Plant fossils in Pennsylvanian strata come from coal-bearing sequences as coal balls or microfossils in coal and as compression fossils from clastic rocks between coal beds. Plant fossils in Pennsylvanian sequences without coal occur primarily as compression fossils in clastic sediments. Comparative studies demonstrate that “roof-shale” floras, from clastic rocks immediately above coals, and “coal” floras, from the coal beds, are distinct both in taxonomic composition and in the relative proportions of major plant groups that comprise the numerically dominant fossils. Thus, the clastic floras were not drawn from the same species pool or vegetation as the floras that characterize peat-forming mires.

The separate species pools for clastic and peat-forming environments each occur throughout tropical Euramerica. When considered in light of the paleogeographic distribution of the respective host lithologies, floras found in clastic sediments had a wider distribution than the peat forming floras. Thus the two species pools had independent paleogeographic distributions. We examined floras from Upper Pennsylvanian shales in New Mexico and found them to be similar to those from Upper Pennsylvanian clastic deposits to the east, from Texas to Europe. The New Mexico floras, however, come from strata deposited in the Ancestral Rocky Mountain foreland on inland floodplains (Missourian) or in coastal deltas/estuaries (Virgilian) in which coal beds are absent or very rare. In contrast, coals are commonly associated with the clastic floras in regions to the east.

Models suggest that the change from peat formation to clastic swamp deposits at any one geographic location reflects climatic change from humid/perhumid to seasonally wet. The biogeographic distribution of coal and clastic floras supports this model, suggesting that western areas were rarely wet enough to support peat formation but were wet enough (at times) to support seasonally wet clastic floodplain vegetation. Thus, clastic floodplain vegetation may have existed contemporaneously with peat-forming vegetation, however, as a general rule, this penecontemporaneity was not local but regional. Clastic floras inhabited areas with reduced precipitation, probably of a seasonal nature; the peats characterized humid to perhumid climatic areas.