Introduction

Interest in understanding the reaction of plant communities to the Cretaceous-Tertiary (K/T) boundary event has focused increased attention on late Maastrichtian and Early Paleocene palynofloras. Both the relative abundances of major groups of palynomorphs and range truncations have been used to infer changes in terrestrial environments coincident with the K/T boundary in mid-continental North America (Orth et al., 1981; Tschudy et al., 1984; Nichols et al., 1986; Bohor et al., 1987; Lerbekmo et al., 1987; Fleming & Nichols, 1990; Nichols, 1990; Sweet et al., 1990; Nichols & Fleming, 1991; Sweet & Braman, 1992). These criteria sometimes are used to emphasize the more catastrophic aspects of floral change at the K/T boundary (Orth et al., 1981; Tschudy et al., 1984; Nichols et al., 1986; Bohor et al., 1987; Fleming & Nichols, 1990). In other publications the relative abundance spikes and range truncations are considered to be part of a continuum of change across the boundary (Tschudy & Tschudy, 1986; Lerbekmo et al., 1987; Sweet et al., 1990; Sweet & Braman, 1992). It now is generally accepted that many morphologically complex species of angiosperm pollen became extinct at or within an interval immediately contiguous to the K/T boundary, and that there is a causal connection between these extinctions and the K/T event.

The dominant late Maastrichtian miospore species often represent flowering plants (Tschudy & Tschudy, 1986; Lerbekmo et al., 1987; Sweet et al., 1990). A dramatic shift occurs in the relative abundances of spores and pollen immediately after the K/T extinction event in most North American terrestrial sections.