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Remarkably preserved fossil sea reptile reveals skin that is still soft

Image: Johan Lindgren
Image: Johan Lindgren

The remains of an 180 million-year-old ichthyosaur (literally ‘fish-lizard’) have been analysed, and the fossil is so well-preserved that its soft-tissues retain some of their original pliability. The study, published in Nature, contributes to our understanding on how convergent evolution works, and shows that ichthyosaurs adapted to marine conditions in a way that is remarkably similar to that of modern whales.

Spectacular soft-tissue fossil (MH 432; Urweltmuseum Hauff, Holzmaden, Germany). Cells, cellular organelles and original biomolecules have been discovered in preserved soft parts of an approximately 180-million-year-old ichthyosaur (literally ‘fish-lizard’). Photographic (top) and diagrammatic (bottom) representation of the 85-cm-long fossil (which corresponds to roughly half of the original length of the animal).

The ichthyosaur lived in what is today southern Germany during the Jurassic Period some 180 million years ago. At that time, the approximately two-metre long reptile swam in a vast ocean that was then covering large parts of present-day Europe.

Johan Lindgren of Lund University, Sweden, led the international collaboration that resulted in the most comprehensive and in-depth examination of a soft-tissue fossil ever undertaken. Among other things, the study reveals that the soft parts have fossilised so quickly that both the original cells and their internal contents are preserved.

“You can clearly see both the body outline and remains of internal organs. We can even distinguish the different cellular layers within the skin”, explains Johan Lindgren.

Fossilized skin forming the trailing edge of the right pelvic fin (Image: Johan Lindgren)
Fossilized skin forming the trailing edge of the right pelvic fin (Image: Johan Lindgren)

The researchers identified blubber underneath the skin. To date, such specialized fat-laden tissue has only been found in modern marine mammals and adult individuals of the leatherback sea turtle. The presence of blubber indicates that ichthyosaurs had metabolic rates that were higher than are those of typical reptiles living today. These data could help explain why ichthyosaurs had an almost global distribution, even in cold waters, and how they could dive to considerable depths, as well as grow as fast as they did.

The team also examined remains of the animal’s liver, which included part of the original biochemistry (e.g., eumelanin pigment and haemoglobin residues). The molecular and imaging analyses were performed in laboratories in Sweden, Germany, Japan and the USA.

“It’s truly remarkable that the biomolecules we discovered so closely match the tissues that we could identify”, says Johan Lindgren.

Pigment cells

One hundred and eighty-million-year-old pigment cells (melanophores). To the left, light micrograph of the branched melanophores. In the centre, synchrotron-radiation X-ray tomographic microscopy (SRXTM) rendering of a melanophore with long dendritic processes. To the right, transmission electron microscopy (TEM) image of a melanophore that contains melanosome organelles. Courtesy of Johan Lindgren (left image), Mats E. Eriksson and Federica Marone (centre images), and Ola Gustafsson (right image).

In the study, the researchers also succeeded in showing that the fossil contains tissues that still retain some of their original pliability, even though 180 million years have passed since the material was fresh. The team used chemicals to remove the mineral phase of the specimen, i.e. the inorganics that once turned the animal carcass into a petrified fossil.

“We then discovered that the soft parts retain a certain degree of elasticity”, says Johan Lindgren.  

Comparison between artificially matured modern porpoise integument and fossil ichthyosaur blubber. To the left, cross-section through porpoise skin and blubber before decay experiments. In the centre, side (top) and internal (bottom) views of heat- and pressure-treated porpoise integument (note severe reduction of the blubber layer). To the right, side (top) and internal (bottom) views of fossilized ichthyosaur skin and blubber. Image: Johan Lindgren and Martin Jarenmark.

The results also reveal the colouring of adult ichthyosaurs: the upper part of the body was dark, whereas the belly was light. This colouration acted either as camouflage or UV protection, or both. It may also have helped the animal to warm up faster in cold climates and/or after long and deep dives.

Not only do the results provide insights into the biology, physiology and ecology of derived ichthyosaurs, they also show how little we know about the fossilisation process and what can actually be preserved in the fossil record. Moreover, they could add to our knowledge on convergent evolution, as ichthyosaurs display an interesting mix of characteristics otherwise found in toothed whales (such as dolphins and porpoises) and the leatherback sea turtle.

Publication: Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur

Johan Lindgren, senior lecturer
Department of Geology, Lund University

+ 46 46 222 46 35 or +46 768 54 14 91
johan [dot] lindgren [at] geol [dot] lu [dot] se