Megafloral change across the Cretaceous/Tertiary boundary in the northern Great Plains and Rocky Mountains, U.S.A.

Intensive collecting of Maastrichtian and early Paleocene plant megafossils (primarily leaves) in the northern Rocky Mountains and Great Plains has resulted in a database of nearly 25,000 specimens from more than 200 localities in eight areas. The most completely sampled section is at Marmarth, in southwestern North Dakota, where 57 latest Cretaceous and 30 early Paleocene localities (11,503 specimens) span the Cretaceous/Tertiary (K/T) boundary. Zonation of the Marmarth megaflora based on floral composition and relative abundance results in four zones: HC I, HC II, HC III, and FU I. This megafloral zonation appears to be applicable to terrestrial sediments across the northern Great Plains and Rocky Mountains and to correlate with the Western Interior ammonite zonation. The zones reflect major megafloral changes before and at the K/T boundary. Megafloral change at the boundary is large (79 percent) and is characterized by the disappearance of most of the Upper Cretaceous dicotyledonous angiosperm taxa. It coincides with a peak in palynofloral extinctions and iridium content and with the occurrence of shocked mineral grains, all of which have become accepted as the characteristic signature of the K/T boundary and as indications of an abrupt causal mechanism. In contrast, the megafloral changes before the boundary appear to have been caused by a regional climate warming. The fact that the megafloral zones are not reflected by palynostratigraphy argues for using an integrated approach to biostratigraphy that combines the high stratigraphic resolution of palynomorphs with the high taxonomic resolution of megafossils. Results of this analysis of the terrestrial plant record are compatible with the hypothesis of a biotic crisis caused by extraterrestrial impact at the end of the Cretaceous.