Now we know the true colours (I had hoped for something a bit more exciting perhaps

Skin pigmentation provides evidence of convergent melanism in extinct marine reptiles

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12899.html

Nice reconstructions here:

http://www.sci-news.com/paleontology/science-color-mosasaurs-ichthyosaurs-turtles-1672.html

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Laura