The well-preserved organisms from the Ordovician period lived in what is now the town of Llandrindod Wells -- which was partially under water -- and shed new light on how ocean communities have evolved, researchers said.
The fossils include a variety of creatures from sponges and worms to nautiloids, something like a squid with a shell, hydroids -- which are related to sea anemones and are also known as the flowers of the sea. The find provides evidence of a fossil community that was entirely new and surprising, they said.
Llanfawr quarry is an area well known for its fossils, "but somehow the important fossils had been missed," researcher Lucy Muir told the BBC.
"Although there are a handful of sites around the world that preserve extremely delicate and soft animals in a similar way, none of them have preserved this type of ecosystem," she said.
"In particular, solitary hydroids are almost unknown in the fossil record, because they are so delicate," she said. "This type of ecological community type was simply unknown from rocks this old, and for it to suddenly appear makes paleontologists wonder what else they've been missing."
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