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Responses of Plant Communities to Environmental Change
The past is an ideal laboratory in which to study the response of ecosystems to changes in the physical environment. Different magnitudes of environmental change, different rates of change, and different types of change also permit the study of how well changes at a smaller spatio-temporal scales can be extrapolated upward to explain larger scale patterns.
With numerous colleagues I have been working on documenting patterns of change in the composition of plant assemblages in the late Paleozoic. We have focused on those time intervals during which major changes in the physical environment had significant impact on the composition and structure of terrestrial plant communities. Where possible, we have also studied the effects of these changes on the vertebrate and invertebrate assemblages. The studies have involved examination of atmospheric composition through geochemical studies, evaluation of climate changes through modeling and through the study of paleosols and sedimentary environments, characterization of local fossil-bearing sedimentary environments, and documentation of the floras and faunas.
Floristic Change during the Pennsylvanian-Permian transition
The Pennsylvanian and Early Permian were times of globally cool climates, with glacial ice developed at the south pole, likely in the north polar region (though as sea ice) and in mountainous regions. The interval of glaciation was a very long one, however, many millions of years, and so was one of considerable dynamism, including periods of global warming, when ice volumes were very low, and cooling, when massive ice developed. Overlain on these long-term patterns were much shorter cycles of glacial advance and retreat, similar to those of our modern ice age, largely occurring on scales of 100,000 to 400,000 years per cycle.
The Pennsylvanian-Permian boundary was a time during which a major pulse of ice has been detected at the South Pole. This correlates with changes in the character of tropical vegetation, which went from oscillating wet-dry vegetation, reflecting the minor , short-term glacial advances and retreats, to a rise to dominance of “dry” vegetation as the dominant over widespread areas for long periods of time.
We have been studying the vegetational and faunistic patterns associated with this in both north-central Texas and New Mexico, where excellent geological exposures across this boundary exist.
A Late Pennsylvanian age outcrop in north-central Texas. Neil Tabor, Southern Methodist University, for scale. Paleosol at base. White line at top of paleosol is a horizon rich in kaolinitic clay and quartz, which contains a flora of xeromorphic plants. Above this is a dark, organic shale, representing a swamp deposit. The gray sediments on top are stream deposits of a wet floodplain containing wetland plants.
An Early Permian age outcrop in north-central Texas. Red beds representing a lake deposit on a seasonally dry floodplain.
Conifer from north-central Texas Early Permian red beds
The Pennsylvanian-Permian transition in central New Mexico. Distant view showing the Pennsylvanian (gray) – Permian (red) contact.
Gray beds of the latest Pennsylvanian Bursum Formation in central New Mexico. Close up showing excavations. Dan Chaney for scale.
Plants from the latest Pennsylvanian Bursum formation in central New Mexico. Field photographs of specimens now housed in the museum. Conifer branch. Callipterid frond.
Detail of the Early Permian redbeds of the Abo Formation. John Nelson, Illinois Geological Survey, for scale.
Fossil conifer from early Permian redbeds.
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