Bomolithus elegans Roth, 1973
Circular heliolith comprised of two discs and a moderate-high column. The species assigned to this genus are regularly observed in both plan-view and side-view because the high column they possess allows the proportionate settling of specimens in either view (e.g. plan vs side). In contrast, the species of Heliotrochus (e.g., H. kleinpellii and H. steurbautii) are observed only in plan view because they possess a low column, which prevents the proportionate settling of specimens. The terminology of Bomolithus is presented in Fig. 9 in Bowman & Varol (2021).
Optical properties: The discs and column appear birefringent in both plan view and side view. In cross-polarised light, the distal side shows a dextrogyre-type extinction pattern (e.g. Pl. 14, figs. 9, 10 in Bowman & Varol, 2021). In contrast, the proximal side is characterised by a laevogyre-type extinction pattern (e.g., Pl. 14, figs. 11, 12 in Bowman & Varol, 2021). When using the gypsum plate, the distal view will show the blue regions corresponding to the horizontal axis. In contrast, the proximal view will show the blue sectors corresponding to the vertical axis.
Bomolithus and Heliotrochus are differentiated based on column height (e.g. Bomolithus possess a high column, Heliotrochus has a low column). The holotype of Bomolithus elegans (type species of Bomolithus) is an SEM micrograph (Roth, 1973; Pl. 15, figs. 1) revealing two different discs of equal diameter and a high column. The mobile mount technique used in this study showed that Bomolithus elegans has birefringent discs, proving that the paratype of Bomolithus elegans (Roth, 1973; Pl. 15, figs. 6a–d) is an entirely different species. Subsequently, specimens comparable to the paratype (but not to the holotype) have been erroneously placed into Bomolithus elegans by many authors, including Perch-Nielsen (1977, 1985), Varol (1989) and Bown (2010, 2016). The mobile mount technique clearly demonstrates that the forms which are comprised of a non-birefringent disc and a birefringent column are very different from Bomolithus, and these forms are now assigned to the new genus Caycedoa.
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.
Roth, P. H. 1973. Calcareous nannofossils: Leg 17 of the Deep Sea Drilling Project. Initial Reports of the Deep Sea Drilling Project. 17: 695-795.