Prehistoric Echinozoa

Helicoplacoidea is an extinct class within the Echinodermata. All known taxa were discovered in sediments dating back to the Cambrian.

Ophiocistioidea is a class of extinct echinoderms from the Palaeozoic and early Mesozoic. They most likely form a paraphyletic grade along sea cucumber stem lineage, although some sources still consider the question of ophiocistioid monophyly unresolved.