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.
Upper Cretaceous marine rocks of the Big Bend Region of trans-Pecos Texas preserve a number of marine-adapted mosasauroids. At least three unnamed taxa of basal mosasauroids are represented by remains from shaly limestones in the middle Turonian portion of the Boquillas Formation. These occur along with remains of larger derived mosasaurs referable to Russellosaurina and an undescribed tylosaurine. Derived mosasaurs from the middle to late Coniacian include the first report of Tylosaurus kansasensis outside of Kansas, T. nepaeolicus, Platecarpus planifrons, and Platecarpus aff. P. planifrons. Clidastes liodontus is found in the latest Coniacian or early Santonian part of the Pen Formation. An undescribed species of Ectenosaurus, Clidastes sp. and an indeterminate plioplatecarpine occurs in the middle Santonian to early Campanian interval of the Pen Formation. The mosasaur fauna from the Big Bend region is quite similar to that from the Smoky Hill Chalk of Kansas, a thousand kilometres to the north. We refine the position of the Cenomanian–Turonian boundary within the Ernst Member of the Boquillas Formation, based on ammonite faunas. We also corroborate previous interpretations describing the time-transgressive nature of the onset of deposition of the Pen Formation based on a west-to-east descending level of the Inoceramus (Cremnoceramus) undulatoplicatus FAD.
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