Abstract

A combination of detailed, intermediate and large scale stratigraphic perspectives is necessary to understand the driving mechanisms for floral change across the Cretaceous–Tertiary (K–T) boundary, because short-term physical and biotic events occur within longer term trends. Most Maastrichtian palynofloral extinctions have been identified within the northern circumpolar, supra-continental, Aquilapollenites Province. In mid-continental North America, the presence of a physically defined K–T boundary provides opportunities to examine contemporaneous terrestrial palynofloras over a latitudinal (also approximately paleolatitudinal) distance of 3200 km. Seventy-five percent of last appearances occur before the K–T boundary at high latitudes and must have been caused by ongoing terrestrial processes. Palynofloral events directly associated with the K–T boundary include both local last appearances, and inferred extinctions, and synchronous and diachronous changes in relative abundances. Two different styles of factors are reflected in these events. The first, possibly attributable to chemical or other profound atmospheric perturbations associated with the worldwide distribution of the “fireball” layer of the K–T claystone, resulted in extinctions of angiosperm with a zoophilous pollination strategy. The second factor, possibly reflecting energy released during the deposition of ballistically transported impact debris or hackly (ejecta) layer of the boundary claystone, resulted in the selective, northward-attenuating removal of the canopy vegetation, best documented for southwestern Canada, which was succeeded by a fern-dominated vegetation in mid-continental North America. These consequences of the boundary event were mediated by independent, preexisting circumstances: climate (regional and through time), local depositional environment, and ongoing background extinctions and origins.

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