title: Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles.
abstract: 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. The most ubiquitous and abundant pigment, melanin, also has a diverse range of non-visual roles, including thermoregulation in ectotherms. 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. Here we present direct chemical evidence of pigmentation in fossilized skin, from three distantly related marine reptiles: a leatherback turtle, a mosasaur and an ichthyosaur. 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 animals, at least some ichthyosaurs were uniformly dark-coloured in life. Our analyses expand current knowledge of pigmentation in fossil integument beyond that of feathers, 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.
authors.affiliation: Department of Geology, Lund University, SE-223 62 Lund, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Lindgren
authors.affiliation: SP Technical Research Institute of Sweden, Chemistry, Materials and Surfaces, SE-501 15 Borås, Sweden.
authors.forename: Peter
authors.initials: P
authors.lastname: Sjövall
authors.affiliation: Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02906, USA.
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Carney
authors.affiliation: 1] MAX-IV laboratory, Lund University, SE-221 00 Lund, Sweden [2] Chemical Physics, Department of Chemistry, Lund University, SE-221 00 Lund, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Uvdal
authors.affiliation: Department of Geology, Lund University, SE-223 62 Lund, Sweden.
authors.forename: Johan A
authors.initials: JA
authors.lastname: Gren
authors.affiliation: 1] Ocean and Earth Sciences, University of Southampton, Southampton SO14 3ZH, UK [2] Institute for Life Sciences, University of Southampton, Southampton SO14 3ZH, UK.
authors.forename: Gareth
authors.initials: G
authors.lastname: Dyke
authors.affiliation: MUSERUM, Natural History Division, Havnevej 14, 7800 Skive, Denmark.
authors.forename: Bo Pagh
authors.initials: BP
authors.lastname: Schultz
authors.affiliation: Integrated Bioscience Program, University of Akron, Akron, Ohio 44325, USA.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Shawkey
authors.affiliation: Mosasaur Ranch Museum, Lajitas, Texas 79852, USA.
authors.forename: Kenneth R
authors.initials: KR
authors.lastname: Barnes
authors.affiliation: Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Polcyn
mesh_terms: D000818:Animals; D059001:Aquatic Organisms / Q000502:physiology*; D005075:Biological Evolution; D001833:Body Temperature Regulation; D003116:Color; D053476:Extinction, Biological; D005580:Fossils; D008543:Melanins / Q000032:analysis; D008548:Melanosis / Q000378:metabolism*; D020460:Melanosomes / Q000737:chemistry; D010802:Phylogeny; D012104:Reptiles / Q000502:physiology*; D012867:Skin / Q000737:chemistry; D012880:Skin Pigmentation; D014426:Turtles / Q000502:physiology
chemical_list: D008543:Melanins; C041877:eumelanin