Discolithus trabeculatus Górka, 1957
Eiffellithus trabeculatus (Górka, 1957) Reinhardt & Górka,1967; lectotype pp. 20-251, pl. 31, figs. 19, 23.
Loxolith has a double wall consisting of a zeugoid outer wall and a protolith inner wall composed of eight large sub-quadrilateral segments. The central area contains a cross whose arms are often composed of parallel longitudinal segments arranged in two halves and lacking a distal process.
For the two loxolith species Discolithus trabeculatus and Discolithus anceps described by Górka (1957), lectotypes were designated by Reinhardt & Górka (1967). However, it is unclear whether these lectotypes represent rephotographs of the original holotypes or different specimens from the original material. Reinhardt & Górka (1967) referred to the specimens as “lectotype” in the text but labelled them as “holotype” in the plate captions. Substantial morphological differences exist between the holotypes illustrated by Górka (1957) and the lectotype/holotype figures published by Reinhardt & Górka (1967). According to Reinhardt & Górka (1967), holotype coordinates for all species were recorded by Górka using a mechanical stage.
Discolithus anceps was designated as the type species of Helicolithus by Noël (1965). The lectotype description explicitly states that “the rims are indistinct between crossed nicols,” indicating that both walls are in constant extinction under cross-polarised light. This interpretation is further supported by the inclusion of Zygodiscus? amphipons and Discolithus fessus in synonymy by Reinhardt & Górka (1967). In addition, the presence of a rectangular pore at the centre of the diagonal cross indicates the base of a distal process. In this study, loxoliths characterised by a double wall, which are in constant extinction and have a distal process, are assigned to Helicolithus.
The lectotype/holotype of Discolithus trabeculatus, illustrated by Reinhardt & Górka (1967), also exhibits a double wall; however, only the outer wall shows constant extinction, whereas the inner wall displays white interference colours. No distal process is present. Subsequent studies (e.g., Varol & Girgis, 1994) demonstrate that the inner wall consists of eight protolith segments that display birefringence.
In this work, loxoliths characterised by an outer wall in constant extinction and an inner wall composed of eight birefringent segments, without a distal process, are assigned to the new genus Aktasia.
The zeugoid outer wall is in constant extinction, whereas the protolith inner wall and central cross exhibit white interference colours.
Górka, H. 1957. Les Coccolithophoridés du Maestrichtien supérieur de Pologne. Acta Palaeontologica Polonica. 2: 239-284
Noël, D. 1965. Sur les Coccolithes du Jurassique Européen et d'Afrique du Nord. Essai de classification des coccolithes fossiles. Éditions du Centre National de la Recherche Scientifique, Paris. 1-129.
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.
Varol, O. & Girgis, M. 1994. New taxa and taxonomy of some Jurassic to Cretaceous calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen. 192: 221-253.