COMPARATIVE BIOGEOGRAPHIC ANALYSIS OF PLANKTIC FORAMINIFERAL SURVIVORSHIP ACROSS THE CRETACEOUS TERTIARY (K/T) BOUNDARY/

It is now widely recognized that a large number of Cretaceous planktic foraminiferal species are commonly found associated with fully Danian faunas in many K/T boundary sections and deep-sea cores. This ''Creta ceous'' fauna has traditionally been regarded as representing the rewo rking of older Cretaceous sediments into younger strata, though recent isotopic data from some species indicates that, at least in these ins tances, the reworking hypothesis is false. To further test this rework ing hypothesis the biogeography of this ''Cretaceous'' fauna is compar ed to the underlying uppermost Maastrichtian biogeography and to the b iogeography of lowermost Danian planktic foraminiferal faunas. Results show that there is no regular decline in species richness, extinction , or faunal co-occurrence values for this ''Cretaceous'' fauna at prog ressively higher (=younger) Danian stratigraphic horizons. Moreover, t here is no compelling association between the stratigraphic persistenc e of this ''Cretaceous'' fauna and shallow depositional settings. Inst ead, this fauna is characterized by: (1) a close (and predictive) asso ciation between ''Cretaceous'' and indigenous Danian species richness values throughout the lower Danian, (2) a close numerical and geograph ic correspondence between Danian speciation and the disappearance of ' 'Cretaceous'' species from the Danian fossil record, and (3) a pronoun ced similarity between changes in the general biogeographic structures of the ''Cretaceous'' and associated Danian faunas throughout the stu dy interval. These data suggest that the K/T planktic foraminiferal ex tinction event exhibited a marked geographic structure with low and mi ddle latitude faunas experiencing differentially high extinction rates in the lowermost Danian zones P0 and P1a and high latitude survivor f aunas persisting more or less unchanged into the overlying zone, P1b a nd P1c. Taken together, these results challenge the traditional concep t of an instantaneous uppermost Cretaceous planktic foraminiferal mass extinction and its proposed causal connection to bolide impact.