Researchers from the Royal Ontario Museum (ROM) have identified the oldest known species of swimming jellyfish
The 505-million-year-old swimming jellyfish from the Burgess Shale highlights diversity in Cambrian ecosystem.
The Royal Ontario Museum announced the oldest swimming jellyfish in the fossil record with the newly named Burgessomedusa phasmiformis.
Jellyfish belong to medusozoans, or animals producing medusae, and include today’s box jellies, hydroids, stalked jellyfish and true jellyfish.
Medusozoans are part of one of the oldest groups of animals to have existed, called Cnidaria, a group which also includes corals and sea anemones.
Burgessomedusa unambiguously shows that large, swimming jellyfish with a typical saucer or bell-shaped body had already evolved more than 500 million years ago.
Burgessomedusa fossils are exceptionally well preserved at the Burgess Shale considering jellyfish are roughly 95% composed of water.
ROM holds close to two hundred specimens from which remarkable details of internal anatomy and tentacles can be observed, with some specimens reaching more than 20cm in length.
These details enable classifying Burgessomedusa as a medusozoan.
By comparison with modern jellyfish, Burgessomedusa would also have been capable of free-swimming and the presence of tentacles would have enabled capturing sizeable prey.
Co-author Joe Moysiuk, a PhD candidate in Ecology & Evolutionary Biology at the University of Toronto, who is based at ROM, said: “Although jellyfish and their relatives are thought to be one of the earliest animal groups to have evolved, they have been remarkably hard to pin down in the Cambrian fossil record.
“This discovery leaves no doubt they were swimming about at that time.”
Large swimming arthropods
This study, identifying Burgessomedusa, is based on fossil specimens discovered at the Burgess Shale and mostly found in the late 1980s and 1990s under former ROM curator of invertebrate palaeontology, Desmond Collins.
They show that the Cambrian food chain was far more complex than previously thought, and that predation was not limited to large swimming arthropods like Anomalocaris.
Co-author, Dr Jean-Bernard Caron, ROM’s Richard Ivey Curator of Invertebrate Palaeontology, said: “Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wonderous discovery.
“Burgessomedusa adds to the complexity of Cambrian foodwebs, and like Anomalocaris which lived in the same environment, 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.”
Cnidarians have complex life cycles with one or two body forms, a vase-shaped body, called a polyp, and in medusozoans, a bell or saucer-shaped body, called a medusa or jellyfish, which can be free-swimming or not.
While fossilised polyps are known in circa 560-million-year-old rocks, the origin of the free-swimming medusa or jellyfish is not well understood.
Fossils of any type of jellyfish are extremely rare. As a consequence, their evolutionary history is based on microscopic fossilised larval stages and the results of molecular studies from living species (modelling of divergence times of DNA sequences).
Though some fossils of comb-jellies have also been found at the Burgess Shale and in other Cambrian deposits, and may superficially resemble medusozoan jellyfish from the phylum Cnidaria, comb-jellies are actually from a quite separate phylum of animals called Ctenophora.
Previous reports of Cambrian swimming jellyfish are reinterpreted as ctenophores.
These findings are published in Proceedings of the Royal Society B.
Image 1: Artistic reconstruction of a group of Burgessomedusa phasmiformis swimming in the Cambrian sea. Credit: Reconstruction by Christian McCall. © Christian McCall.
Image 2: Slab showing one large and one small (rotated 180 degrees) bell-shaped specimens with preservation of tentacles. ROMIP 65789. Credit: Photo by Jean-Bernard Caron © Royal Ontario Museum.
