However, the rock record is capable of exceptional preservation, and occasionally decay-prone tissues and organs are conserved as mineralized or carbonized residues with a high degree of morphological fidelity. But what is the nature of these remains, and which taphonomic processes have enabled their survival across geological time? We sought to investigate this by applying a novel multidisciplinary experimental approach to soft-tissue structures in a remarkably well-preserved ichthyosaur fossil. Surprisingly, demineralization of the lithified soft parts revealed still flexible remnants of the original skin and subcutaneous tissue, retaining endogenous cellular, sub-cellular and biomolecular constituents.

My passion (or some might say obsession) for Mesozoic marine reptiles (that is, ichthyosaurs, mosasaurs, plesiosaurs and their kin) began more than twenty years ago when I was looking for a subject for my master’s thesis. When asked for advice, my then supervisor, Dr. Mikael Siversson, kindly informed me that although I could not work with dinosaurs (my initial choice after having been blown away by the science fiction movie Jurassic Park), there was perhaps a chance to study mosasaurs. So, with his encouragement, I initiated my research on these magnificent (but sadly now long extinct) lizards, an endeavour that was later—during my postdoctoral tenure at University of California in Berkeley—expanded to include also their soft-tissue remains and potential to inform on large-scale evolutionary patterns and processes.

Over the years, I have had the opportunity (and great pleasure) to collaborate with many leading scientists from other fields, and their expertise has been invaluable for the successful outcome of the cross-disciplinary study published this week in Nature. In that paper, we present multiple lines of evidence for the retention of not only endogenous ultrastructural features, such as cells and cellular organelles, but also biomolecular traces in relict skin and deeper anatomies recovered from an approximately 180-million-year-old ichthyosaur (literally ‘fish-lizard’) fossil (MH 432; Urweltmuseum Hauff, Holzmaden, Germany) from the Posidonia Shale Formation of Germany. What is perhaps even more astonishing is that the soft-tissues, upon demineralization, appear to retain some of their original pliability! From our discoveries, we are able to conclude that adult ichthyosaurs were countershaded in life; that is, had a light belly and dark upper surface. This simple but practical colour pattern is still used by many pelagic tetrapods around today, and functions to provide camouflage, protect against harmful ultraviolet radiation during sea surfacing behaviours, and/or infer thermoregulatory advantages in cold climates.

Ichthyosaur pigment cells (melanophores). a, Photograph of the anterior end of our 180-million-year-old ichthyosaur fossil (MH 432) in oblique ventral view. b, Demineralized flank integument showing multiple subsurface pigment cells (dark dots) embedded in the semi-transparent epidermis. c, Light microscopy image of epidermal melanophores embedded in phosphatic matrix. d, Enlargement of the fossilized pigment cells and their branching processes. e, SRXTM rendering of a fossil melanophore with dendritic processes.

We have also been able to establish that the inside of the skin in our fossil originally was lined with blubber, a specialized fibro-adipose tissue layer that until now only has been found in living whales, sea cows, seals, and adult individuals of the leatherback sea turtle, to suggest that derived ichthyosaurs were warm-blooded.

Comparison between experimentally treated porpoise integument and preserved ichthyosaur cutis and subcutis. a, Untreated harbour porpoise (Phocoena phocoena) body integument (the outer skin surface is seen in oblique aspect). b, Side (top) and internal (bottom) views of artificially heated and compressed P. phocoena integument. c, Side (top) and internal (bottom) views of fossilized ichthyosaur (MH 432) skin and blubber.

Overall, our fossil provides the most complete insight into ichthyosaur skin and subcutaneous tissue to date, and thus greatly expands our understanding of the anatomy and physiology of these seagoing reptiles. However, although the preservation of MH 432 truly is remarkable, we suspect that it may not be that unusual, especially since the Posidonia Shale Formation has yielded other examples of soft tissue residues from marine amniotes, fishes and invertebrates. Comprehensive analyses, such as those performed in our study, will thus likely uncover additional ultrastructural and biomolecular remains in the future, and open the door for a much more detailed approach towards understanding the biology of marine biotas from the deep past.