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Planktonic foraminiferal assemblages from the Late Cretaceous Niobrara Formation of the Western Interior, U.S.A., can be divided into a lower temperate fauna, a middle subtropical fauna and an upper temperate fauna. The subtropical fauna is marked by conspicuous keeled planktonic foraminifera. The presence of Marginotruncana concavata indicates that the subtropical fauna is of late Coniacian to early Santonian age. Faunal studies of formations correlative with the Niobrara Formation indicate that the subtropical fauna is an easily traced assemblage which has been noted as far north as Montana.

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