Cec. Gemmill et Kr. Johnson, PALEOECOLOGY OF A LATE PALEOCENE (TIFFANIAN) MEGAFLORA FROM THE NORTHERN GREAT DIVIDE BASIN, WYOMING, Palaios, 12(5), 1997, pp. 439-448
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).