An order of echinoderms (spiny-skinned sea creatures) that includes crown-like sea urchins.
From diadema + -oid (resembling) + -a (Latin plural). Names a group of sea creatures whose appearance resembles a crown or diadem.
Diadematoida sea urchins are ancient—fossil records show they've been wearing their crown-like spines in the ocean for over 200 million years!
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