Macroecological patterns of the terrestrial vegetation history during the end-Triassic biotic crisis in the central European Basin: A palynological study of the Bonenburg section (NW-Germany) and its supra-regional implications

The end-Triassic mass extinction is often linked to environmental and climate change triggered by the activity of the Central Atlantic Magmatic Province ca. 201 Mya. In the German Triassic Basin, the transition from pre-extinction to the so-called extinction interval is documented from the fossil-rich Contorta Beds to the mostly barren Triletes Beds. However, despite the lack of macrofossils, plant microfossils are present and studying palynomorph diversity and assemblages still give us a detailed insight into the impact of environmental changes on the flora.

Here, we present data from 64 samples taken from the new Triassic-Jurassic section 'Bonenburg', which originates from a brick quarry in North Rhine-Westphalia (Germany), and encompasses the Triassic-Jurassic transition in the Germanic Basin. Using palynofacies analysis, we document changes in the depositional environment. Based on terrestrial and marine palynological analysis, we also document quantitative changes in four assemblage zones for correlation with other European sections. Further, we discuss the vegetation history with special focus on the transition from the pre-extinction to extinction interval (Contorta to Triletes Beds). Additionally, we investigate palynofloral diversity patterns especially prior to the Triassic-Jurassic transition, to evaluate the ecological impact of environmental upheaval on the flora. Furthermore, we document intraspecific palynomorph variability, indicating a variety of aberrant spore, pollen, and tetrad formation, in the middle Rhaetian, the lowermost upper Rhaetian, and the lowest Hettangian.

Our study supports existing paleogeographical reconstructions for the region and reveals, that vegetation underwent gradual changes with intermediate successional stages rather than dramatic extinction or drastic turnover as documented for animals. Diversity patterns coinciding with aberrant palynomorph occurrences, potentially associated with episodes of increased environmental stress, suggest three disturbances pulses of probably increasing severity. Although environmental stress in the context of diversity patterns seems a likely explanation for the occurrence of aberrant palynomorphs, further investigations are needed to better understand the underlying mechanisms and their evolutionary significance and temporal connection with biotic crises.