1914 illustration of a Jurassic marine reptile of the genus Metriorhynchus, by S.W. Williston. Metriorhynchus comprised about a dozen known species, making this one of the most common marine reptiles of the late Jurassic period. This prehistoric crocodile was characterized by its lack of scales (its smooth skin probably resembled that of its fellow marine reptiles, the ichthyosaurs) and its lightweight, porous skull, which presumably enabled it to poke its head out of the surface of the water while the rest of its body floated underneath at a 45-degree angle. This image has been color enhanced.