Heliolithus amfivolos

Heliolithus amfivolos Varol & Bowman, 2021

A medium-sized and circular species of Heliolithus featuring a large central opening, and a birefringent disc and column in plan and side views.

From Greek amfivolos, dubious—indicating the strange general appearance of the species in the side-view.

Overall height: 2.60μm; Column height: 0.94μm, Column diameter: 5.26μm; Disc diameter: 6.41μm; Central Opening diameter: 2.81μm.

Medium-sized (6.0–8.0μm) circular heliolith possessing a birefringent disc and a column. The diameter of the disc is slightly greater than the diameter of the column. The segments composing the disc and column appear indistinct.

On the distal side, the extinction lines are dextrogyre (e.g. Pl. 45, figs. 21–22 in Bowman & Varol, 2021), but the extinction lines are laevogyre on the proximal side (e.g. Pl. 45, figs. 23–24 in Bowman & Varol, 2021). The horizontal axis exists within the blue portion on the distal side. The vertical axis lies in the blue sector on the proximal side when observed using the gypsum plate. The disc and column are birefringent in side-view and plan-view. In plan view, the disc and column display yellowish-white birefringence colours.

This delicate species appears strange in general. In the side-view, the very low column. Heliolithus amfivolos displays a large and circular opening in the central area, whereas other species of Heliolithus lack this feature. This species differs from the species of Caycedoa based on the diagnostic birefringent pattern of its disc in plan-view. The disc is nonbirefringent in plan-view in the species of 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.