ASSESSING CHANGES IN COMMUNITY DIVERSITY: PORTRAIT OF A PERMIAN GONDWANAN FLOODPLAIN ECOSYSTEM

The Permian-Triassic has been long recognized as an interval of massive turnover in the taxonomic composition of plant communities. However, in contrast to evidence of a major marine mass extinction event, the timing of the transition in terrestrial ecosystems is more controversial. Recent work suggests a decline in within-community plant diversity across the P/Tr boundary, but the paucity of sites studied with consistent sampling protocols makes this interpretation uncertain. The Clouston Farm site is a new Upper Permian (Estcourt Formation, Karoo Supergroup) locality located in KwaZulu-Natal, South Africa. Plant fossils are preserved at the top of a 16m section consisting of a single, upwards-fining channel-fill, with outcrop extending for approximately 56m along both sides of a seasonally dry stream cut. Foliage, axes and reproductive organs are preserved as impressions in finely laminated mudstones in the uppermost 2m of the section. The dense leaf mats are interpreted to have accumulated in an abandoned fluvial channel over <1000 years and the quality of preservation suggests limited transport prior to deposition. To assess changes in the diversity and composition of the local community on ecological time scales within a controlled taphonomic context, the macloflora was sampled on ten bedding horizons at ca. 10cm intervals across one meter of vertical outcrop, using a half-meter square grid. All leaves, axes, reproductive organs, rhizomes and other parts larger than 1cm2 were tabulated. The macroflora is dominated by Glossopteris foliage but also contains abundant sphenophytes, as well as rare ferns and lycopods. Initial analyses suggest that although the alpha diversity is relatively consistent, the relative abundance of higher groups (i.e., glossopterids, sphenophytes) varies. Preliminary results from a second section 45m to the SE indicate significant spatial differences in the vegetational signature, particularly in the relative abundance of sphenophytes. Taken as a whole, the Clouston Farm site is taxonomically diverse relative to other Upper Permian macrofloras, but is still significantly less diverse than the angiosperm-dominated communities of the Cretaceous and Cenozoic, suggesting considerable differences in the ecosystem structure.