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The origin of the Phacopida order is uncertain. The three suborders Phacopina, Calymenina, and Cheirurina, all share a distinctive protaspis (developmental stage lacking segments) type. This sort of similarity in development suggests phylogenetic unity. The Calymenina suborder is the most primitive of the Phacopida order.

Phacops rana is one of the most well-known members of this order.