Unravelling a Late Ordovician pentameride (Brachiopoda) hotspot from the Boda Limestone, Siljan district, central Sweden

摘要

The correlation of the uppermost Boda Core Member of the Boda Limestone is reassessed and the beds assigned to the upper Katian. This is based on the identification of the brachiopod species Brevilamnulella kjerulfi (Kiaer) and Amphiplecia tardicostata Wright & Jaanusson from the highest beds in the unit. The former species is overwhelmingly abundant in the uppermost beds of the Boda Core Member in Osmundsberget quarry, forming an almost monospecific coquina. Also occurring in the coquina, is Clorilamnulella osmundsbergensis gen. et sp. nov., which ranges into the overlying Hirnantian Glisstjarn Formation. The new genus is here regarded as a transitional form between Brevilamnulella and Clorinda. The oldest occurrence of this latter genus is here revised and not found in rocks older than the Aeronian. Instead, previous Rhuddanian occurrences of ‘Clorinda’ are transferred to Clorilamnulella based on a dorsal cardinalia that differs from both that of Brevilamnulella, as well as that of Clorinda. Moreover, gigantic shells of a new species of Brevilamnulella, B. umbosulcata sp. nov, are described from the nearby Solberga quarry where the species occurs in great abundance in a local fossil pocket, that also is correlated with the upper Katian. These new virgianid and clorindoid occurrences, together with previously described occurrences of Holorhynchus and Costilamnulella, demonstrate that during the late Katian, the Boda Limestone was a virgianid hotspot crucial in the evolution of the earliest pentameroid family, the Virgianidae, as well as a possible early cradle for the evolution of the hitherto exclusively Silurian superfamily, the Clorindoidea.