To reconstruct a late Paleocene (Tiffanian) vegetation, we intensively sampled and censused plant megafossils of the Fort Union Formation from Bison Basin, Wyoming. Extensive sampling of this deposit at the local scale was undertaken to assess the species richness and spatial distribution of leaf megafossils in a well-dated Paleocene assemblage. The megafossils were recovered from a series of ten quarries in a single fossiliferous horizon composed of fine-grained claystone that represents deposition in a shallow floodplain pond. Early and middle Paleocene leaf localities from the northern Great Plains are typically depauperate; this deposit was no exception, yielding only 28 distinct leaf morphotypes from a collection of 5,650 specimens (average: 10.6 morphotypes/quarry). Significant differences in the numbers of specimens per morphotype were observed. Four species, Corylites sp. (37.66%), Archeampelos acerifolia (33.93%), Metasequoia occidentalis (18.35%), and Fortuna cf. F. marsilioides (5.29%) comprised >95% of the specimens recovered. Megafloral dominants were fairly consistent from quarry to quarry indicating that this vegetation was relatively homogeneous at the scale of the local community. Detrended Correspondence Analysis indicates that there is no environmental gradient apparent within this community. This extremely well-sampled and laterally extensive leaf site supports the view that the late Paleocene (Tiffanian) flora of the northern Rockies is depauperate compared to megafloras of the Late Cretaceous and late Paleocene (Clarkforkian).

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