Middle Ordovician (middle Darriwilian) Dirafinesquina and Jigunsania gen. nov. (Rafinesquinidae

Abstract

Three rafinesquinid brachiopod (Strophomenoidea) species, Dirafinesquina chosenensis, Jigunsania guraeriensis gen. et sp. nov., and Jigunsania hambaeksanensis gen. et sp. nov. are described from the Middle Ordovician (middle Darriwilian) Jigunsan and Duwibong formations in the Taebaeksan Basin, South Korea of the Sino-Korean platform of North China. Dirafinesquina chosenensis is distinguished from the other two Dirafinesquina species, D. globosa and D. antiqua by having postero-lateral ridges parallel to socket ridges in the dorsal valve. Jigunsania gen. nov. is distinctive among the rafinesquinids in having strongly developed and posteriorly curved alae at cardinal extremities and characterized by having dorsal geniculation, prominent postero-lateral rugae, and subperipheral rim on the dorsal interior. Compared with the two rafinesquinid subfamilies (Rafinesquininae and Leptaeninae), Jigunsania bears typical rafinesquinine features such as bifid triangular cardinal process lobes and straight socket ridges and is most similar to Kjaerina, whereas it has the subperipheral rim which is diagnostic to the leptaenines but generally absent in the rafinesquinines, and thus considered to be taxonomically significant in separating the two subfamilies. Jigunsania is tentatively assigned to the Rafinesquininae due to its great resemblance to Kjaerina. The spatio-temporal distribution of Kjaerina suggests that once originated from the middle Darriwilian Jigunsania in the low-latitude province in the late Darriwilian, it rapidly diversified into other regions including the high-latitude province during the Sandbian and Katian. The onset of diversification and paleobiogeographic expansion of Kjaerina appears to be late among the strophomenoids which first began to diversify during the Darriwilian at species and generic level. The leptaenines, a predominantly post-Ordovician rafinesquinid group, is also considered to have been derived from Jigunsania and not undergone the Darriwilian diversification onset. In addition, the occurrence of Dirafinesquina suggests that the South Korean brachiopod fauna including orthides may be closely linked with the paleobiogeographically important Saucrorthis Fauna recognized in South China, Alborz (northern Iran), and Sibumasu (Myanmar).