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.
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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.).
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Backscattered electron images and single-element EDX maps of fossil ‘skin’ samples.
a, Leatherback turtle Eosphargis breineri, FUM-N-1450. b, Mosasaur Tylosaurus nepaeolicus, SMU 76532. c, Ichthyosaur, YORYM 1993.338. Energy-dispersive X-ray (EDX) maps: white, high intensity; black, low intensity. Note relatively high levels of carbon (C) in the fossil ‘skin’ structures, represented by the dark region in the backscattered electron images. Scale bars, 1 mm.
Extended Data Figure 2 Negative-ion ToF-SIMS spectra of ‘skin’ from FUM-N-1450, SMU 76532 and YORYM 1993.338, and natural eumelanin.
Note close agreement between fossil spectra, as well as between fossil spectra and that of the natural eumelanin standard (from Sepia officinalis). This similarity, both with regard to relative intensity distribution and precise mass of the eumelanin-related peaks in the entire mass range up to about 175 u (see also Extended Data Table 1), provides compelling evidence for high amounts of eumelanin pigment on the surface of the fossil microbodies. Differences in absolute signal intensities are caused by variations in instrument set up and data acquisition parameters, and are thus not related to the chemical composition of the samples. +, peaks in the natural eumelanin spectrum originating from impurities and not the eumelanin structure.
Extended Data Figure 3 Negative-ion ToF-SIMS spectra from selected regions of the mosasaur ‘skin’ sample together with natural eumelanin.
The spectra were obtained from an area containing primarily sedimentary matrix (top panel; red outline in Fig. 3b) and an area with abundant fossil melanosomes (middle panel; yellow outline in Fig. 3b). The spectrum acquired from the melanosome-rich area shows close agreement with the natural eumelanin standard spectrum (bottom panel), whereas the spectrum obtained from the sedimentary matrix is dominated by peaks representing ions of SixOy− and SixOyH− type, indicating silicate-rich minerals. Differences in peak widths are caused by variations in the data acquisition parameters and are thus not related to chemical composition. Specifically, the fossil spectra were acquired with the ToF-SIMS instrument optimised for high spatial resolution (resulting in broad peaks), whereas the eumelanin standard spectrum was acquired with the instrument optimised for high mass resolution (resulting in narrow peaks). +, peaks in the natural eumelanin standard spectrum originating from impurities and not the eumelanin structure.
Extended Data Figure 4 Negative-ion ToF-SIMS images of peaks representing eumelanin, sulphur-containing organic fragments and silicon dioxide.
a–p, Peaks representing eumelanin (a–c, e–h), sulphur-containing organic fragments (i–l) and silicon dioxide (m–p). The data were collected from a single measurement of the mosasaur ‘skin’. Note similar spatial distributions obtained for characteristic eumelanin peaks, sulphur-containing organic fragment peaks and silicon dioxide peaks, respectively. Note also comparable spatial distributions of eumelanin and sulphur-containing organic fragment peaks, suggesting diagenetic incorporation of sulphur with the eumelanin structure (Extended Data Fig. 6; see also Supplementary Information). Finally, note different spatial distribution of silicon dioxide peaks, representing the sedimentary matrix. The images in the right-hand column show the combined signal intensity for all peaks representing eumelanin (h), sulphur-containing organic fragments (l), silicon dioxide (p), and a colour overlay of these three images (d) in which green represents eumelanin, red represents silicon dioxide and blue represents sulphur-containing organic fragments. Peak mass is indicated beneath each image. MC, maximum count in one pixel; TC, total counts in the entire image.
Extended Data Figure 5 Comparison of negative-ion ToF-SIMS spectra from compounds with a molecular structure similar to that of eumelanin.
Note that the two lower spectra (natural and synthetic eumelanin) are very similar to one another, with the only substantial differences relating to peaks representing impurities in the natural eumelanin standard (marked with +). The spectra from ‘phaeomelanin’ (see Supplementary Information) and the two porphyrins (coproporphyrin I dihydrochloride and copper (II) phthalocyanine) show some similarities with eumelanin in the mass range up to 100 u, although substantial differences also do occur. Above 100 u, the ‘phaeomelanin’ and porphyrin spectra lack several features that characterize the eumelanin spectra, including prominent peaks at 121, 122, 145 and 146 u.
Extended Data Figure 6 Principal component analysis comparing negative-ion ToF-SIMS spectra from our fossil samples, eumelanin, phaeomelanin and other molecular standards.
a, Score plot of principal component 1 (PC1) and PC2, in which each spectrum is represented by a data point. The position of each point reflects characteristic features of the spectrum. b, Loadings plot for PC1 and PC2, in which each point represents a specific peak included in the analysis. The position of each peak indicates that it has a relatively high signal intensity in the spectra located at a corresponding position in the score plot (and, conversely, that spectra located in other areas have relatively lower intensities of this particular peak). Note substantial separation between different samples and molecular compounds in the score plot (see Supplementary Information). c, Peaks included in the analysis. These were selected based on their prominence, as well as assignment to organic fragments in the synthetic eumelanin (Eu) and natural phaeomelanin (Ph) spectra, respectively (see Extended Data Fig. 5).
Extended Data Figure 7 Light micrographs of histological sections from unstained skin tissue of extant leatherback turtle, Dermochelys coriacea.
a–c, Sections taken from the hip region of a hatchling (Saint Croix, US Virgin Islands) (a), carapace of a juvenile (Palm Beach County, Florida, USA) (b) and periocular region of an adult (Hutchinson Island, Florida, USA) (c). Note the unusually dark melanised layer of the dermis directly under the cornified layer (red arrows denote basal membrane of epidermis). Samples had been fixed (within a few hours post mortem) in 10% buffered formalin for months (hip, periocular) to years (carapace), stored in 70% ethanol for approximately 3 and 22 years, respectively, and then embedded with Tissue-Tek O.C.T. Compound (Sakura Finetek) and sectioned into ∼5-μm-thick slices using a Leica CM3050 S cryostat. Samples were transported to R.M.C. under authorisation of the US Fish and Wildlife Service with approval from the Florida Fish and Wildlife Conservation Commission pursuant to Marine Turtle Permit no. 073. Scale bars, 100 μm.
Note full ‘skin’ envelope preserved as amorphous black material (PMU R435; Museum of Evolution, Uppsala, Sweden), indicating that the animal was uniformly dark-coloured in life. Scale bar, 5 cm.
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Lindgren, J., Sjövall, P., Carney, R. et al. Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles. Nature 506, 484–488 (2014). https://doi.org/10.1038/nature12899
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