Biometric insights on the calcareous nannofossil genus Aspidolithus across the Santonian–Campanian boundary: Western Interior (Niobrara, USA) vs. Tethys (Loibichl and Postalm, Austria) sections

The taxonomic subdivision of Aspidolithus parcus has long relied on biometric parameters, particularly the ratio between the central area width and the outer rim width (b/a). However, its reliability as a taxonomic criterion remains uncertain. This study applies biometric and statistical analyses to specimens from the Niobrara Formation (Western Interior Seaway, USA) and the Postalm and Loibichl sections (Tethyan Realm, Austria) to reassess the validity of arbitrary taxonomic subdivisions and their biostratigraphic implications. We also examine the distribution of calcareous nannofossil assemblages to explore palaeoenvironmental influences on biometric variation. Mixture analysis of b/a values reveals three statistical groups in the Postalm and Niobrara sections. However, overlapping ranges suggest that b/a is influenced by palaeoenvironmental factors rather than reflecting stable taxonomic boundaries. In contrast, total coccolith length (L), as previously shown in the published data from the Loibichl section, provides a more robust criterion for distinguishing the precursor species Aspidolithus enormis from A. parcus, with a statistically significant threshold at ∼ 8.5 µm. Our biometric analysis refines previous subspecies definitions as follows: A. parcus constrictus (b/a < 1.6), A. parcus expansus (1.6 ≤ b/a < 2.2), and A. parcus parcus (b/a ≥ 2.2). Statistical results indicate a correlation between the b/a ratio and surface water temperature, supporting the idea that environmental factors play a role in shaping Aspidolithus morphology. Nevertheless, similarities with data reported from other Tethyan sections (Austria) suggest that evolutionary trends also contributed to the observed biometric variations. The unexpected stratigraphic pattern at Bottaccione (Italy), where smaller specimens appear later than larger ones, contrasts with trends in Niobrara and Postalm. This suggests that, while palaeoenvironmental factors influence A. parcus morphology, its overall evolutionary trajectory may not be strictly controlled by local environmental conditions. These findings emphasise the need for reassessing A. parcus taxonomy and standardising biometric criteria to improve biostratigraphic resolution and ensure consistency in Santonian–Campanian stage and age correlations.