Late Ordovician ostracods from the "butter shale" Lagerstätten of the Cincinnati Arch region, Ohio, USA

The "butter shale" Lagerstätten of the Cincinnati Arch region is renowned for its well-preserved fossil records, documenting the dynamics of the Late Ordovician "Richmondian Invasion,"during which faunal migration from multiple source basins into the region led to a drastic increase in diversity. Among the diverse fossil groups, ostracods, an important component in marine benthic communities, have been substantially neglected, despite their great potential for paleobiogeographical, paleoecological, and macroevolutionary reconstructions. Here we systematically document ostracods from the butter shale for the first time, with 29 species of 24 genera being described. Two new species, Milleratia binodosa n. sp. and Daleiella? cincinnatiensis n. sp., are examined in detail. Analyses on faunal composition indicate that ostracods also participated in the process of the invasion during the early and main stages, as evidenced by the presence of both native and migrated taxa from other paleobasins in Laurentia. The studied ostracod fauna, characterized by the dominance of the suborder Binodicopina with relatively low diversity but high abundance, is associated with siliciclastic depositional environments and is consistent with the interpreted depositional setting of the butter shales. We also found strong biogeographical connections of the butter shale ostracod fauna with those within Laurentia as well as with faunas from adjacent paleocontinents, especially Baltica and Avalonia. UUID: http://zoobank.org/1718afb4-a453-4b6a-b5d8-3c36dad835fd. © The Author(s), 2026. Published by Cambridge University Press on behalf of Paleontological Society.