Lower Campanian calcareous nannofossils: Morphometry and palaeoenvironmental implications of the Aspidolithus group (Rhenodanubian Flysch Zone, Austria)

Abstract

Our calcareous nannofossil study focused on eleven samples from the lower Campanian (UC14a–UC14b) of the Loibichl section, located in the Rhenodanubian Flysch Zone of the Austrian Alps. Relative counts of calcareous nannofossil assemblages were followed by a morphometrical analysis on 1021 specimens belonging to the Aspidolithus genus (Aspidolithus enormis and Aspidolithus parcus). For this analysis, a light microscope was utilized to improve our understanding of the taxonomic concepts of this group during a phase of rapid morphological evolution. The analysis included the maximum length (L), the width of the outer rim versus the small diameter of the central area (b/a), and the number of perforations in the central area. The CaCO₃ content and bulk carbonate stable isotope ratios of δ¹³C and δ¹⁸O were analysed to facilitate stratigraphic and palaeoecological interpretations. Additionally, this study was to determine whether variations in nannofossil morphometrics were driven by changing palaeoenvironmental conditions. This study identified five morphotypes: A. enormis subsp. 1, A. enormis subsp. 2, A. parcus expansus, A. parcus parcus, and A. parcus constrictus. The morphometric analysis substantiated the differentiation between “small” morphotypes (L < 8.5 μm; A. enormis) and the “large” A. parcus group (L > 8.5 μm). However, the results did not show any significant patterns that would justify a natural separation of the “subspecies” within the A. enormis and A. parcus group. Our statistical analysis suggests that these morphometric changes may be related to various palaeoenvironmental proxies. Further high-resolution morphometric analysis is required to verify the impact of contrasting environmental factors on Aspidolithus-size variation.