Some of Johnson's initial interpretations of the megafossil paleobotanical record in southwestern North Dakota have changed somewhat since 2002 through the benefit of the application of quantitative methods in collaboration with paleobotanist Peter Wilf. Wilf and Johnson (2004) conducted a definitive quantitative analysis of megafloral turnover across the K-T boundary based on Johnson's data (more than 22 000 specimens from 161 localities). They found that the megaflora changed gradually during the Maastrichtian, shifted sharply at the K-T boundary, and then was static in the early Paleocene. All dominant species were lost at the boundary (verifying Johnson's 2002 conclusion), and species richness did not recover during the Paleocene. The disappearance of species more than 5 m below the boundary is attributed to normal turnover and climate change; megafloral extinction at the boundary was recalculated as 57% (cf. earlier estimates of 79-80%). The new estimate is based on only the uppermost 5 m below the boundary, and it excludes taxa that are present in only one sample. The new numbers are conservative and quite robust.
Nichols and Johnson (2002) cited the same 30% figure for palynological extinction across the boundary given in Nichols (2002a), but we have subsequently re-evaluated that number. Wilf and Johnson (2004) restricted their calculations of extinction percentage to the 5 m just below the K-T boundary. We recalculated the extinction at each of the palynological localities using this standard and found a range of 17-30%. This range is almost identical to Hotton's (2002) estimate for eastern Montana (15-30%). The plant fossil record at the K-T boundary in the eastern Montana part of the Williston Basin is discussed below.
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