Scientists have discovered Burgessomedusa phasmiformis, the oldest swimming jellyfish species ever, that lived over 500 million years ago. The specimens were remarkably well-preserved, unearthed at Canada's Burgess Shale fossil site in the late 1980s and 1990s. This finding sheds light on the history of medusozoans, the group to which modern-day jellyfish, box jellies, and hydroids belong.