Zygodiscus amphipons Bramlette and Martini, 1964
Loxoliths are characterised by a single wall, commonly with a proto-inner wall, and a diagonal cross that may support a distal process. Species are distinguished primarily by the morphology, structural organisation, and optical behaviour of the central cross.
In plan view under cross-polarised light, the single wall and proto inner wall generally remain in constant extinction, although faint whitish-grey interference colours may occasionally occur. The optical behaviour of the central cross varies according to its morphology and structural organisation.
The type species of Helicolithus is characterised by a wall and a proto inner wall and exhibits constant extinction when observed in plan view under crossed polars. Reinhardt & Górka (1967) noted that, in the lectotype, “the rims are indistinct under crossed nicols”, an observation corroborated by published images of the lectotype. On this basis, Helicolithus is therefore a senior synonym of Chiastozygus.
In practice, however, Helicolithus has been applied to forms possessing a double wall, a birefringent inner wall, and a cross structure. In the present study, species with a protolith inner wall composed of eight birefringent segments are assigned to the new genus Aktasia. In contrast, those with a birefringent zeugoid inner wall are retained within Helicolithus.
Species concepts of Helicolithus and Chiastozygus differ considerably among authors, indicating that a comprehensive revision of both genera remains necessary.
Bramlette, M. N. & Martini, E. 1964. The great change in calcareous nannoplankton fossils between the Maastrichtian and Danian. Micropaleontology. 10(2): 291-322.
Gartner, S., 1968. Coccoliths and related calcareous nannofossils from Upper Cretaceous deposits of Texas and Arkansas. University of Kansas Paleontological Contributions, Articles 48 (Protista 1), 1-56.
Reinhardt, P. & Górka, H. 1967. Revision of some Upper Cretaceous Coccoliths from Poland and Germany. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 129(3): 240-256.