ABSTRACT
Evolution in the Pentamerida is characterized primarily by changes in hinge line length and the degree and nature of development of a spondylium in the ventral valve, and a septalium in the dorsal valve. Two suborders are recognized: the primarily Ordovician, more primitive, paraphyletic Syntrophiidina, and the Siluro-Devonian, derived, monophyletic Pentameridina. Rhynchonellida originates within the Syntrophiidina between two superfamilies, the older Porambonitoidea and the younger Camerelloidea, both paraphyletic. In general, the syntrophiidines transform from primitive orthide-like morphologies to derived, rhynchonellide-like morphologies. Pentameridines tend to be larger in size, ventribiconvex, rather than dorsibiconvex, and strongly rostrate. The suborder Pentameridina now includes three superfamilies: Pentameroidea, Stricklandioidea, and Gypiduloidea, that may be distinguished primarily by variation in their dorsal interiors.
