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Illustration: This is what the Burgessomedusa phasmiformis might have looked like

Photo: Christian McCall

Jellyfish are among the oldest groups of animals – researchers at the Royal Ontario Museum (ROM) have now identified the earliest swimming jellyfish. The previously unknown species Burgessomedusa phasmiformis lived 505 million years ago and belongs to the so-called Medusozoa: In its complex life cycle, this group forms a stage as Medusa, colloquially called jellyfish. These include box jellyfish (Cubozoa) and umbrella jellyfish (Scyphozoa). The work was published in the journal Proceedings of the Royal Society.

The evolutionary history of jellyfish is mostly based on microscopically small, fossilized larval stages and molecular studies of living species, according to the researchers. Fossils of jellyfish species are rare because they consist largely of water and do not contain hard parts. But the researchers were lucky: For the study, they examined fossils that were discovered in the Burgess Shale deposit in Canada – and are exceptionally well preserved. In the study, the team describes the Burgessomedusa species, named after the site, of which about 170 fossils are known.

The jellyfish has a bell-shaped umbrella that is up to 20 centimeters high and up to about 8 centimeters wide. More than 90 short finger-like tentacles emanate from the edge of the umbrella, which are used to catch prey. In the middle of the umbrella hangs – as with today's jellyfish – an elongated stomach stalk, the so-called manubrium, including a mouth opening at the lower end.

B. phasmiformis is the earliest clearly free-swimming Medusa known to date, the group writes. It is possible that it is a very early form of the box jellyfish (Cubozoa) or a similar group.

In contrast, other fossils from China and the US state of Utah, which have been discussed as jellyfish, are probably animals that resemble comb jellyfish (Ctenophora). These are not real jellyfish, the group points out, pointing to a number of anatomical differences and in particular to the absence of tentacles.

In their life cycle, jellyfish develop from a fertilized egg into a larva that attaches itself to the substrate, grows and becomes a polyp. The Medusa – the actual jellyfish – detaches itself from this and then swims through the sea.

The researchers see the new species as evidence of species richness more than half a billion years ago. In the Cambrian, which began roughly 545 million years ago, many animal phyla emerged within a short period of time that still exist today. 505 million years ago, there were already several groups of predatory animals in the oceans, including arrowworms (Chaetognatha) and various representatives of arthropods (Arthropoda) such as the shrimp-like species Anomalocaris canadensis. This even appears in some impressions of the Burgess Shale in the immediate vicinity of B. phasmiformis.