TROPICAL LATITUDINAL CLIMATIC GRADIENTS IN THE PENNSYLVANIAN OF NORTH AMERICA: EVIDENCE FROM SWAMP COMMUNITIES AND LEAF MORPHOLOGY

Plants are sensitive to their habitat and, as a result, are excellent environmental indicators. Using macrofloral and microfloral assemblages, coal macerals, coal abundance, and pyrite abundance, a community gradient spanning deeply flooded to drained peat substrates has been established for the Pennsylvanian. Lycopsids flourished in flooded peat swamps; although some appear adapted to drained peat. Tree ferns possibly required exposed substrates for reproduction. Tree ferns, seed ferns, and sphenopsids flourished in areas with high siliciclastic influx. Cordaites may have been adapted to peat swamps that experienced seasonal saltwater influx linked to seasonally dry climates. The habitat preferences of these Pennsylvanian plants has been used to reconstruct environments within a single coal bench or seam, and to trace paleoclimatic change in the Pennsylvanian. They can also be used to reconstruct latitudinal climatic gradients during the Pennsylvanian. During the Late Atokan, tree ferns and lycopsids predominated in the paleo-equatorial Illinois Basin. Cordaites predominated in both the Western Interior Basin to the north and in the Appalachian Basin to the south, suggesting seasonal rainfall in both locations.

Paleobotanists have long sought to use leaf morphology to determine Paleozoic paleoclimates. Alethopteris leaves, derived from medullosan seed ferns occur in most Pennsylvanian coals with permineralized peat. The ratio of laminar thickness to surface area and the relative abundance of fibers in Alethopteris leaves may correlate with paleoclimate. Alethopteris species from the early Desmoinesian of the Western Interior Basin, interpreted as a relatively dry interval, have thicker lamina, smaller surface areas, and more fibers than Alethopteris species from the wet middle to late Desmoinesian. Alethopteris from the early Desmoinesian of the Western Interior Basin appear to be xeromorphic. However, seasonal saltwater influx into the swamp rather than aridity may account for xeromorphy in these leaves.