It took decades of struggling with the weather on a small, desolate island off the Antarctic Peninsula. But now, scientists have finally unearthed the heaviest known elasmosaur, an ancient aquatic reptile that swam the seas of the Cretaceous period alongside the dinosaurs. The animal would have weighed as much as 15 tons, and it is now one of the most complete ancient reptile fossils ever discovered in Antarctica.

Elasmosaurs make up a family of the plesiosaurs, which represent some of the largest sea creatures of the Cretaceous. Plesiosaurs generally look a little like large manatees with giraffe necks and snake-like heads, though they have four flippers rather than a manatee’s three. (Find out about a plesiosaur fossil found