Morphometrical analysis of the Aspidolithus/Broinsonia group around the Santonian–Campanian boundary in the Loibichl section (Austrian Eastern Alps)

The first occurrence of large Aspidolithus parcus parcus (Stradner 1963) Noël 1969 or Broinsonia parca subsp. parca (Stradner, 1963) Bukry, 1969 (of Nannotax3) defines the base of the Campanian. This bioevent is generally near the paleomagnetic reversal from C34n (Long Cretaceous Normal Superchron) to C33r, the primary marker for the base of the Campanian, with a proposed GSSP at Bottachione/Gubbio (Miniati et al., 2021). Changes in coccolith morphometry of the Aspidolithus/Broinsonia group are used to identify subspecies of the group relevant in the Upper Cretaceous. However, there is a taxonomic problem in the generic attribution of Aspidolithus (without central cross) or Broinsonia (with central cross) (see also Nannotax 3 and Miniati et al., 2020).

The present study focuses on the light-microscope morphometric analysis of Broinsonia/Aspidolithus group, around the Santonian–Campanian boundary (UC13-UC14a-UC14b) in the Loibichl section (Austrian Eastern Alps). A total of 1021 specimens with a moderate to good state of preservation of Broinsonia/Aspidolithus spp. have been measured using JMicroVision software. For each specimen, the length (L) and width (W) of the coccolith, the b/a ratio of the width of the outer rim/shield (b) versus the small diameter of the central area (a) (e.g. Gardin et al., 2001), as well as the number of perforations in the central area (e.g. Lauer, 1975) have been measured.

Five morphotypes were distinguished at Loibichl section as well as Wolfgring et al. (2018, Postalm section). Including the number of perforations: (1) Broinsonia enormis subs. 1 (L < 9 µm, b/a ≥ 2 and 8-16 perforations); (2) Broinsonia enormis subs. 2 (L < 9 µm, b/a < 2 and 4-12 perforations); (3) A. parcus expansus (Wise & Watkins in Wise 1983) Perch-Nielsen 1984 (L > 9 µm, b/a ≥ 2 and 8-16 perforations); (4) A. parcus parcus (L > 9 µm, b/a < 2 to ≥ 1 and 4-16 perforations; (5) A. parcus constrictus (Hattner et al., 1980) Perch-Nielsen 1984 (L > 9 µm, b/a <1 and 4-8 perforations). Throughout the section a trend in the distribution of morphospecies is observed, marked by an increase in the abundance of Aspidolithus parcus parcus and a decrease in the abundance of Aspidolithus parcus expansus in the upper part of the section.

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

Lauer, G. 1975. Archives des Sciences de Genève 28, 259–262.

Miniati, F., et al. 2020. Rivista Italiana di Paleontologia e Stratigrafia 126, 783–801.

Wolfgring, E., et al. 2018.. Newsletters on Stratigraphy 51.

Young, J.R., et al. 2017. Nannotax3 website. Accessed 2 May 2021. URL: https://www.mikrotax.org/Nannotax3.