Bomolithus universus

Heliolithus universus Wind & Wise in Wise & Wind, 1977

Emended by Bowman & Varol, 2021: Moderate to large (6.5–11.0μm) circular species of Bomolithus consisting of two discs and a column. The discs are of unequal diameter (e.g. proximal disc >median disc) and appear convex (observed in the side-view). The high column is wide and flaring out distally. The narrowest part of the species occurs at the median disc, causing the species to resemble a “butterfly” in the side-view. The column and disc consist of a relatively wide canal closed by a smooth central plug. The discs possess numerous segments (about 30–60).

In plan view, the discs and column appear yellowish under polarised light. In the distal view, the extinction lines are dextrogyre, whereas the extinction lines are laevogyre in the proximal view. When viewed under a gypsum plate, the horizontal axis of the coccolith lies within the blue sector on the distal/column-side. Still, the vertical axis of the coccolith lies within the blue sector on the proximal/disc side. The discs and column are birefringent in plan and side views.

The narrow median disc and distally flaring column produce a “butterfly-like” appearance in the side-view. The relatively high column of Bomolithus universus is the diagnostic criterion for separating species from Heliotrochus. Bomolithus universus possesses a proximal disc and column of approximately equal diameter. In contrast, the diameter of the proximal disc is much greater than the diameter of the column in Bomolithus cantabriae. In the side-view, Bomolithus universus demonstrates a distally flaring column, but the column appears cylindrical or parallel-sided in Bomolithus cantabriae. Bomolithus universus looks superficially similar to Heliolithus riedelii but possesses a median disc, whereas Heliolithus riedelii lacks a median disc.

Wise, S. W. & Wind, F. H. 1977. Mesozoic and Cenozoic calcareous nannofossils recovered by DSDP Leg 36 drilling on the Falkland Plateau, south-west Atlantic sector of the Southern Ocean. Initial Reports of the Deep Sea Drilling Project 36: 269-491