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Oldest known swimming jellyfish date back 505 million years

A team of researchers led by the Royal Ontario Museum (ROM) has recently identified and described the oldest known species of swimming jellyfish.

Burgessomedusa phasmiformis is a 505-million-year-old swimming jellyfish, highlighting the biodiversity of Cambrian ecosystems.

The fossilized specimens of Burgessomedusa were discovered at the Burgess Shale – one of Earth’s most important fossil deposits, located within the Yoho and Kootenay National Parks and managed by Parks Canada.


Jellyfish belong to medusozoans – a group of marine organisms within the phylum Cnidaria (one of the oldest group of animals to have existed that also includes corals and sea anemones). 

Medusozoans are still represented in today’s ecosystems by species such as box jellies, hydroids, stalked jellyfish, and true jellyfish. 

The discovery of Burgessomedusa provides clear evidence that large, swimming jellyfish with a typical saucer- or bell-shaped body were already present over 500 million years ago.

Focus of the study

According to the experts, cnidarians exhibit intricate life cycles featuring two distinct body forms: a polyp, resembling a vase-shaped structure, and a medusa, resembling a bell- or saucer-shaped structure. The medusa may be free-swimming or stationary. 

Although fossilized polyps have been discovered in rocks dating back approximately 560 million years, the origins of free-swimming medusae or jellyfish remain poorly understood. 

Scarce evidence

Fossil evidence of jellyfish is exceptionally scarce, necessitating a reliance on microscopic fossilized larval stages and molecular studies of present-day species (using DNA sequence divergence time models) to comprehend their evolutionary history.

Interestingly, some fossils resembling medusozoan jellyfish have been found in the Burgess Shale and other Cambrian deposits. However, most of these fossils actually belong to a distinct phylum of animals called Ctenophora, commonly known as comb-jellies. 

These creatures may share superficial similarities with medusozoans, but they are separate entities, making previous reports of Cambrian swimming jellyfish to be currently reinterpreted as ctenophores.

Exceptional fossils

Considering that jellyfish are 95 percent composed of water, the fossils discovered at the Burgess Shale are exceptionally well preserved. 

Currently, ROM has nearly two hundred specimens of Burgessomedusa (some of them reaching over 20 centimeters in length), allowing scientists to examine in great detail their internal anatomy and tentacles and classify them as medusozoans. 

However, compared to modern jellyfish, Burgessomedusa was likely capable of free-swimming and capture sizeable prey with its tentacles.

A wondrous discovery 

“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,” said co-author Joe Moysiuk, a doctoral student in Ecology and Evolutionary Biology at the University of Toronto.

“Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wondrous discovery. 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,” concluded co-author Jean-Bernard Caron, a Curator of Invertebrate Palaeontology at ROM.

Modern jellyfish

Jellyfish are fascinating marine animals found in every ocean around the world. Their bodies are composed of a gel-like substance that’s about 95% water, which makes them incredibly lightweight and helps them float.


Jellyfish come in a wide variety of shapes, sizes, and colors, with some being just a few millimeters in diameter, while others can reach several meters across. 

Their tentacles contain specialized cells called cnidocytes that they use for defense and to capture prey. These cells contain venomous stingers known as nematocysts, which can be harmful or even deadly to other animals, including humans.


One of the most remarkable traits of some jellyfish species is their ability to bioluminesce, or create light. This is thought to help them scare away predators or attract prey.

Life cycle

Jellyfish are also known for their unique life cycle, which includes both a sexual and an asexual phase. The life cycle begins with a free-swimming planula that eventually attaches itself to a rock or other hard surface and becomes a polyp. The polyp can then bud off clones of itself in a process known as strobilation. These new organisms, known as ephyrae, grow into the adult, or medusa, stage.

As for modern jellyfish, they are very similar to their ancient counterparts, with fossil evidence suggesting that jellyfish have been around for more than 500 million years. 


Despite their simple structure, jellyfish have proven to be incredibly resilient and adaptable, capable of surviving in a wide range of oceanic conditions. Some species, like the infamous box jellyfish and the Irukandji jellyfish, are known for their extremely potent venom.

Jellyfish blooms

Unfortunately, due to factors like climate change and overfishing of their predators, jellyfish populations have been increasing in some areas, leading to phenomena known as jellyfish blooms. These can be problematic for fishing, swimming, and power generation, as they can clog up the cooling systems of power plants.

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

Image Credit: Reconstruction by Christian McCall. © Christian McCall


By Andrei Ionescu, Staff Writer

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