Early Jurassic Ichthyosaur Was Warm-Blooded, Had Blubber and Camouflage, Well-Preserved Soft Tissues Show

An international team of paleontologists has found the exceptionally preserved remains of a Stenopterygius ichthyosaur that lived 180 million years ago (Early Jurassic epoch). The fossil is so well-preserved that its soft-tissues retain some of their original flexibility. Molecular and microstructural analyses of the fossil have revealed that this creature was warm-blooded, had insulating blubber and used its coloration as camouflage from predators.

Cells, cellular organelles and original biomolecules were found in preserved soft parts of a Stenopterygius ichthyosaur; photographic (top) and diagrammatic (bottom) representation of the fossil. Image credit: Johan Lindgren.

“Ichthyosaurs are interesting because they have many traits in common with dolphins, but are not at all closely related to those sea-dwelling mammals,” said senior author Professor Mary Schweitzer, from the North Carolina Museum of Natural Sciences and Lund University.

“We aren’t exactly sure of their biology either. They have many features in common with living marine reptiles like sea turtles, but we know from the fossil record that they gave live birth, which is associated with warm-bloodedness. This study reveals some of those biological mysteries.”

In the study, Professor Schweitzer and co-authors analyzed a Stenopterygius fossil from the Holzmaden quarry in Germany.

The ichthyosaur lived in what is today southern Germany some 180 million years ago. At that time, the approximately 6.5-foot (2 m) long reptile swam in a vast ocean that was then covering large parts of present-day Europe.

“Both the body outline and remnants of internal organs are clearly visible,” said lead author Dr. Johan Lindgren, of Lund University.

“Remarkably, the fossil is so well-preserved that it is possible to observe individual cellular layers within its skin.”

Stenopterygius ichthyosaur’s fossilized skin forming the trailing edge of the right pelvic fin. Image credit: Johan Lindgren.

The paleontologists identified cell-like microstructures that held pigment organelles within the fossil’s skin, as well as traces of an internal organ thought to be the liver.

They observed material chemically consistent with vertebrate blubber, which is only found in animals capable of maintaining body temperatures independent of ambient conditions.

They also extracted soft tissues from the samples and performed multiple, high-resolution immunohistochemical analyses.

“We developed a panel of antibodies that we applied to all of the samples, and saw differential binding, meaning the antibodies for a particular protein — like keratin or hemoglobin — only bound to particular areas,” Professor Schweitzer said.

“This demonstrates the specificity of these antibodies and is strong evidence that different proteins persist in different tissues. You wouldn’t expect to find keratin in the liver, for example, but you would expect hemoglobin. And that’s what we saw in the responses of these samples to different antibodies and other chemical tools.”

The team also found chemical evidence for subcutaneous blubber.

“This is the first direct, chemical evidence for warm-bloodedness in an ichthyosaur, because blubber is a feature of warm-blooded animals,” Professor Schweitzer said.

Taken together, the findings indicate that Stenopterygius ichthyosaurs had skin similar to that of a whale, and coloration similar to many living marine animals — dark on top and lighter on the bottom — which would provide camouflage from predators, like pterosaurs from above, or pliosaurs from below.

“Both morphologically and chemically, we found that although Stenopterygius would be loosely considered ‘reptiles,’ they lost the scaly skin associated with these animals — just as the modern leatherback sea turtle has,” Professor Schweitzer said.

“Losing the scales reduces drag and increases maneuverability underwater.”

The research was published in the journal Nature.

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Johan Lindgren et al. Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature, published online December 5, 2018; doi: 10.1038/s41586-018-0775-x