Craniopsida

This type of morphology is present already in the earliest trimerellides, the Ussuniidae of Llandeilo age; these shells are among the largest known Ordovician brachiopods. According to Gorjansky and Popov (1986) the Ussuniidae may be related closely to the craniopsides, and although the former can be over ten times larger than the latter, they show similarities in the shape of the pseudo-interareas as well as the development of the visceral areas and main muscle scars.