The shells are fairly flat and are generally wider than they are long. It is known from Middle and Upper Ordovician rocks of the northern hemisphere. It has a central stalk, from which extend a series of feather-like branches. About 20 secondary branches extend from the main branches. Rangea is thought to be related to corals.
Sea pens inhabit soft-bottomed aquatic environments.
They are made up of a cylindrical primary polyp with a stem-like base that anchors it to the sand. The body is fleshy and secondary polyps are attached to the large central stalk. Rhamphodopsis Unlike other placoderms, Rhamphodopsis and its close relatives possessed much reduced head and body shields. The need for protective armor in Rhamphodopsis was apparently less critical than in other placoderms. Rhamphodopsis was also specialized in possessing tooth plates that were capable of crushing shellfish.