Hirnantian (Late Ordovician) through Aeronian (Llandovery, early Silurian) conodonts from the Whippoorwill, Brassfield, and golden Brassfield formations of the North American Midwest Basins and Arches region, USA

The age of the Ordovician–Silurian boundary interval beds, as well as the position of the boundary itself, has been difficult to determine in the North American Midwest Basins and Arches (NAMBA) region. Previous conodont studies considered the boundary to occur at a large, systemic-boundary-spanning unconformity that removed the entire Hirnantian and uppermost part of the Katian stages. However, Ozarkodina hassi Zone conodonts recovered from a newly discovered section of the Whippoorwill formation in Decatur County, Indiana indicate that upper Hirnantian strata are present in the eastern NAMBA region. Conodonts reviewed from a previous study in Adams County, Ohio indicate that the Ozarkodina hassi , Distomodus kentuckyensis, and Pseudolonchodina expansa Zones are present in the Whippoorwill and Brassfield formations, which had previously been assigned to only the Distomodus kenuckyensis Zone. The lowest occurrence of Distomodus kentuckyensis indicates that the Ordovician–Silurian systemic boundary occurs within the upper part of the Whippoorwill formation, and not at a large unconformity. The conodont biostratigraphic data presented here add to a growing body of evidence that late Hirnantian strata are present throughout the North American continental interior, opening a new region for study of the recovery interval after the final pulse of the first major mass extinction.