Throughout the animal kingdom, adaptive colouration serves critical functions ranging from inconspicuous camouflage to ostentatious sexual display, and can provide important information about the environment and biology of a particular organism1,2. The most ubiquitous and abundant pigment, melanin, also has a diverse range of non-visual roles, including thermoregulation in ectotherms3,4. However, little is known about the functional evolution of this important biochrome through deep time, owing to our limited ability to unambiguously identify traces of it in the fossil record2. Here we present direct chemical evidence of pigmentation in fossilized skin, from three distantly related marine reptiles: a leatherback turtle5, a mosasaur6 and an ichthyosaur7. We demonstrate that dark traces of soft tissue in these fossils are dominated by molecularly preserved eumelanin, in intimate association with fossilized melanosomes. In addition, we suggest that contrary to the countershading of many pelagic animals8,9, at least some ichthyosaurs were uniformly dark-coloured in life. Our analyses expand current knowledge of pigmentation in fossil integument beyond that of feathers2,10, allowing for the reconstruction of colour over much greater ranges of extinct taxa and anatomy. In turn, our results provide evidence of convergent melanism in three disparate lineages of secondarily aquatic tetrapods. Based on extant marine analogues, we propose that the benefits of thermoregulation and/or crypsis are likely to have contributed to this melanisation, with the former having implications for the ability of each group to exploit cold environments.
We thank I. Gladstone, S. King and the Yorkshire Museum for permission to sample YORYM 1993.338, as well as J. Wyneken, P. Weston and L. Alibardi for providing and sectioning the extant leatherback turtle skin samples, respectively. B. P. Kear took the photograph of PMU R435 (Extended Data Fig. 8). This research was supported by grants from the Swedish Research Council, the Crafoord Foundation, the Royal Swedish Academy of Sciences (J.L.), VINNOVA Swedish Governmental Agency for Innovation Systems (P.S.), the National Geographic Society/Waitt Foundation (R.M.C.), the National Science Foundation, Human Frontiers Science Program, and Air Force Office of Scientific Research (M.D.S.).
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