Wednesday, February 12, 2014

[Paleontology • 2014] Skin Pigmentation provides evidence of Convergent Melanism in Extinct Marine Reptiles

Fossil pigments reveal dark coloration of extinct marine reptiles.
The leatherback turtle (top) and mosasaur (bottom) have a dark back and light belly, a camouflage pattern, and the ichthyosaur (center) is uniformly dark.
illustration: Stefan Sølberg
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 mosasaur6 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.

Johan Lindgren et al. 2014Skin Pigmentation provides evidence of Convergent Melanism in Extinct Marine Reptiles. Nature. doi:

Fossils Reveal True Color of Mosasaurs, Ichthyosaurs, Prehistoric Turtles 
: Fossilized skin pigments from an 85 million-year-old mosasaur, a 193 million-year-old ichthyosaur and a 55 million-year-old leatherback turtle have revealed the skin color of these extinct marine reptiles.