Five hundred million years ago, the ancient, shallow sea in what is now British Columbia teemed with unusual creatures unlike any alive today. But there's one you'd recognize if it swam by: A jellyfish much like those that pulse through today's oceans.

Scientists say fossils found in Canada's Burgess Shale are the oldest-known creatures that we would recognize as jellyfish — and they were likely the terrors of the sea during the Cambrian geological period.