Hemicidaroida

(paleontology) An order of extinct echinoderms in the superorder Echinacea characterized by one very large tubercle on each interambulacral plate.

An extinct paraphyletic order of regular sea urchins (Echinoidea), identified by having a plain, unsculp-tured test and tubercles that are perforate and crenulate. They almost certainly include ancestors of later groups that developed imperforate and noncrenulate tuberculation and are thus paraphyletic. Two families are generally placed within the order, Hemicidaridae and Pseudodiadematidae. The oldest hemicidaroid is Late Triassic (Upper Norian) and the youngest comes from the Upper Cretaceous (Campanian). They appear to have been epifaunal grazers. See also Echinoidea.