Morphometry of the Aspidolithus group in the lower Campanian at the Loibichl section (Austrian Eastern Alps). Comparison with palaeoenvironmental proxies.

A morphometric analysis of the Aspidolithus group was carried out on samples from the lower Campanian of Loibichl (Rhenodanubian Flysch Zone in the Austrian Eastern Alps). Semi-quantitative analysis of the nannofossil assemblages was used to calculate palaeoenvironmental indexes. In addition, 1021 specimens of the Aspidolithus group with a good state of preservation were considered for measurement of different key parameters: (1) the maximum length of the coccolith (L); (2) the maximum width of the coccolith (W); (3) the width of the outer rim versus the small diameter of the central area (b/a); and (4) the number and arrangement of perforations in the central area. The aim of this study is to observe if there are significant differences between Aspidolithus morphotypes and to explore if there are any relationships that may control the size variation of this group. Furthermore, we analyzed the CaCO₃ composition and the stable isotope ratios of δ¹³C and δ¹⁸O in order to assist in the interpretation of stratigraphy and palaeoecology. 

This study revealed the presence of five morphotypes: A. enormis subsp. 1, A. enormis subsp. 2, A. parcus expansus, A. parcus parcus, and A. parcus constrictus. The morphometric analysis provided evidence of a significant taxonomic differentiation between "small" morphotypes (with a length of < 8–8.5µm; identified as A. enormis or "small" A. parcus according to Gardin et al. (2001)) and the "large" A. parcus group (with a length of ≥ 8–8.5µm). The observations revealed a slight increase in the overall dimensions, a decrease in the size of the central area, and a general reduction in the number of perforations. Nevertheless, our research did not uncover clear patterns that facilitate the natural categorization of "subspecies" (parcus expansus, parcus parcus, and parcus constrictus) within the A. parcus group. Our statistical study indicates a possible correlation between the size variation of the Aspidolithus group and the different paleoenvironmental proxies, suggesting that they may reflect palaeoecological changes.

Additional research is required to confirm the impact of size variation. This can be achieved through a comparative analysis that includes more material from the Santonian–Campanian boundary. An in-depth investigation of the impact of global vs. regional environmental elements is essential for comprehending these results.

Gardin, S., et al. 2001. Developments in Palaeontology and Stratigraphy 19, 745–757.