TRANSITIONAL FORMS IN THE EPROLITHUS-LITHASTRINUS LINEAGE FROM THE USGS KURE BEACH COREHOLE, NC AND OTHER KEY LOCALITIES: A TAXONOMIC REVISION OF TURONIAN THROUGH SANTONIAN SPECIES

Confusion among the transitional forms of the calcareous nannofossil genera Eprolithus and Lithastrinus has not been resolved, despite their frequent use as key middle and late Cretaceous marker species in marine sediments. These genera are similar in having a series of wall elements in a dual cycle that surround a central depression connected by a thin diaphragm of overlapping elements. The evolution of Lithastrinids is recorded in morphotypes with 7 wall elements found in the Turonian through Santonian as they were reduced through time from 9 in Eprolithus floralis to 6 in Lithastrinus grillii. Inconsistency arises with the specific criteria for separating Eprolithus from Lithastrinus in the light microscope, especially of the forms with 7 elements, which has included: a change in the morphology of projections off the wall elements (or rays) from short and flower petal like to long and curved; reduction in the size of the diaphragm; and inclination and twisting about the central area of the wall elements. Exceptionally well preserved material from the USGS Kure Beach Corehole NH-C-1-2001 from North Carolina has been most important for observing ideal specimens across this transition. Using scanning electron photomicrographs and morphological measurements obtained through light microscopy a set of reliable criteria is subsequently defined for distinguishing two different taxa, Eprolithus moratus and Lithastrinus septenarius. This allows for more accurate biostratigraphic interpretation within the Turonian to Santonian range of these intermediate forms and correlation to other key sites along the western North Atlantic margin and around the world.

The younger upper Turonian to middle Santonian L. septenarius is separated from the older E. moratus by having a reduced central depression (<50% the width of the entire central area) and rays that are typically longer (>50% the width of the central area) and angled away from the ring of elements (>15°). In Lithastrinus the two cycles of wall elements taper inwards at the connection with the diaphragm giving the appearance of a rounded collar when in focus, which can be used to differentiate between the two taxa in poorly preserved material.