Morphometric and Palaeoenvironmental Analysis of Aspidolithus (Calcareous Nannofossils): Rhenodanubian Flysch Zone (Eastern Alps, Austria) vs. Niobrara Formation (Kansas, USA)

Calcareous nannofossils are essential tools for reconstructing palaeoenvironmental conditions and understanding evolutionary trends in plankton organisms during the Late Cretaceous. This study presents a comparative morphometric analysis of the Aspidolithus enormis - parcus group from the Lower Campanian and distant sections of the deep-water Rhenodanubian Supergroup (Loibichl, Eastern Alps, Austria) and the pelagic Smoky Hill Member of the Niobrara Formation (Western Interior Seaway, Kansas, USA). The aim is to refine our understanding of the evolutionary trends within this group, particularly in response to changing palaeoceanographic and climatic conditions.

Morphometric analysis of Aspidolithus taxa in both sections reveals that A. enormis (“small A. parcus”) can be clearly distinguished from A. parcus based on the total length of the coccolith, with a threshold of approximately 8.5 µm. A. enormis consistently exhibits a smaller coccolith size (< 8.5 µm), while A. parcus is characterized by a larger size (> 8.5 µm). However, despite clear size differences between A. enormis and A. parcus, statistical differentiation between the subspecies of A. parcus (A. parcus parcus, A. parcus expansus, and A. parcus constrictus) based on the ratio of central area width to external ring width (b/a) was not observed in either of the studied sections.

Interestingly, both the sections exhibit a similar pattern in which size variation in the b/a ratio correlates with sea surface temperature, with larger central areas associated with higher temperatures.

These findings reinforce the hypothesis that the Aspidolithus group exhibits regional consistency in response to environmental fluctuations, highlighting the applicability of nannofossil morphometry for palaeoenvironmental reconstructions and stratigraphic correlation.