World's oldest jellyfish? Fossils found in Canada are 1st of their kind

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.

The jellyfish had a bell about 20 centimetres high — as large as a loaf of bread — making it one of the largest creatures at that time, said Joe Moysiuk, a PhD student at the University of Toronto and the Royal Ontario Museum (ROM) who helped describe the species in a new study published Tuesday in the Proceedings of the Royal Society B.

The rectangular shape of the bell was similar to that of deadly, venomous, modern-day box jellies that live in northern Australian and Indo-Pacific waters, suggesting that the ancient jellyfish was also a fast and powerful swimmer. 

"This may have been a pretty aggressive predatory species of jellyfish," said Moysiuk, who worked on the study with two other colleagues, Justin Moon and Jean-Bernard Caron.

Its bell was fringed with more than 90 tentacles, resembling those of the harmless moon jellyfish.

A couple of the fossilized jellyfish — among nearly 200 unearthed at the Burgess Shale — have been on public display as part of the Dawn of Life exhibit at the Toronto-based ROM since the exhibit first opened in 2021.

But this week, a name plate was added underneath: Burgessomedusa phasmiformis — their new official scientific name.

The name means "the Burgess Shale jellyfish with a ghostly form," Moysiuk explained. "Specifically, we thought it looked like the ghost from the game Pac-Man."

Frankie Dunn, a senior researcher at the Oxford University Museum of Natural History in the U.K., called the new discovery "an amazing fossil."

Dunn published a discovery last year of an even older creature that was technically a jellyfish, but not as most people know them.

Jellyfish are related to corals, beginning their lives as coral-like polyps that have a stalk and are stuck to things like rocks or the sea floor — a stage where very ancient jellyfish relatives may have remained for their whole lives. 

The fossil Dunn studied, Auroralumina attenboroughii, was a huge jellyfish polyp that lived 557 million to 562 million years ago, during the Ediacaran period, before the Cambrian.

It wasn't clear if Auroralumina ever matured into a swimming jellyfish, Dunn said.