The Royal Ontario Museum (ROM), in Canada, has announced that it has identified the oldest swimming jellyfish in the fossil record with the newly named Burgessomedusa phasmiformis, according to the journal Proceedings of the Royal Society B. Jellyfish belong to the medusozoans, one of the oldest groups of animals that have existed on our planet and that also includes corals and sea anemones.
Burgessomedusa was already known of antiquity through its fossils, which are the oldest on Earth. However, the new study shows that large, swimming jellyfish with the typical saucer- or bell-shaped body had already evolved more than 500 million years ago.
The team of scientists from Canada has found close to two hundred specimens of which remarkable details of the internal anatomy and tentacles can be observed, with some specimens reaching more than 20 centimeters in length.
These details allow the Burgessomedusa to be classified as a medusozoa. Compared to modern jellyfish, Burgessomedusa would also have been able to swim freely, and the presence of tentacles would have allowed it to capture large prey.
Burgessomedusa fossils are exceptionally well preserved in the Burgess Shale considering that jellyfish are approximately 95% composed of water.
“Although jellyfish and their relatives are thought to be one of the earliest groups of animals to have evolved, they have been very difficult to pin down in the Cambrian fossil record. This discovery leaves no doubt that they were swimming at that time”, explained the co-author of this discovery, Joe Moysiuk, a researcher at the University of Toronto.
This study, which identifies Burgessomedusa as the oldest, is based on fossil specimens discovered in the Burgess Shale (Canada) and found mostly in the late 1980s and 1990s.
"Finding such incredibly delicate animals preserved in layers of rock on top of these mountains is a marvelous discovery. Burgessomedusa adds to the complexity of Cambrian food webs, and like Anomalocaris, which lived in the same environment, these jellyfish were efficient swimming predators," says Dr. Jean-Bernard Caron, co-author of the study.
Cnidarians have complex life cycles with one or two body shapes, a vase-shaped body, called a polyp, and in medusozoans, a bell- or saucer-shaped body, called simply a medusa, which may or may not be free-swimming. Although fossilized polyps are known in rocks that are about 560 million years old, the origin of the jellyfish is not well understood. Fossils of any type of jellyfish are extremely rare.
Consequently, their evolutionary history is based on fossilized microscopic larval stages and on the results of molecular studies of living species (modeling the divergence times of DNA sequences).
The Burgess Shale fossil beds are located in Yoho and Kootenay National Parks and are managed by Parks Canada. The Burgess Shale was designated a UNESCO World Heritage Site in 1980 due to its Outstanding Universal Value and is now part of the largest Canadian Rocky Mountain Parks World Heritage Site.
