(CC BY-SA 3.0)
These animal groups lived in the ocean over half a billion years ago but were buried by a subterranean mudflow:
Paleontologists found thousands of fossils in rocks on the bank of the Danshui river in Hubei province in southern China, where primitive forms of jellyfish, sponges, algae, anemones, worms and arthropods with thin whip-like feelers were entombed in an ancient underwater mudslide. The creatures are so well preserved in the fossils that the soft tissues of their bodies, including the muscles, guts, eyes, gills, mouths and other openings are all still visible. The 4,351 separate fossils excavated so far represent 101 species, 53 of them new.Ian Sample, “‘Mindblowing’ haul of fossils over 500m years old unearthed in China” at The Guardian
We are told that this Qingjiang discovery is important for its diversity, especially of cnidarians (corals and jellies). The new fossils represent a different ecology from previous discoveries in China at Chengjiang. The Cambrian era seems to have been more complex than thought.
Paper. (paywall)
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See also: Surprise Superhighway: Cambrian Worms Lived In “Unsustainable” Ocean 500 Mya
and
Complex Worm Find From Cambrian (541-485 Mya) “Helps Rewrite” Our Understanding Of Annelid Head Evolution