May 28 : Tyrannosaurus rex was a huge land predator during the twilight of the age of dinosaurs. But it was not the only T. rex that terrorized the Cretaceous Period. There also was a marine reptile named Tylosaurus rex - the T. rex of the sea - that rivaled its land counterpart in size and ferocity.Scientists said they have identified Tylosaurus rex as a distinct species after a wide-ranging examination of previously discovered fossils. They made the judgment based on its enormous size, fine serrations along the edges of its teeth to cut flesh more efficiently, heavy-duty musculature of the jaws and neck for subduing large prey, and certain other anatomical nuances.Roughly 66 million years ago, Tyrannosaurus rex roamed the land in Western North America. Approximately 80 million years ago, Tylosaurus rex prowled an inland sea that at the time split North America in two. The seagoing T. rex had a streamlined body, elongated snout with large teeth, four large paddle-like flippers and a powerful tail.The genus Tylosaurus has been known since fossils were first unearthed during the 19th century. A handful of species are recognized, including Tylosaurus proriger. A genus is a grouping of closely related species sharing similar characteristics. For example, lions and tigers are from the same genus but represent different species.