REGIONAL EXTIRPATIONS AND SELECTIVITY PATTERNS IN THE LATE PALEOZOIC TERRESTRIAL TROPICS

The Late Paleozoic tropics experienced a series of geographically complex floristic changes beginning during the Pennsylvanian and continuing into the Permian. The driving mechanism of these vegetational changes appears to have been climate change associated with changes in ice volume in the south polar region, perhaps ultimately driven by increasing atmospheric CO2. Two basic patterns emerge in the basinal lowlands (where the samples are). 1) Within species pool patterns. Example: Near the Moscovian-Kasimovian boundary, in western and central Pangea, dominant taxa in wetland communities changed significantly from lycopsids to tree ferns during a major ecological reorganization. Tree ferns began rising in abundance in wetlands during the late Moscovian, prior to the regional extirpation, effectively “invading” lycopsid-dominated assemblages, possibly reflecting the beginnings of environmental change. In eastern Pangea (China) wetland assemblages similar to those of the Moscovian persisted into the Late Permian. 2) Between species pool patterns. Example: At the beginning of the Permian in western and central Pangea, seasonally dry assemblages became most common in basinal lowlands as wetlands became increasingly rare and species diversity therein declined. These patterns and associated physical indicators suggest the following. 1) Within species pools, weedy opportunists were significantly advantaged during times of extreme ecological disruption, ultimately giving rise to new ecological dominants. 2) As the areal extent of suitable conditions attenuated, the species richness of wetlands followed suit, with opportunistic and highly specialized taxa retaining dominance. 3) Changes in species-pool-level landscape dominance involved spatial replacement rather than displacement and reflect strong tracking of climatic conditions at the species-pool level.