“A wonderous discovery” – The 505-million-year-old fossil record reveals the earliest known species of swimming jellyfish

The Royal Ontario Museum (ROM) has unveiled the discovery of the oldest fossilized swimming jellyfish known to date, which has been given the name Burgessomedusa phasmiformis. This exciting revelation was published in the esteemed journal, Proceedings of the Royal Society B.

The jellyfish, part of a group known as medusozoans, is part of a broader group called Cnidaria, one of the earliest known groups of animals to have lived, and includes current species such as box jellies, hydroids, stalked jellyfish, true jellyfish, as well as corals and sea anemones. Evidence from Burgessomedusa confirms that large, bell-shaped swimming jellyfish had already developed over half a billion years ago.

Remarkably preserved Burgessomedusa fossils have been found at the Burgess Shale, a testament to the preservation process given that jellyfish are almost entirely water-based. The ROM possesses nearly two hundred of these specimens, revealing intricate details of their internal anatomy and tentacles.

Some of the specimens even measure over 20 centimeters in length. These findings provide solid evidence for classifying Burgessomedusa within the medusozoan group. Given its similarities with modern jellyfish, it’s reasonable to infer that Burgessomedusa would have had free-swimming abilities and tentacles capable of catching substantial prey.

“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,” added Joe Moysiuk, co-author of the study and a Ph.D. candidate in Ecology & Evolutionary Biology at the University of Toronto, based at ROM.

The identification of Burgessomedusa is primarily based on the fossils discovered at the Burgess Shale, most of which were found in the late 20th century under the guidance of the former ROM Curator of Invertebrate Palaeontology, Desmond Collins.

These discoveries shed new light on the complexity of the Cambrian food chain and indicate that predatory behavior wasn’t limited to large swimming arthropods like Anomalocaris.

Co-author, Dr. Jean-Bernard Caron, ROM’s Richard Ivey Curator of Invertebrate Palaeontology, reflected on the impact of these findings, “Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wonderous 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.”

The evolutionary history of jellyfish, part of the Cnidaria group with their unique two-form life cycle (a vase-shaped body, or polyp, and a bell-shaped body, or medusa), is still a mystery.

Although fossilized polyps have been found in roughly 560-million-year-old rocks, the emergence of the free-swimming medusa form remains unclear. Fossils of jellyfish are extraordinarily rare, leading to a reliance on microscopic fossilized larval stages and molecular studies of living species to trace their evolution.

Fossils resembling medusozoan jellyfish, known as comb-jellies, have been discovered at the Burgess Shale. Despite superficial similarities, comb-jellies are part of a separate animal group, Ctenophora. Previous findings suggesting Cambrian swimming jellyfish are now being reinterpreted as ctenophores.

The Burgess Shale fossil sites, situated within Yoho and Kootenay National Parks and managed by Parks Canada, have been recognized as a UNESCO World Heritage Site since 1980 due to their exceptional significance.

Parks Canada continues to collaborate with leading scientists to further our understanding of this pivotal era in Earth’s history and to share these sites with the public through award-winning guided tours.

Source: 10.1098/rspb.2022.2490

Image Credit: Christian McCall. © Christian McCall