Sea turtles are magnificent swimmers, migrating thousands of miles each year.
But when the first turtles moved from the land to the sea more than a hundred million years ago, they didn't have this ability, says Ren Hirayama, a paleontologist at Teikyo Heisei University in Chiba, Japan. The little turtle, barely eight inches long, was probably a young adult. Hirayama noticed that the bones in its limbs were not fused into rigid paddles as they are in living sea turtles. The fossil turtle had movable digits, like a freshwater turtle's. When swimming in salty seas, a marine animal with land ancestry runs the risk of dehydration because its body fluids are much less salty than the surrounding water.