Record-Old Jellyfish Fossils

Record-Old Jellyfish Fossils

Artist’s impression of the Cambrian jellyfish Burgessomedusa phasmiformis. © Christian McCall

They glide majestically through the seas – and have done so for ages, as the oldest known remains of jellyfish now prove: Researchers report on the amazingly well-preserved fossils of bell-shaped, free-swimming cnidarians from the Cambrian age. About 505 million years ago, these medusae, which can be up to 20 centimeters in size, preyed on small aquatic animals with their tentacles. The development of the “jellyfish concept” goes back to the so-called Cambrian explosion, the scientists say.

Pulsating, bell-shaped creatures made of gelatinous substance: Due to their bizarre-looking characteristics, jellyfish are prominent in the marine environment. Actually, however, it is not a specific animal form, but a life stage of special cnidarians. These so-called medusozoa sometimes live sessile as polyps and then produce their free-living medusa stage as part of what is known as metagenesis. These usually have a bell-shaped structure and move with the help of a water current generated by contractions. Many jellyfish also have tentacles with stinging cells, which they use to capture small aquatic animals and transport them to their central mouth opening.

On the trail of the Medusa concept

As can be seen from the great diversity and distribution of the Medusozoa among the cnidarians, it is a question of a decidedly successful concept in the history of development. This is already very old, because the cnidarians already emerged when the animal world developed strongly as part of the so-called Cambrian explosion. While fossilized polyps are known from rocks that are 560 million years old, the origin of the medusae in the cnidarians has so far remained unclear.

“Although jellyfish and their relatives are believed to be very ancient, they are extremely difficult to identify in the fossil record,” says senior author Joe Moysiuk of the University of Toronto. Because the jelly beings, which mainly consist of water, have hardly left any traces. Surprisingly, this was not the case with the finds that the team is now presenting. They come from the Burgess Shale of the Canadian Rocky Mountains, which is already famous for its detailed fossils of Cambrian creatures. The most prominent creature is probably the shrimp-like Anomalocaris with its bizarre gripping tools.

Amazingly detailed preserved jellyfish

As the researchers report, their results are not based on a

Fossil of two Burgessomedusa jellyfish with tentacles visible © Jean-Bernard Caron, Royal Ontario Museum

but also on over 170 extraordinarily well-preserved fossil finds, which are dated to be around 505 million years old. They detail the structures of what they now describe as the species Burgessomedusa phasmiformis. According to the investigations, the animal had a slightly cuboid screen of up to 20 centimeters in height. It was equipped with over 90 finger-like tentacles on the edge, which were probably used to capture sea creatures. The morphological features also suggest that the creature moved using pulsating movements, the researchers say.

According to them, the characteristics clearly show that this was the free-living developmental stage of a cnidarian – a jellyfish. The animal already had similarities with today’s representatives of the box and umbrella jellyfish, say the researchers. As part of the study, they also show that fossils from the Cambrian previously interpreted as possible jellyfish remains can be assigned to other life forms. “We now define Burgessomedusa phasmiformis as the oldest macroscopic and free-swimming medusa in the fossil record,” the scientists write. Apparently, the evolution of the life cycle with the medusa stage goes back to the time of the Cambrian explosion, the team concludes.

The discovery is rounded off by a special highlight among the

Jellyfish fossils (right) and tracks of the top arthropod predator Anomalocaris canadensis. © Desmond Collins, Royal Ontario Museum

Burgessomedusa fossils, the researchers report: The jellyfish were also preserved along with traces of the era’s famous “predator” – Anomalocaris. This is made particularly impressive: “Alongside Anomalocaris, these jellyfish were also efficient swimming predators in the Cambrian Seas,” says co-author Jean-Bernard Caron from the University of Toronto.

Source: Royal Ontario Museum, Specialist article: Proceedings of the Royal Society B Biological Sciences. Doi: 10.1098/rspb.2022.2490

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