Whether they were mosasaurs or long-necked plesiosaurs, the seas of the Late Cretaceous Period teemed with gigantic marine reptiles. For example, plesiosaurs could grow up to 60 feet, or18 meters, long. Many of these enormous creatures did not survive the great extinction, but one marine reptile did — the sea turtle.
The first sea turtles appeared more than 245 million years ago. Most scientists believe they were land creatures that evolved to become sea dwellers during the Cretaceous. They were much larger than today's sea turtles with longer necks. One of the largest ancient sea turtles was the now extinct Atlantochelys mortoni. It lived about 75 million years ago in North America. It was 10 feet (3 meters) long and resembled a modern loggerhead [sources: Sci-News, Amos, SCISTP].