Earliest jellyfish

The earliest free-swimming jellyfish, as known from unequivocal fossil evidence, is the macroscopic medusa form of Burgessomedusa phasmiformis. This prehistoric species of medusozoan cnidarian was formally named and described in Proceedings of the Royal Society B: Biological Sciences on 2 August 2023, and existed approximately 505 million years ago, during the middle Cambrian. It is currently known from 182 exceptionally well-preserved body fossils obtained from the famous Burgess Shale fossil beds in Raymond Quarry, British Columbia, Canada. Each Burgessomedusa specimen resembles a cuboidal umbrella or lampshade up to 20 centimetres (7.9 inches) high, with a fringe of more than 90 short, finger-like tentacles around its edge. This species is likely to have been one of the top predators of its time.

Medusozoans constitute a major taxonomic group (subphylum) of the invertebrate phylum Cnidaria, and include among their modern-day members the scyphozoans or true jellyfishes, the hydrozoans (which include hydras and siphonophores), the staurozoans (stalked jellyfishes), and the cubozoans (cube jellies), all of which include species that contain a medusa form in their lifecycle.

Certain previously described ancient fossil species thought by some to have been macroscopic free-swimming medusozoan medusae are now believed to have been misidentified ancient ctenophores (comb jellies) instead, creatures that belong to an entirely separate invertebrate phylum from cnidarians. Even earlier sessile jellyfish ancestors have been identified in the fossil record, such as Auroralumina attenboroughii dating back 560 million years to the Ediacaran period making it the earliest-known animal predator, but it can't yet be known for sure if these anchored sea creatures later had a free-swimming life stage.