2023-08-03 12:16:26
Researchers from the Royal Ontario Museum (ROM, in Canada) have announced the discovery of the oldest swimming jellyfish in the fossil record: Burgessomedusa phasmiformis. The finding and details are published in the magazine Proceedings of the Royal Society B.
Its remains have been dated to 505 million yearss and have been located in the Shale or Burgess shalesa huge deposit that extends through the Yoho and Kootenay National Parks in British Columbia (Canada).
Fossils of Burgessomedusa phasmiformis, from 505 million years ago, have been found in the Burgess Shale (Canada)
Given their age and the fact that jellyfish are made up of approximately 95% water, the fossils of this Burgessomedusa they are exceptionally preserved.
Jellyfish belong to the group of Medusozoos which, in addition to true jellyfish, include cubozoans (because of their cubic shape), hydrozoans, and sessile jellyfish. They are part, in turn, of one of the oldest types of animals that have existed, the cnidariansto which corals and sea anemones also belong.
The Burgessomedusa demonstrates unequivocally that large, swimming jellyfish with a typical saucer- or bell-shaped body had already evolved more than 500 million years ago.
Fossil of a large and a small specimen (rotated 180 degrees) in the shape of a bell and with preserved tentacles. / Jean-Bernard Caron/Royal Ontario Museum
The ROM keeps about two hundred copies in which its internal anatomy and tentacles can be observed, with some specimens reaching more than 20 centimeters in length. These details allow classifying Burgessomedusa like a jellyfish. Compared to modern jellyfish, it was also free-swimming and could capture large prey with its tentacles.
Cambrian predators
“Although jellyfish and their relatives are believed to be one of the earliest animal groups to have evolved, it has been very difficult to fit them into the fossil record of the Cambrian (between about 539 and 485 million years ago), but this discovery leaves no doubt that they were already swimming at that time,” says the co-author. Joe Moysiuk from the University of Toronto.
More than 500 million years ago, these jellyfish were already swimming freely and could capture large prey with their tentacles.
This study is based on fossil specimens of the Burgess Shale found mostly in the late eighties and nineties under the direction of former ROM curator of invertebrate paleontology, Desmond Collins.
They show that the Cambrian food chain was much more complex than previously thought, and that predation was not limited to large swimming arthropods such as Anomalocaris.
“To find such incredibly delicate animals preserved in layers of rock on top of these mountains is a wonderful discovery. Burgessomedusa contributes to the complexity of Cambrian food webs and, like Anomalocariswhich lived in the same environment, these jellyfish were effective swimming predators”, insists another of the authors, Jean Bernard Caron of the ROM, “and this adds another remarkable lineage of animals that the Burgess Shale has preserved as a Time.news of the evolution of life on Earth.”
Field images of specimens of the ‘Burgessomedusa phasmiformis’ jellyfish (center right) and the predatory arthropod ‘Anomalocaris canadensis’ preserved on the same rocky surface. scale hammer / Desmond Collins/Royal Ontario Museum
Cnidarians have complex life cycles with one or two body shapes, a vase-shaped body called a polyp and, in cases like medusozoans, a bell- or saucer-shaped body called a medusa, which may or may not be free-swimming. Although fossilized polyps have been discovered in rocks about 560 million years old, the origin of free-swimming jellyfish is not well understood, as their fossils are extremely rare.
Consequently, their evolutionary history is based on fossilized microscopic larval stages and on the results of molecular studies of living species (models with divergence times in DNA sequences). Although fossils of animals reminiscent of jellyfish in some respects have also been found in the Burgess Shale and other Cambrian sites, the ctenophores, these actually belong to a quite different phylum. Previous studies on Cambrian swimming jellies are now reinterpreted as ctenophores.
The Burgess Shale was designated a UNESCO World Heritage Site in 1980 and the fossils of Burgessomedusa phasmiformis found in it are on public display in a newly opened room of the Royal Ontario Museum.
Fieldwork in the Burgess Shale in Yoho National Park during 1992, and ‘Burgessomedusa phasmiformis’ exhibit at the Royal Ontario Museum. / Desmond Collins/ David McKay/Royal Ontario Museum
Reference:
Justin Moon et al. “A macroscopic free-swimming medusa from the middle Cambrian burgess sale”. Proceedings of the Royal Society B Biological Sciences2023.
Fuente: Royal Ontario Museum
Rights: Creative Commons.
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