BROAD PHYTOGEOGRAPHIC PATTERNS ACROSS NORTHERN PANGEA THROUGH THE PERMIAN

The proportional contribution of different plant groups to overall land plant diversity varied through the Paleozoic. Diversification of lycopod and progymnosperm groups through the middle Paleozoic overlaps with, and is then taxonomically dominated by, the ascendence of what has become known as the "paleophytic" flora. This flora is defined using factor analysis of co-occuring plant groups from mostly North American and Eurasian data and is typified by the cordaites, glossopterids, gigantopterids, some pteridosperm groups, and the lycopsids. These groups contribute to the well-known Carboniferous forests with complex canopy structure, ecological succession, and multitypic stands. By Late Carboniferous and Permian time, plant morphytypes more typical of Mesozoic floral assemblages (including ginkgophytes, cycadophytes, peltasperms, conifers, and some ferns) appear in greater diversity. The gradual rise of the "mesophytic" floral components at this time coincides with a drop in both raw and proportional richness of the "paleophytes". This turnover, however, appears to be assynchronous across Permian Pangea, occurring earlier in the paleotropics (Euramerica) than in either the northern (Angaran) or southern (Gondwanan) extratropical regions of the supercontinent. This intraregional study examines this spatio-temporal transition within the northern extratropics of Pangea using a large genus occurrence dataset over three sub-equal time bins in the Permian. The results show that within this phytogeographic region fossil plant assemblages of Asselian, Sakmarian, and Artinskian age show greater "mesophytic" richness at lower paleolatitudes than higher paleolatitudes. By Kungurian and Middle Permian time, assemblages at middle to high northern paleolatitudes exhibit greater numbers of typically "mesophytic" plant groups. By Late Permian some assemblages at paleolatitudes >50 degrees north are taxonomically dominated by "mesophytes" and the data show that typically "mesophytic" groups gain dominance at ever higher paleolatitudes at a faster rate through the Permian than the northward continental motion of Pangea itself through the same time interval. This pattern may, in part, be a result of dynamic climate belt migration northward coincident with shift of global climate from an Icehouse to Hothouse phase in the Late Paleozoic.