These 508-Million-Year-Old Fossils May Be Earth’s Oldest Swimming Jellyfish

Whether spotted pulsing under the waves or stranded on a sandy shore, jellyfish are a common sight on a trip to the beach. The soft-bodied animals are also a connection to the ancient past. Now, researchers have uncovered a 508-million-year-old fossil of a bygone jellyfish, possibly representing the oldest swimming jelly paleontologists have found.

In life, the umbrella-like animal, which paleontologists named Burgessomedusaphasmiformis in a new paper released Tuesday, was about seven inches across and draped with more than 90 short tentacles trailing beneath. Uncovered in the mountains of British Columbia, the fossil confirms that prehistoric jellies floated over reefs where their tentacles could capture prey swimming in the water column, just like living jellyfish.

“Sometimes in paleontology you know a great discovery as soon as it’s made in the field,” says Royal Ontario Museum paleontologist and study author Joseph Moysiuk.

During the 1980s and 1990s, field teams from the Royal Ontario Museum were looking for fossils of early animal life among the layers of the Burgess Shale, fine-grained rocks that contain reef life from more than 508 million years ago. The ancient layers are world-famous for containing fossils of strange, soft-bodied animals from a time when creatures such as arthropods and mollusks were just beginning to evolve. But “unlike a lot of Burgess Shale fossils,” which have gone through multiple interpretations due to their unusual anatomy, “these ones are quite large and obvious, even when they’re covered in mud,” Moysiuk says. Paleontologists knew almost immediately that they had found early jellyfish, but it took years before a formal description of the fossils was undertaken, culminating in the new identification published Tuesday in Proceedings of the Royal Society B.

All told, Moysiuk and colleagues have counted 182 fossils of Burgessomedusa uncovered in a particular spot called the Raymond Quarry, the only place they have been found. The fossils eluded detection for a long time because they seem relatively rare compared to their neighbors, although the reason for that is unclear. “Is it that they tended to live high in the water column, out of reach from the mud flows that buried these ancient seafloor communities?” Moysiuk wonders.

Paleontologists have been searching for Cambrian Period jellies for over a century, with some false starts along the way. In 1911, then-administrator of the Smithsonian Institution Charles Doolittle Walcott identified what he thought was a small, circular jellyfish from a curious, disc-shaped fossil found in the Burgess Shale. The find led artists to draw modern-looking jellyfish trailing tentacles in their theoretical depictions of Cambrian life. But later research revealed that these fossils were not jellyfish but the mouthparts of a strange arthropod named Peytoia that grasped soft prey with large, flexible appendages trailing from its head.

Despite the case of mistaken identity, however, paleontologists still expected that jellies were among the animals that originated in the Cambrian. “Given their presumably ancient roots, we would expect to see jellyfish preserved alongside other Cambrian animals,” Moysiuk says. Now, after more than a century of searching, paleontologists are beginning to find the missing medusas. In 2007, University of Kansas researcher Bruce Lieberman and colleagues described medusa jellyfish from Utah from the 505-million-year-old Marjum Formation, and a different team of paleontologists described similar fossils in 2016 from 521-million-year-old rocks in China’s Yunnan Province.

But researchers disagree about the identities of these previously described jellyfish. Moysiuk and colleagues propose the previous finds represent comb jellies, animals that belong to a different phylum and that push themselves through the water with hundreds of tiny cilia rather than pumping a large bell-shaped body like free-swimming jellyfish. Despite being superficially similar and about equally as old, jellyfish and comb jellies belong to two different branches on the tree of life.

Lieberman, who was not involved in the new study, disagrees. “I’ve been working with the description and analysis of soft-bodied fossils like these for decades, and [the Utah fossils] are among the best-preserved specimens that I’ve ever encountered.” The fossils from Utah and China, Lieberman says, are definitely jellyfish and not the comb jellies the new paper suggests.

Disagreement over the Utah and China fossils aside, Lieberman notes that the new study helps underscore the Cambrian as a critical time for jellyfish evolution. “It is always nice to see new Cambrian fossil finds presented,” Lieberman says, noting that “this study adds to the compelling body of evidence indicating that medusozoans were already an established and important clade by the Cambrian fossils.”

Even though Burgessomedusa is recognizable as a jellyfish, that doesn’t mean it was just like its living counterparts. “Burgessomedusa shares features with multiple modern jellyfish groups but doesn’t fit neatly into any of them,” Moysiuk says. The animal has a mix of features seen among modern jellies, its body roughly resembling today’s infamous box jellyfish while the tentacles look like those of moon jellies. The mix of traits, Moysiuk hypothesizes, might mean that Burgessomedusa was a close relative of the common ancestor from which our familiar modern jellies diverged.

The Cambrian, which ran from 485 million to 541 million years ago, was a critical time for life on Earth. Many of the animal body shapes we see around us today, including our own, began to coalesce during the time period, setting the stage for the next 500 million years of evolution.

“Burgessomedusa represents one more major animal body plan that can trace its roots back to the Cambrian,” Moysiuk says, “a striking and humbling reminder of how much biodiversity had already taken shape by half a billion years ago.”