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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 organism (1,2). The most ubiquitous and abundant pigment, melanin, also has a diverse range of non-visual roles, including thermoregulation in ectotherms (3,4). 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 record (2). Here we present direct chemical evidence of pigmentation in fossilized skin, from three distantly related marine reptiles: a leatherback turtle (5), a mosasaur (6) and an ichthyosaur (7). 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 counter shading of many pelagic animals (8,9), at least some ichthyosaurs were uniformly dark-coloured in life. Our analyses expand current knowledge of pigmentation in fossil integument beyond that of feathers (2,10), 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., On rare occasions, the fossil record reveals examples of exceptional preservation, in which decay-prone tissues, such as skin, are preserved as 'an organic film' (11) with a high degree of [...]

An isolated, yet virtually intact contour feather ( FUM-1980) from the lower Eocene Fur Formation of Denmark was analysed using multiple imaging and molecular techniques, including field emission gun scanning electron microscopy ( FEG- SEM), X-ray absorption spectroscopy and time-of-flight secondary ion mass spectrometry (ToF- SIMS). Additionally, synchrotron radiation X-ray tomographic microscopy ( SRXTM) was employed in order to produce a digital reconstruction of the fossil. Under FEG- SEM, the proximal, plumulaceous part of the feather revealed masses of ovoid microstructures, about 1.7 μm long and 0.5 μm wide. Microbodies in the distal, pennaceous portion were substantially smaller (averaging 0.9 × 0.2 μm), highly elongate, and more densely packed. Generally, the microbodies in both the plumulaceous and pennaceous segments were aligned along the barbs and located within shallow depressions on the exposed surfaces. Biomarkers consistent with animal eumelanins were co-localized with the microstructures, to suggest that they represent remnant eumelanosomes (i.e. eumelanin-housing cellular organelles). Additionally, ToF- SIMS analysis revealed the presence of sulfur-containing organics - potentially indicative of pheomelanins - associated with eumelanin-like compounds. However, since there was no correlation between melanosome morphology and sulfur content, we conclude these molecular structures derive from diagenetically incorporated sulfur rather than pheomelanin. Melanosomes corresponding roughly in both size and morphology with those in the proximal part of FUM-1980 are known from contour feathers of extant parrots (Psittaciformes), an avian clade that has previously been reported from the Fur Formation. [ABSTRACT FROM AUTHOR]

Feathers are amongst the most complex epidermal structures known, and they have a well-documented evolutionary trajectory across non-avian dinosaurs and basal birds. Moreover, melanosome-like microbodies preserved in association with fossil plumage have been used to reconstruct original colour, behaviour and physiology. However, these putative ancient melanosomes might alternatively represent microorganismal residues, a conflicting interpretation compounded by a lack of unambiguous chemical data. We therefore used sensitive molecular imaging, supported by multiple independent analytical tests, to demonstrate that the filamentous epidermal appendages in a new specimen of the Jurassic paravian Anchiornis comprise remnant eumelanosomes and fibril-like microstructures, preserved as endogenous eumelanin and authigenic calcium phosphate. These results provide novel insights into the early evolution of feathers at the sub-cellular level, and unequivocally determine that melanosomes can be preserved in fossil feathers. [ABSTRACT FROM AUTHOR]