Jellyfish: Earth’s Timeless Swimmers

Jellyfish, those fascinating slimy sea creatures, have been gliding through the oceans for an astonishingly long time. Despite their lack of blood, a heart, or a brain, they are some of the oldest animals on Earth, existing for over 500 million years. Recently, scientists from Toronto’s Royal Ontario Museum made an exciting discovery – they found the oldest swimming jellyfish in the fossil record, named Burgessomedusa phasmiformis. Let’s dive into the world of jellyfish and learn more about their ancient history.

Jellyfish: Part of the Medusozoans

An Ancient Clade

Jellyfish belong to a group called medusozoans, which includes various sea creatures like box jellies, hydroids, stalked jellyfish, and true jellyfish. Medusozoans are part of a larger group known as Cnidaria, which also includes sea anemones and corals. The discovery of Burgessomedusa shows that large, bell-shaped or saucer-shaped swimming jellyfish had already evolved more than 500 million years ago.

Remarkably Preserved Fossils

A Glimpse into the Past

Jellyfish are primarily made of water, which makes it challenging for their remains to be preserved as fossils. However, the Burgessomedusa fossils are an exception, with exceptional preservation in the Burgess Shale located in the Canadian Rockies. The Royal Ontario Museum holds nearly 200 specimens that offer insights into the internal anatomy and tentacles of these ancient jellyfish. Some of these fossils measure more than seven inches long, and they demonstrate that Burgessomedusa could freely swim and catch relatively large prey using its tentacles.

Complex Life in the Cambrian Era

More Than Just Swimming Arthropods

The discovery of Burgessomedusa in the Burgess Shale has provided valuable information about the complexity of the Cambrian food chain. Contrary to previous beliefs, the large swimming arthropods like Anomalocaris were not the only predators during that time. Jellyfish like Burgessomedusa were also efficient swimming predators, contributing to the diverse ecosystem of the Cambrian era.

Unraveling the Evolutionary History

From Polyps to Medusa

Cnidarians, the group that jellyfish belong to, have a complex life cycle that includes multiple body forms. The vase-shaped, non-free-swimming body is called a polyp, while medusozoans, like jellyfish, have a bell or saucer-shaped body. Fossilized polyps have been found in rocks dating back around 560 million years, but the origin of the more free-swimming medusa or jellyfish is not entirely clear. However, this recent discovery of Burgessomedusa adds more pieces to the puzzle of jellyfish evolution and their place in Earth’s history.


Jellyfish continue to amaze us with their timeless presence in Earth’s oceans. Thanks to discoveries like Burgessomedusa, we can better understand their ancient origins and their significant role in shaping the ecosystems of the past. As scientists uncover more about these enigmatic creatures, the mysteries of their evolutionary history are slowly being unraveled, adding to the wondrous diversity of life on our planet.

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