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Abstract
Stegerhynchus eoborealis sp. nov. from the Dalmanitina Beds (Hirnantian, uppermost Ordovician), Borenshult, Östergötland, Sweden, is described in this study as the earliest known species of Stegerhynchus, within the family Rhynchotrematidae. This implies that Stegerhynchus originated during the latest Ordovician mass extinction event, and radiated in the Silurian Period to become one of most common rhynchonellide brachiopod genera in shallow tropical seas. In the early evolutionary stage of the genus, Stegerhynchus eoborealis already exhibited a notable reduction in the size of septalium compared to the closely related genus Rhynchotrema. Compared to its Silurian counterparts (e.g. Stegerhynchus borealis and S. peneborealis), Stegerhynchus eoborealis has some variable characters within single populations, particularly in the number of costae in the sulcus and on the fold, implying a morphological plasticity during the early evolutionary stage of the genus. During the Hirnantian mass extinction events, five rhynchonellide families were represented by small taxonomic groups, but only the Rhynchotrematidae and Trigonirhynchiidae diversified into many highly successful clades of rhynchonellides during the Silurian and Devonian. However, the Ancistrorhynchidae, Orthorhynchulidae, and Sphenotretidae largely disappeared by the end of the Ordovician, with a few species (such as Plectothyrella sp.) surviving into the earliest Silurian but shortly becoming extinct.