Asinithistimorfi paraxenos Bowman & Varol, 2021
From Greek Asinithisti, unusual, and morfi, form—referring to the bizarre appearance that characterises the general structure of the genus.
Two non-birefringent discs and a column characterise this moderately large and elliptical heliolith. The moderate-high column and plug possess a rough appearance. The extinction lines are dextrogyre in the distal view but levogyre in the proximal view. When viewed using the gypsum plate, the horizontal/short axis of the coccolith lies within the blue sector on the distal/column-side, but the vertical/long axis of the coccolith lies within the blue sector on the proximal/disc-side as the length of the species is oriented in the north-south direction. In the side-view, the median disc is slightly wider than the proximal disc, and a narrow canal may be visible in some specimens. The discs may only be observed under phase-contrast and bright-field settings (Plate 2, Figs. 20 and 24 of Bowman & Varol, 2021). In plan view, the column and plug artificially resemble overgrown Dictyococcites under polarised light.
Optical properties: In plan-view, the discs appear non-birefringent, whereas the column and plug appear strongly birefringent. The entire species appears birefringent in the side-view. In cross-polarised light, the distal side shows a dextrogyre-type extinction pattern (e.g. Pl. 3, figs. 10, 14 of Bowman & Varol, 2021). In contrast, the proximal side is characterised by a laevogyre-type extinction pattern (e.g. Pl. 3, figs. 11, 15 of Bowman & Varol, 2021). When using the gypsum plate, with the species oriented in the north-south position, the distal view will show the blue regions corresponding to the horizontal axis, while the proximal view will show the blue sectors corresponding to the vertical axis. The terminology of Asinithistimorfi is presented in Fig. 8 of Bowman & Varol, 2021.
The bizarre rough appearance of the plug and the elliptical shape of the species are diagnostic identification features. Asinithistimorfi is distinguished from Caycedoae by the following: Asinithistimorfi is an elliptical form composed of two discs, whereas Caycedoae is circular/broadly elliptical forms consisting of one disc. In both genera, the disc(s) are non-birefringent in plan-view, separating these genera from other helioliths.
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.