Paleophysiology of Permian and Triassic Seed Plants

Abstract

This study utilized both anatomically and morphologically preserved fossil plants to investigate plant paleophysiology using known form/function relationships. The fossils examined in this project come from fossil localities ideal for studying various paleophysiological relationships. At the beginning of the Permian Period (~299 Ma), atmospheric CO2 and O2 concentrations were comparable to current day values. By the end of the Permian (~251 Ma), atmospheric CO2 concentration and temperature had risen sharply as the Earth underwent a time of rapid global warming. The distinctive leaf of Glossopteris plants can be found at southern high paleolatitude localities throughout the Permian, allowing for changes in plant physiology to be tracked through a drastically shifting climate. The environmental conditions at the beginning of the Permian are also the same as those that are thought to have favored the evolution of the C4 photosynthetic pathway in the Oligocene (~25 Ma). Using known relationships between leaf anatomy and the C4 pathway, along with stable carbon isotope analysis, the presence or absence of this pathway was tested. The combination of both approaches demonstrated the C3-C4 intermediate photosynthetic pathway was present in Glossopteris during the Late Permian. In the ancient past, plants existed in warm environments at high paleolatitudes where they were subjected to light regimes not experienced by plants today (4 months of continuous light and 4 months of continuous dark). A study of leaf economics of Permian Glossopteris leaves reveals that the plant possessed deciduous leaves and adaptations to continuous light environments. Analysis of Permian and Triassic leaf hydraulic conductance demonstrates that leaf venation density in Glossopteris decreases in response to increasing CO2 but does not change in response to latitude. Glossopteris leaves, which dominated the Permian landscapes of Antarctica, demonstrated a higher leaf venation density than any co-occurring leaves. Such an advantage would benefit leaf hydraulic conductance. In contrast, the Dicroidium leaf type, which dominated the Triassic, had leaf hydraulic values similar to co-occuring leaf morphotypes.