Abstract
A new micromorphic rhynchonellide brachiopod Parasphenarina cavernicola gen. et sp. nov. is described from submarine caves on the outer slopes of coral reefs in the ryukyu Islands, Japan. Based on the presence of spinuliform crura, the new genus is included in the Family Frieleiidae Cooper, the diagnosis of which is emended. Detailed morphological observations of different-sized shells and intraspecific variability have shown that the morphology of the hinge plates changes considerably during ontogeny. It is suggested that the new genus Parasphenarina could have evolved from forms close to the extremely rare bathyal Pliocene genus Sphenarina Cooper. The diagnostic characteristics of Parasphenarina such as diminutive adult size and lack of septalium and median septum may represent paedomorphic evolution.
