ABSTRACT

Multiple zones of exceptional preservation within the Niobrara Formation (Upper Cretaceous) have revealed much new morphologic information on coccolithophores. Nine species were found with coccospheres intact, including a species of Biscutum with extraordinary cylindrical rather than spherical coccospheres. Cylindrical coccospheres have not been reported from the fossil record, and are rare in modern oceans. An unusual feature of these cylindrical tests is the orderly, repetitive arrangement of the individual coccoliths. Most cylinders are open at both ends, but one specimen illustrated by electron micrographs with one end nearly closed reveals significantly smaller coccoliths at the ends. These smaller, simpler coccoliths probably did not articulate as tightly as those on the rest of the cell and were easily detached. Because of the difference in size and morphology, these terminal coccoliths may have previously been assigned to other taxa. A coccosphere of Braarudosphaera bigelowi, not previously illustrated from the fossil record, is also documented in this study.

The stagnant, anoxic benthic environment prevalent during these intervals of Niobrara deposition also fostered the preservation of monospecific coccolith clusters that represent coccospheres formed of nonarticulated coccoliths. These clusters, representing 35 different species, including Lithraphidites carniolensis, Bolevetelum sp., Microrhabdulus belgicus, and Rhagodiscus angustus, provide information on the minimum number of coccoliths originally present on the living cell. Such may prove valuable for determining the total biomass of the living populations based on the abundance of individual coccoliths in the fossil record.

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