Heliolithus knoxii

Heliolithus knoxii Steurbaut, 1998

Emended by Bowman & Varol, 2021: Small to very large (6.0–15.0μm) circular species of Heliolithus comprised a disc and a column. The relatively narrow, high-to-very high column is typically parallel-sided. A narrow central canal closed by an indistinct plug/tube cycle and diagnostic depressions characterise the ends of the canal [e.g. distal and proximal]. The disc and column are composed of a similar number of segments (about 24–40), and the segments of the disc and column are of similar width.

The extinction lines display a dextrogyre pattern in the distal view (e.g. Pl. 62, figs. 9, 13), whereas the extinction lines follow a laevogyre pattern in the proximal view (e.g. Pl. 62, figs. 11, 15). Through the use of the gypsum plate, the blue section corresponds to the horizontal axis when viewed from the distal side, but the blue portion relates to the vertical axis if observed from the proximal side. The disc and column are birefringent in side-view and plan-view. In the polarised light, the disc and column display yellowish-whiteish birefringence colour in the plan-view. The disc has blueish-reddish-yellowish birefringence colour, but the column has blueish-reddish birefringence colour in the side-view.

Heliolithus knoxii differs from other species of Heliolithus by having a much higher column with a very narrow canal and an indistinct plug/tube cycle. In plan view, this species differs from those of Caycedoa by the diagnostic birefringent pattern of its disc. The disc is non-birefringent in plan-view in the species of Caycedoa. The individual species of Heliolithus are differentiated from Heliolithus knoxii based on the following: Heliolithus bukryi possesses a hollow column that appears comparably higher and narrower, a distinct plug/tube cycle, and a disc with pointed ends in side view. Heliolithus iptamenosdiskos has a disc that is only slightly wider than the column (e.g., almost equal width), a smooth plug/tube cycle, and a disc that displays rounded ends in side view. Heliolithus nefroeidis exhibits a kidney-shaped column with a narrow central canal. Heliolithus kolovos has a truncated/wedge-shaped column that lacks free length (e.g. does not extend above the height of the disc) in the side-view. Heliolithus magnificus possesses a disc that is only slightly wider than the column, a central canal and a plug/tube cycle occupying the column and a disc/column of similar height. In plan-view, Heliolithus knoxii falsely resembles Heliotrochus kleinpellii (i.e. double-disc form), but these separate species may be easily differentiated by 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.

Steurbaut, E. 1998. High–resolution holostratigraphy of Middle Paleocene to Early Eocene strata in Belgium and adjacent areas. Palaeontographica Abteilung A Palaeozoologie Stratigraphie. 247: 91-256.