Heliolithus kleinpellii Sullivan, 1964
A medium- to very large, circular heliolith comprises two discs, a column, and a central canal with a plug/tube cycle. The discs and column are very thin or compressed, and about equal in height/thickness.
In cross-polarised light, the distal view yields dextrogyre extinction lines, whereas the proximal view shows laevogyre extinction lines. When viewed with a gypsum plate, the alignment of the horizontal axis and the blue sector indicates the distal side. In contrast, the proximal side corresponds to the orientation of the vertical axis and the blue region. The extinction lines are dextrogyre in the distal view (column side) and laevogyre in the proximal view (disc side). Romein (1979; see p. 155) also determined that the disc side of specimens is associated with the appearance of laevogyre extinction lines, as well as alignment of the vertical axis and the blue sector. However, for unknown reasons, he mistakenly referred to this side as distal, possibly because the disc side has traditionally been accepted as distal, but this interpretation lacks evidence. All species appear fully birefringent (i.e. birefringent discs and a column) in side-view and plan-view. The terminology of Heliotrochus is presented in Fig. 12 in Bowman & Varol (2021).
Heliotrochus and Bomolithus are differentiated by column height (e.g., Heliotrochus has a low column, while Bomolithus has a high column). Species of Heliotrochus show significant variation in the character of critical features (e.g. size relationships between the discs and column). The species of Heliotrochus only settle naturally in the plan-view because their thin discs and low columns lack the necessary height needed to distribute mass more evenly throughout the species, which ultimately prevents specimens from appearing in the side-view. By comparison, the relatively higher column exhibited by species of Bomolithus allows these forms to be observed in plan view and side view. Tonromeinia possesses two discs and a column of similar height and width, with a large, featureless central opening that appears featureless. In contrast, Heliotrochus display two discs and a column of similar height but unequal width, as well as a central plug/tube cycle. Species of Heliotrochus are constructed of two discs and a column, whereas Heliolithus and Caycedoa are comprised of one disc and a column. Moreover, species of Caycedoa exhibit a disc that appears non-birefringent in plan-view, but the discs and column of Heliotrochus are birefringent in plan-view and side-view.
Aubry, M. -P. 2015. Cenozoic Coccolithophores: Discoasterales. Micropaleontology Press, American Museum of Natural History, New York. pp. 1-455.
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.
Romein, A. J. T., 1979. Lineages in Early Paleogene calcareous nannoplankton. Utrecht Micropaleontological Bulletin 22, 1-231.
Sullivan, F. R., 1964. Lower Tertiary nannoplankton from the California Coast Ranges. I. Paleocene. University of California Publications in Geological Sciences 44, 163-227.