PALEOECOLOGY OF A LATE PALEOCENE (TIFFANIAN) MEGAFLORA FROM THE NORTHERN GREAT DIVIDE BASIN, WYOMING

To reconstruct a late Paleocene (Tiffanian) vegetation, we intensively sampled and censused plant megafossils of the Fort Union Formation fr om Bison Basin, Wyoming. Extensive sampling of this deposit at the loc al scale teas undertaken to assess the species richness and spatial di stribution of leaf megafossils in a well-dated Paleocene assemblage. T he megafossils were recovered from a series often quarries in a single fossiliferous horizon, composed of fine-grained claystone that repres ents deposition in a shallow floodplain pond. Early and middle Paleoce ne leaf localities from the northern Great Plains are typically depaup erate; this deposit was no exception, yielding only 28 distinct leaf m orphotypes from a collection of 5,650 specimens (average: 10.6 morphot ypes/quarry). Significant differences in, the numbers of specimens per morphotype were observed. Four species, Corylites sp. (37.66%), Arche ampelos acerifolia (33.93%), Metasequoia occidentalis (18.35%), and Fo rtuna cf. F. marsilioides (5.29%) comprised >95% of the specimens reco vered. Megafloral dominants were fairly consistent from quarry to quar ry indicating that this vegetation was relatively homogeneous at the s cale of the local community Detrended Correspondence Analysis indicate s that there is no environmental gradient apparent within this communi ty. This extremely well-sampled and laterally extensive leaf site supp orts the view that the Late Paleocene (Tiffanian) flora of the norther n Rockies is depauperate compared to megafloras of the Late Cretaceous and late Paleocene (Clarkforkian).