THE EARLY TRIASSIC PANTHALASSAN CONTINENTAL MARGIN OF TROPICAL PANGEA; A BARREN WASTELAND, OR WAS IT?

The end-Permian biotic crisis had a major impact on the Earth's marine as well as the terrestrial ecosystems as is evidenced by fossil records from many parts of the world. While the effect of the end-Permian crisis is generally better preserved in oceans, what transpired on land is oftentimes less clear. In the vast area of western and central Pangea, evidence for terrestrial ecosystem dynamics during these times of biosphere reorganization has been severely limited due to the absence of plant macro- and microfossils. Whereas Permian-Triassic (PTb) paleobotanical studies traditionally focused on macrofossil and palynological records, these fossil types were not preserved in the oxidized rocks that almost entirely make up the sedimentary record in the western parts of equatorial Pangea. In contrast, phytoliths – plant-produced microfossils that consist of robust concretions of hydrated silica – are resistant to decay under oxic conditions. Their chemical composition makes them an excellent tool for the reconstruction of plant communities that grew outside the anoxic depositional environments.

Sediments deposited on the coastal Panthalassa margin of Pangaea crop out in Caprock Canyon State Park and Palo Duro State Park in western Texas, USA. Here, fluvial sandstones and the overbank mudstones of the Quartermaster formation are known to include the PTb, as determined by U-Pb dating, and biogeochemical changes associated with the end-Permian crisis. The well-oxidized sedimentary rocks collected at these localities, shown in an earlier study to be void of palynomorphs, proved to be productive when the same samples were processed for phytolith extraction.

Almost all samples contain diverse assemblages of phytoliths. The most interesting morphotypes encountered are characteristic for coniferous wood, suggesting that vegetation rich in conifers was present at the coastal Panthalassa margin of Pangaea. This study provides new insight into the vegetational changes across the PTb in western Pangaea. It also demonstrates that phytoliths may be of use for vegetational reconstructions or biostratigraphical purposes at localities where the traditional approaches fail.