Palaeontologists uncover oldest known jellyfish fossils

Fossils of the oldest known species of jellyfish – which bobbed around ancient Cambrian seas some 505 million years ago – have been found in the Canadian Rockies.

The finding is particularly remarkable. While palaeontologists find fossilised remains all the time, jellyfish fossils are rarer than most.

After all, it’s tough to fossilise an animal that’s 95% water.

But traces of Burgessomedusa phasmiformis embossed on the face of the Burgess Shale – a World Heritage site within Canada’s Yoho National Park – are “exceptionally well preserved” according to the Royal Ontario Museum (ROM), which announced its findings in the Proceedings of the Royal Society B Biological Sciences.

Since fossil records from the area were first published in 1888 and the Burgess Shale was identified in 1909, a long list of ancient animals has been described from their rocky echoes. It is one of the best records of Cambrian invertebrates known.

Medusae – the grouping of cnidarians that Burgessomedusa belongs to – still exist today as freely swimming jellyfish like the Portuguese man o’ war, box jellyfish and tiny Irukandji.

The fossil analysis from those preserved in the Burgess Shale reveals anatomical features associated with these animals, including the telltale signs of tentacles.

B. phasmiformis probably grew to lengths of 20cm, including about 90 stubby tentacles emerging from its bell-shaped body.

Artistic reconstruction of a group of burgessomedusa phasmiformis swimming in the cambrian sea
Artistic reconstruction of a group of Burgessomedusa phasmiformis swimming in the Cambrian sea. Credit: Christian McCall

Most fossil specimens the ROM studied were identified in the Burgess rock layers three to four decades ago, but only now have evolutionary biologists pinned them to a genus.

“Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wonderous discovery,” says study co-author Dr Jean-Bernard Caron, ROM’s curator of invertebrate palaeontology.

Cnidarians like this newfound jellyfish are among the oldest groups of animals. While B. phasmiformis is not the oldest cnidarian discovered, many older specimens are of polyps – an early asexual stage of these animals still seen in the lifecycles of those that exist today.

Ancient comb-jellies have emerged from several Cambrian deposits but belong to a different animal category distinct from the cnidarian phylum.

Finding examples of the saucer-like medusa is almost unheard of and the discovery of B. phasmiformis helps researchers to draw evolutionary lines between jellyfish alive today and their poorly understood ancestors.

“Although jellyfish and their relatives are thought to be one of the earliest animal groups to have evolved, they have been remarkably hard to pin down in the Cambrian fossil record,” says research co-author Joe Moysiuk, a University of Toronto evolutionary biologist.

“This discovery leaves no doubt they were swimming about at that time.”

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