505-million-year-old fossils upset primeval food chain
Paleontologists have discovered the world’s oldest free-swimming jellyfish in fossil form. Burgess Medusa Phasmiformis existed 505 million years ago and moved through the water column using a bell-shaped screen up to 20 centimeters high. It caught larger and more agile prey with it’s over 90 finger-like tentacles. As a result, contrary to popular belief, jellyfish were at the top of the former food chains, not just large arthropods.
Life began in the water over 500 million years ago. Aside from squid and trilobites, there were also large predators like Anomalocaris up to no good. However, it was not entirely clear when the first free-swimming jellyfish with umbrellas and tentacles appeared in the primordial ocean. It’s also difficult to find clues because jellyfish are made of soft tissue and rarely fossilize.
The chances of fossilization are much higher in the first phase of these cnidarians’ lives, which they spend as a sessile polyp with a mineral shell. However, simply looking at a fossil polyp does not reveal whether it would have evolved into a free-swimming jellyfish medusa or not.
Jellyfish hunting in prehistoric times
Many fossils that are said to show the first free-swimming Medusa, as we know it today, have been presented in the search for one. However, the paleontological community is divided about these specimens. Some scientists believe that these discoveries are comb jellyfish, but not jellyfish in the traditional sense.
However, paleontologists led by Justin Moon of the University of Toronto are now convinced that they have discovered the world’s oldest free-swimming jellyfish. They examined over 170 very well-preserved body fossils from the 505-million-year-old Burgess Shale in Raymond Quarry, Canada, for this classification. The majority of the fossils, some of which include the jellyfish’s stomach cavity, pedicle, and gonads, were discovered during expeditions in the late 1980s and 1990s.
Moon and his colleagues believe that all of these fossil jellyfish are members of a previously unknown species, Burgess Medusa phasmiformis. Paleontologists believe that this jellyfish’s bell-shaped umbrella could have reached a height of 20 centimeters based on the fossil remains. According to the researchers, she was probably able to “row” in the water with its assistance.
Burgess Medusa also had over 90 short, finger-like tentacles, which Moon and colleagues believe could be used to catch larger, more agile prey. This contradicts the previous model of the food webs in the primordial ocean. Large swimming arthropods, such as the infamous Anomalocaris, as well as large jellyfish, could have been at the top of the food chain.
Moon and his colleagues hypothesize that Burgess Medusa was hunting in both the open water column and near the seafloor. This is due to the fact that most fossilized specimens were once buried alive by mudslides, which would not have occurred in open water.
The progenitor of the box jellyfish
But what is the connection between Burgess Medusa and today’s jellyfish? The early free-swimmer may have belonged to the extinct ancestral group of modern-day Acraspeda, which includes stalk, umbrella, and box jellyfish, according to Moon’s team’s pedigree analysis. It’s also possible that Burgess Medusa was the only ancestor of the box jellyfish.