ABSTRACT: The calcareous nannofossil genus Biscutum is ubiquitous in mid to Upper Cretaceous pelagic sediments. Biscutum constans is an important paleoceanographic proxy for surface water fertility and Biscutum species are important components in several Upper Cretaceous zonation schemes. Species concepts within Biscutum have inconsistently been applied and interpreted owing to a convoluted taxonomic history, thus limiting its utility. Most work on the evolution of Biscutum has been conducted on high latitude sections or Lower Cretaceous or Jurassic sediments. As such, Biscutum evolution in mid-latitudes of the mid to Late Cretaceous is poorly understood. This study documents the evolution of Biscutum during the mid to Late Cretaceous in a mid-latitude composite section from North America. Samples from the Western Interior Basin, North Atlantic Ocean and Gulf of Mexico were examined. Six new species of Biscutum are presented: B. ubiquem, B. anthracenum, B. shamrockiae, B. dehiscum, B. subditivum and B. aura. Review of the taxonomic history of Biscutum constans indicates that this species has been systematically misinterpreted. An emendation to the Biscutum constans species concept is provided based on the original work of Görka (1957). A diversification interval is documented in the mid-Cretaceous during which five new species of Biscutum evolved in approximately six million years. Evidence from previous work indicates the occurrence of a second diversification interval in Campanian high latitudes. These results suggest a distinct shift in the site of evolutionary activity within Biscutum from the mid-latitudes in the mid Cretaceous to high latitudes in the Late Cretaceous.
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