The smooth, spire-bearing brachiopods after the terminal Ordovician extinction through lower Llandovery in the central Oslo region, Norway

Strata of the Solvik Formation in the central Oslo region (upper Hirnantian to upper Aeronian) are rich in atrypides that elucidate the evolution of this group. A few athyridide brachiopods are also present. Eleven taxa of smooth spire-bearers are described taxonomically using fossils and peels. Among these occur one new genus, Eisaella, and five new species: Eisaella uniplicata, Thulatrypa huangi, T. vikenensis, ?Becscia pentagona, and Shelvothyris bivittata. Atrypides make up 30% (24 genera) of the total brachiopod genera in the Solvik Formation, more than one third of the known global atrypide fauna during that interval. Most are members of the family Lissatrypidae, illustrating the evolution of this group from upper Hirnantian and lower Silurian strata. These show close affinity to each other attesting to ongoing radiation. The new genus Eisaella is a likely ancestor of Lissatrypa, while Meifodia displays a gradual evolution through the formation. Atrypides were most diverse at the base and top of the Solvik Formation due to heterogony in environments and water depth, but were less diverse in the comparatively constant environmental setting of the late Rhuddanian time. The basal assemblages with few atrypides from shallow facies may be compared to the shallow Cathay Fauna of China. Most of the abundant Norwegian atrypide fauna is found in deeper water with few comparable faunas known globally. The Oslo region may have served as a center for the evolution and spread of atrypides immediately following the end-Ordovician glacial drawdown in sea level.