With over a century of accumulated data on Carboniferous paleobotany, it is expected that these data should converge upon emergent patterns. Inferences involving: 1) disparity in evolutionary rates, 2) segregation into floristic biomes, 3) species richness as a proxy for climate change, and 4) geographic ranges as evidence for paleogeographic reconstructions, require real cladistic genera having unique times of origin, dispersal rates, and extinctions. However, taxonomic assignments, particularly throughout the Mississippian, are often based on broadly defined form genera. These heterogeneous groupings do not have a unique stratigraphic or geographic range and thus say little toward broader issues. It is apparent that taxonomic assignments are often biased toward preconceived age determinations. Nowhere was this more evident than in the form genus Triphyllopteris, the historic single index fossil for the Visean. The revelation that North American taxa assigned to Triphyllopteris were of entirely different architecture, perhaps belonging to different orders of plants, underscores the circularity in considering stratigraphic ranges in generic determinations. A converse example, Sphenopteris cheathami (Lesquereux) Read, clearly has a tri-lobed leaflet outline but was not assigned to Triphyllopteris perhaps because of a Pottsvillian age. The problem lies in the nature of the plants. During this early evolutionary stage, there was apparently little control over leaflet outline through the activity of marginal meristems. Leaves at this level, like ultimate axes, may have had indeterminate growth. The result is very different plants having superficially similar foliage, and also many different-shaped leaves possibly occurring on the same plant. It is a wonder that plant biostratigraphy ever worked at all. Perhaps this is because form genera divert focus to developmental grade transitions. For purposes of age determination, is it possible to proceed toward an enhanced understanding of the sequence and timing of grade transitions; but then pursue taxonomic precision for diversity counts and phytogeography?

Key words: biostratigraphy, Carboniferous paleobotany, form genera, phytogeography