Any sea urchin belonging to the family Cidaridae, a primitive group of sea urchins with thick spines that have existed since the Paleozoic era.
From Cidaris (genus) plus -id suffix. Cidaris comes from Greek kidaris, meaning a type of headdress or crown, possibly referring to the urchin's spiny appearance like a crown.
Cidarid sea urchins are living fossils that barely changed for over 300 million years—they survived the same mass extinction that killed the dinosaurs because they were already so perfectly adapted to the ocean.
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