The influence of taphonomy and time on the paleobotanical record of the Permian-Triassic transition of the Karoo basin (and elsewhere)

Terrestrial fossil assemblages preserved in the upper Permian-Lower Triassic strata of the Karoo Basin, South Africa, have played a central role in the interpretation of ecosystem patterns and end-Permian extinction models. However, these models need to be carefully reconsidered because of the limitations of the rock record. Four lessons learned from a multidisciplinary approach to the rocks, lithology, stratigraphy, and dating are relevant to other paleofloras in large continental basins. In reality, the Karoo paleofloral record is very sparse. Hence, reports of a near continuous fossil record in this basin should be considered as the near continuous record of erosion and time lost with sporadic plant-fossil assemblages.A review of the debate over the rate and timing of the Permian-Triassic in the Karoo Basin reinforces the need for extensive stratigraphic mapping, the analysis of depositional environments of the plants, as well as the application of a variety of dating methods. First, Late Permian to Early Triassic paleobotanical assemblages are extremely rare in the basin with only a handful of sites in the Free State and Eastern Cape Provinces. These fossil data originate from >3750 m of total measured section wherein megafloral remains are preserved in <1% of the available rock record (0.9% all megafloral elements; permineralized wood = 0.1%, adpressions = 0.8%), with spore-and-pollen assemblages only slightly more frequently encountered at 1.3%. This low occurrence is comparable with other basins. Thus, any continental fossil assemblage represents a very short temporal window into the paleobiosphere because of taphonomic effects of the soils, pedogenesis, and controls on depositional environments. Second, geochronometric and rock magnetic data, developed in a sequence stratigraphic context, are critical to constrain time and biological trends in continental successions. The missing time, diastems and hiatuses, are critically important. Third, the spatial relationships of plant-fossil assemblages are not easily correlated across the basin without an extensive dataset of the paleolandscape. In general, the Late Permian Beaufort rocks represent channels, floodplains, and braided streams rather than lakes and oxbows that are conducive to the preservation of plant parts. Finally, the temporal distribution of paleobotanical assemblages is complicated by the missing time (sediments) that has resulted in the apparent scarcity of vegetation before and after the end Permian extinction. The reported uncharacteristic diversity and abundance of plants in the Carnian-Norian Molteno Formation is most likely due to an environment conducive to preserving fossil-plant assemblages combined with a record of intensive collecting. Overall, the large inland Karoo Basin, without any marine influence or extensive volcanic deposits, has favored the preservation of vertebrate assemblages.