this and other similarities difficult to confirm.

Johan Lindgren and colleagues studied the composition of skin tissues from a well-preserved ichthyosaur specimen, of the genus Stenopterygius, dating from around 180 million years ago. They found remnants of the animal’s original, smooth skin that was still flexible and comprised of distinct inner (dermal) and outer (epidermal) layers, with blubber underneath. The latter is characteristic of present-day marine mammals and insulates against the cold, aids buoyancy, and acts as a fat store. This is the first time fossil ichthyosaur blubber has been identified, and confirms that ichthyosaurs were warm-blooded.

In addition, the authors find that the ichthyosaur skin is pigmented in a pattern that suggests the reptiles were ‘countershaded’ - having a light underbelly and a darker upper surface. This colouration is seen in many modern marine mammals and can act as camouflage, protect against UV light, and aid heat regulation.