Heliolithus nefroeidis

Heliolithus nefroeidis Bowman and Varol, 2021

A medium to large-sized circular species of Heliolithus, having a low column, typically appears relatively wide and elongated (but may appear narrow and/or truncated), generally resembling a “kidney” in the side-view. The column contains a very narrow central canal and plug/tube cycle.

From Greek nefroeidis, kidney-based on the kidney-shaped column depicting the species in side-view.

Overall height: 3.49μm; Column height: 1.20μm, Column diameter: 5.41μm; Disc diameter: 6.38μm, Disc height: 2.49μm.

Medium to large size (6.0–10.0μm) circular species of Heliolithus possessing one disc and a column. The diameter of the disc is slightly wider than the width of the column or almost equal. The low column creates a kidney-shaped appearance in the side view. The column may appear severed or extended somewhat above the height of the disc. The narrow central canal with plug/tube cycle displays minor depressions on both ends of the canal (e.g. distal and proximal). An approximately equal number of segments (about 20–40) constitute the disc and column, and the narrow segments are of equal width.

The distal view corresponds to the dextrogyre-type extinction lines (e.g. Pl. 73, figs. 9–10 in Bowman & Varol, 2021), whereas the proximal view demonstrates the laevogyre-type extinction lines (e.g. Pl. 73, figs. 11–12 in Bowman & Varol, 2021). When enhancing the view by applying the gypsum filter, the horizontal axis relates to the blue section (on the distal side). Still, the vertical axis correlates to the blue portion on the proximal side. The disc and column are birefringent in side-view and plan-view. A whiteish-yellowish birefringence colour characterises the disc and column in plan-view, but a yellowish-blueish birefringence colour is associated with the disc and column in the side-view.

Heliolithus nefroeidis displays some variation in overall size, as well as within the width/height ratio of the disc and column. This species is differentiated from the Caycedoa species based on the diagnostic birefringent pattern of the disc in plan-view. The disc is birefringent in Heliolithus but non-birefringent in Caycedoa in the plan view. Heliolithus nefroeidis possesses a kidney-shaped column, which differentiates this species from other species of Heliolithus. Heliolithus bukryi has a hollow column that is contrastingly taller, a distinct plug/tube cycle, and a disc with pointed ends in side view. Heliolithus iptamenosdiskos exhibits a disc that is only slightly wider than the column (e.g. almost equal width), a smooth plug/tube cycle and a disc that possesses rounded ends in side-view. Heliolithus knoxii demonstrates a comparably much higher column and a narrow central canal with an indistinct plug/tube cycle. Heliolithus kolovos has a truncated/wedge-shaped column that does not extend above the height of the disc (e.g. lacks free
length). Heliolithus magnificus consists of a slightly wider disc than the column (e.g. almost equal width), a central canal with plug/tube cycle and lacks extension above the height of the disc (e.g. lacks free length), and a disc and column that show similar height. Also, in plan-view, Heliolithus
nefroeidis superficially resembles Heliotrochus kleinpellii (i.e. double-disc form), but the two species may be easily separated by using careful focusing.

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.