'Swimming Predators' That Lived 500 Million Years Ago Found in Rockies

Researchers have uncovered a previously unknown species of ancient jellyfish that lived more than 500 million years ago.

The now-extinct creature is the oldest swimming jellyfish discovered to date and sheds light on the evolutionary history of these animals, according to a study Proceedings of the Royal Society B.

Jellyfish are regarded as some of the oldest animals ever to have existed. But because their soft bodies are prone to decay, it is extremely difficult for them to become fossilized. As a result, their evolutionary history remains largely hypothetical.

But in the latest study, a team of researchers describes a new species, named Burgessomedusa phasmiformis—an exceptionally well-preserved large jellyfish dated to the middle of the Cambrian Period around 505 million years ago.

Researchers described the new species from more than 170 fossil specimens found in the Burgess Shale—a geological formation in the Canadian Rockies of British Columbia.

"It is one of the best-preserved fossil jellyfish in the entire history of this group of animals," Jean-Bernard Caron, a paleontologist and author of the study from the University of Toronto, Canada, told Newsweek.

Most jellyfish fossils uncovered to date are simply imprints of the animals, meaning that the specimens themselves are not preserved. Only a few details of anatomy can be gathered from such imprints, and therefore, such fossils cannot provide much information about the evolutionary history or ecology of these animals.

"The body of a jellyfish is filled with water; this is why it is so hard to see these animals when diving, and it is therefore little surprise that fossilized jellyfish are extremely rare!" Caron said.

The latest study examined fossils that were mostly found in the late 1980s and 1990s by researchers with the Royal Ontario Museum, Toronto. The fossils were being kept in the museum's collections but had never formally described until the recent research was conducted.

Scientists still do not know for sure how and why these jellyfish fossils were so preserved so well at the Burgess Shale. The formation, which was once underwater, is rich in fossils from the mid-Cambrian, including many with soft body parts preserved.

"These fossils certainly highlight the exceptional conditions for soft-tissue preservation that the Burgess Shale is already famous for," Caron said. "I would even say that this is a new high for the Burgess Shale with these fossils pushing the limits of what can be preserved at that site."

The jellyfish that the scientists documented are generally bell-shaped, but some box-like and other intermediate forms also appear, demonstrating the flexibility in the body of these animals.

The specimens vary in size from just under one inch to almost eight inches in length, according to Caron.

"Considering that most animals at the Burgess Shale are smaller than an adult finger, the largest jellyfish specimens were giants compared to other organisms living at the same time in the Burgess Shale community," he said.

Remarkable details have also been preserved in the fossils. For example, the jellyfish have more than 90, short, finger-like tentacles all around the margin of the bell. The mouth is located at the end of a long structure within the bell, which is connected to a large stomach cavity. Some specimens also display four elongated gonads located inside and along the bell.

The evidence indicates that Burgessomedusa would have been capable of free-swimming, while the presence of tentacles suggests that it could capture sizeable prey.

"This discovery suggests that the Cambrian food-web was more complex than previously thought, with this new jellyfish being one of the largest predators known during that time"

Jellyfish belong to an animal group known as medusozoans, which includes today's true jellyfish, box jellyfish, hydrozoans, and stalked jellyfish. Medusozoans, in turn, form part of a larger group known as Cnidaria, which also includes corals and sea anemones.

Cnidarians have complex life cycles that may involve two principal body forms: a vase-shaped body form known as a polyp, and a bell- or saucer-shaped body called the medusa—the typical body form of a jellyfish. The medusa, which is the reproductive stage, can be free-swimming. Some members of Cnidaria only exhibit the polyp body form, but others cycle through both stages.

While fossilized polyps have previously been found in rocks dated to around 560-million-years-ago—even earlier than the Burgessomedusa fossils—the origin of free-swimming medusa, or jellyfish, is not well understood.

The jellyfish fossil record is controversial, according to Caron. Before the discovery of Burgessomedusa, the oldest published jellyfish body fossil was a fragmentary and poorly preserved specimen from the 518-million-year-old Chengjiang deposit in China.

But in the latest study, the authors reinterpret this specimen as a type of comb jelly. While comb jellies superficially resemble jellyfish from the Cnidaria group, they actually belong to a separate branch of animals known as Ctenophora.

Older microscopic fossils thought to represent medusa from 535 million years ago are already known, but it is likely that they were not capable of swimming.

As a result, Burgessomedusa represents the oldest unambiguous, large-scale swimming jellyfish in the fossil record, the researchers said.

"The key term here is 'swimming,'" Caron said. "This discovery shows that a large swimming jellyfish life-stage had already evolved by the middle Cambrian period in the group called medusozoans."

"Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wondrous discovery. 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," Caron said in a statement.