Cretaceous marine predators. Computer artwork of three primary marine predators that shared the waters of the Western Interior Seaway of North America 75 million years ago, during the Cretaceous period. Left to right is a nondescript invertebrate pursued by a 4 foot (1.2 metres) long Enchodus, followed by a 17 foot (5 metres) long Dolichorhynchops, followed by a 55 foot (17 metres) long Mosasaur.