Diadelithus robustus

Cyclolithus? robustus Bramlette & Sullivan, 1961

Large (8–12 μm) circular placolith with almost equal-sized shields and a narrow tube cycle surrounding the large vacant central opening. The weak birefringence leads to low interference colours (greyish-white). It is challenging to determine extinction lines in Diadelithus robustus. Diadelithus displays extinction lines similar to Denisea, showing dextrogyre extinction lines on the distal side and laevogyre extinction lines on the proximal side. When using the gypsum plate, the distal side exhibits alignment of the horizontal axis and blue interference colours. However, the vertical axis is associated with the blue interference colours appearing on the proximal side. The entire species is birefringent in the side view.

The species of Tonromenia and the large species of Denisea are erroneously placed in Diadelithus robustus (see Bowman & Varol, 2021). The inclination angle of the distal shield in Diadelithus robustus is between 40–55°, but discs and columns are almost 0° in Tonromeinia. The inclination angle of the distal shield (55–75°) is much higher in Denisea than that of Diadelithus robustus (40–55°). A narrow tube cycle and low interference colours in the plan view further distinguish Diadelithus robustus from Tonromenia, which displays high interference colours and lacks a tube cycle. Denisea species displays high interference colours and has a wide tube cycle. The tube cycle is triangular in the side view, but it is indistinct in Diadelithus robustus.

Aubry, M. -P. & Bord, D. 2009. Reshuffling the cards in the photic zone at the Eocene/Oligocene boundary. In: Koeberl, C., Montanari, A. (eds), The Late Eocene Earth-Hothouse, Icehouse, and Impacts. Geological Society of America, Special Papers 452: 279-301.

Bowman, A. R. & Varol, O. 2021. A Taxonomic Revision of Heliolithaceae - Applications in Resolving the Problematic Calcareous Nannofossil Biostratigraphy of the Paleocene. In: M. Montenary, M. (Ed.). Calcareous nannofossil biostratigraphy of the Stratigraphy and timescales. 6: 43-223.

Bramlette, M. N. & Sullivan, F. R. 1961, Coccolithophorids and related nannoplankton of the Early Tertiary in California. Micropaleontology 7(2): 129-188.

Varol, O. 2022. Denisea, a new genus of Paleocene calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 304/2 (2022): 159-186.