Heliolithus riedelii Bramlette & Sullivan, 1961
Emended by Bowman & Varol, 2021: Small to very large and circular to slightly subcircular heliolith composed of one disc and a column, both of which display variable character. In the side-view, the column appears parallel-sided, truncated, slightly tapering (distally or proximally), or flaring out distally, and is filled by a central canal. The disc appears concave, convex or flat. The central area also demonstrates variable features in plan view (e.g., open vs plug/tube cycle).
The species of this genus are regularly observed in plan view and side view because they possess a tall column that allows for natural settling in both views. The terminology of Heliolithus is presented in Fig. 11 in Bowman & Varol (2021).
Optical properties: The disc and column are birefringent in both plan and side views. In cross-polarised light, the distal side shows a dextrogyre-type extinction pattern (e.g. Pl. 26, figs. 9, 10 in Bowman & Varol, 2021). In contrast, the proximal side is characterised by a laevogyre-type extinction pattern (e.g., Pl. 26, figs. 11, 12 in Bowman & Varol, 2021). When using the gypsum plate, the distal view shows the blue regions corresponding to the horizontal axis, while the proximal view shows the blue sectors corresponding to the vertical axis.
Heliolithus and Caycedoa are differentiated by the birefringence of their discs in plan view. Heliolithus display a birefringent disc, but the disc of Caycedoa is non-birefringent. Heliolithus and Caycedoa possess a single disc, whereas Heliotrochus and Bomolithus are constructed of two discs.
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.