Leaf-fossil evidence for extensive floral extinction at the Cretaceous-Tertiary boundary, North Dakota, USA

Abstract

Forty-seven collections of plant macrofossils (megaflora) from the upper 17 m of the Hell Creek Formation (late Maastrichtian) and the lower 80 m of the Ludlow Member of the Fort Union Formation (Paleocene; Puercan) in south-western North Dakota document floral change across the Cretaceous-Tertiary (K-T) boundary. These collections were made within a detailed stratigraphic framework based on 37 measured sections in eight areas along a 60 km transect and were supplemented with a microstratigraphic palynological analysis of the K-T boundary. The boundary is defined by the extinction of 30% of the late Maastrichtian Wodehouseia spinata Assemblage Zone palynomorphs and occurs within a few meters above or below the Hell Creek-Fort Union formational contact. The boundary is documented palynologically in seven sections and, in one section, coincides with an iridium anomaly and shocked mineral grains. The megaflora of the uppermost 17 m of the Hell Creek Formation (megafloral zone HC III) is a diverse (65 taxa) angiosperm-dominated assemblage that is taxonomically distinct from the megaflora of the lower Hell Creek. The HC III megaflora suffers 80% extinction across the K-T boundary and is replaced by the lower Fort Union megaflora (FU I) which, with 73 taxa, appears more diverse than HC III but has fewer taxa per locality. The Paleocene flora, with abundant aquatic angiosperms and taxodiaceous conifers, reflects the increased abundance of pond and swamp environments. This indicates that some of the floral change near the boundary was due to sedimentological facies change. The number, abruptness and regional extent of extinctions combined with the presence of iridium and shocked minerals at the extinction horizon support an extinction scenario compatible with an extraterrestrial impact. The mechanisms of that extinction, however, remain unclear.