This may look like a swimming cap, but it's actually an ancient predator

The oldest-ever jellyfish fossil from more than half a billion years ago has been discovered.

The free-swimming predatory jellyfish, named Burgessomedusa phasmiformis, was discovered from fossil records extracted from the Burgess Shale – a famous fossil-bearing deposit in the Canadian Rockies.

The 505 million year old fossils of the jellyfish are remarkably well-maintained at the site and show the animal – comprised 95% of water – had tentacles capable of seizing sizeable prey.

Jellyfish belong to the ‘medusozoans’ group of animals, named after their similarities in appearance with the Greek myth of Medusa; an evil woman who had her hair turned to snakes and a face so hideous it would turn people into stone after upsetting the goddess Athena.

Today’s medusozoans include box jellyfish, hydroids, stalked jellyfish and ‘true’ jellyfish.

They are part of one of the oldest groups of animals ever to have existed on Earth – called Cnidaria – which also includes corals and sea anemones.

The Burgessomedusa unarguably show that large, swimming jellyfish with typical saucer or bell-shaped bodies had already evolved more than 500 million years ago.

The Burgess Shale fossil site, in the Yoho and Kootenay National Parks in southwestern Canada, is renowned for the exceptional preservation of the soft parts of its fossils and is one of the world’s earliest fossil beds containing soft-part imprints.

The site, which was designated a UNESCO World Heritage Site in 1980, also contains exceptionally well-preserved Burgessomedusa fossils.

The Royal Ontario Museum (ROM) holds close to 200 specimens, mostly found in the late 1980s and 90s, from which intricate details of internal anatomy and tentacles can be observed – with some specimens measuring more than 20 centimetres in length.

These details enabled researchers to identify Burgessomedusa as a medusozoan.

The findings show the Cambrian food chain was far more complex than previously thought, and that predation was not limited to large swimming arthropods like Anomalocaris – a strange-looking ‘abnormal shrimp’, fossils for which have also been found in the Burgess Shale.

Joe Moysiuk, a PhD candidate in Ecology & Evolutionary Biology at the University of Toronto, explained that early jellyfish have, until today, always proven difficult to track.

Mr Moysiuk, who is based at ROM, said: ‘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.

‘This discovery leaves no doubt they were swimming about at that time.’

Dr Jean-Bernard Caron, a co-author of the study, added the fossil records showed the jellyfish were ‘efficient predators’.

‘Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wonderous discovery,’ he said.

‘Burgessomedusa adds to the complexity of Cambrian foodwebs, and like Anomalocaris which lived in the same environment, these jellyfish were efficient swimming predators.’

‘This adds yet another remarkable lineage of animals that the Burgess Shale has preserved chronicling the evolution of life on Earth.’

Cnidarians, a group containing more than 11,000 species of aquatic animals found both in freshwater and marine environments, have complex life cycles with one or two body forms: a vase-shaped body, called a polyp, and in medusozoans, a bell or saucer-shaped body, called a medusa or jellyfish.

While fossilised polyps are known from rocks 560 million years old, the origin of the free-swimming medusa or jellyfish is comparatively not well understood.

Fossils of any type of jellyfish are extremely rare and, as a consequence, their evolutionary history is based on microscopic, fossilised larval stages and the results of molecular studies of living species.

Visitors to the ROM in Toronto, southeastern Canada, can see fossils of Burgessomedusa phasmiformis on display in the Burgess Shale section of the recently opened Willner Madge Gallery, Dawn of Life.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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