Yet these benthic besties disappeared from the fossil record around 273 million years ago, after the specific crinoids and corals in question went extinct.

Other species of crinoids and corals emerged in the Mesozoic, following the Permian-Triassic extinction - but never again have we seen them together in a symbiotic relationship.

It's also unclear what benefit the crinoids gain from a relationship with coral, but one interesting thing did emerge: unlike the Paleozoic corals, the new specimens did not modify the structure of the crinoids' skeleton.

The Paleozoic fossils of symbiotic corals and crinoids involve corals that have a calcite skeleton, such as Rugosa and Tabulata.

If these corals don't modify the host, and leave no fossil record, perhaps they have had a long relationship with crinoids that has simply not been recorded.