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Precursors to the Cretaceous-Tertiary Boundary Event: Evidence for Terrestrial Environmental Instability

STROMBERG, C. A. E., THOMPSON, A., ARENS, N. C., and CLEMENS, W. A., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780

Facies homogeneity and relatively coarse-scale stratigraphic sampling have traditionally been interpreted to indicate that the late Cretaceous terrestrial environment was relatively stable; some suggest that this stability was permanently altered by the terminal Cretaceous bolide impact and perhaps other environmental threshold effects. New data from finer-scale stratigraphic sampling suggest, at least on a regional scale, more environmental and biotic instability than previously assumed. For example, the Aquilapollenites-Wodehousia palynological zone is often invoked to be characteristic for the uppermost Cretaceous, but data from detailed palynological sampling of the Upper Hell Creek Formation in McCone County, eastern Montana, indicate that Aquilapollenites sp. were rare and Wodehousia sp. were absent in the latest Maastrichtian. Instead we find well-preserved palynomorphs representing taxa found both above and below the K/T boundary, as well as many as yet undescribed forms.

Rapid and regular sedimentation in the latest Cretaceous and early Paleocene in McCone and Garfield counties, eastern Montana, yield the most complete and detailed terrestrial sequences crossing the K/T boundary. Therefore, this area offers the opportunity to resolve time scales reflecting ecological time-scale changes, and to tease apart Earth-based (climate, atmosphere, transgression) and extraterrestrial factors in faunal extinction.

Carbon isotope data from the Hell Creek and Tullock formations suggests a decrease in global biomass followed by an overall increase of biomass during the latest Maastrichtian. This is witnessed regionally by changes in plant community composition, possibly reflecting a transition from more open to more moist and closed habitats. Data from quantitative palynological sampling with high time-stratigraphic resolution show substantial floral turnover in the latest Maastrichtian, a conclusion supported by plant macrofossils. Palynomorph species richness also declines before the K-T boundary. Upper Maastrichtian palynofloras are also characterized by unusually small pollen, which may indicate two things: (1) Plants were environmentally stressed; (2) Plants were at the edges of their biogeographic ranges, suggesting rapid migration in and out of the region.

Climate-vegetational change on the scale of thousands to hundreds of thousands of years in the latest Cretaceous suggests that late Maastrichtian faunal change might be compared to models of late Pleistocene extinction, in which landscape-scale vegetation change drives differential faunal extinction. Climate-vegetation change inferred from the palynological and isotopic records of the latest Cretaceous may have affected some animals more than others, either causing their extinction or rendering them vulnerable to other disturbance. Specifically, large herbivores and their predators dependent on open habitats may have suffered differentially in contrast to aquatic animals and those adapted to more closed and mesic habitats.

75/125 YEARS