The Pennsylvanian-permian Vegetational Transition: a Terrestrial Analogue to the Onshore-offshore Hypothesis.

An analysis of 68 floras from the Pennsylvanian and Early Permian of Euramerica reveals distinct patterns of environmental distribution. Wetland assemblages are the most commonly encountered floras from the Early and Middle Pennsylvanian. Floras from drier habitats characterize the Permian. Both wetland and dry-site floras occur in the Late Pennsylvanian, but floristic overlap is minimal, which implies strong environmental controls on the distributions of the component species. Drier habitats appear to be the sites of first appearance of orders that become prominent during the Late Permian and Mesozoic. Higher taxa originated in physically heterogeneous, drier habitats, which were geographically marginal throughout most of the Pennsylvanian. They then moved into the lowlands during periods of climatic drying in the Permian, replacing older wetland vegetation. This pattern is analogous to the marine onshore-offshore pattern of origination and migration. The derivation of Mesozoic wetland clades from the Permian dry-lowland vegetation completes the parallel. The similarities of the marine and terrestrial patterns suggest that the combination of evolutionary opportunity, created by physical heterogeneity of the environment, and migrational opportunity, created by changing extrinsic conditions, may be underlying factors that transcend the specifics of organism and environment.