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. Hirayama found a nearly complete fossil of the oldest known sea turtle in a 110-million-year-old rock collected from the site of an ancient Brazilian sea. 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. "It would not have been a very strong swimmer," says Hirayama. The turtle's skull, however, showed that it was indeed a marine animal: Hirayama found traces of enormous tear glands. 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. Sea turtles have large tear glands that concentrate salt and shed it in huge, salty tears. "If such glands are not present," says Hirayama, "the turtles could not thrive in a marine environment."