Nannotax Enabling nannofossil research

Description: Coccoliths circular or elliptical, with Calcidiscus structure but central area open, distal shield elements usually show complex kinked sutures. Proximal shield may be monocyclic (e.g. U. sibogae, U. rotula) or bicyclic (e.g. U. foliosa, U. hulburtiana) and may be larger or smaller than distal shield.

Remarks: The separation of Calcidiscus and Umbilicosphaera is essentially based on width of central opening, which is not a very satisfactory criterion, but so far molecular genetic data has supported separation of the extant species assigned to these genera (Saez et al. 2003). The larger size of the central opening also means that the proto-coccolith ring is relatively wide in Umbilicosphaera and so that the numbers of elements for a given coccolith size is higher in Umbilicosphaera and so they are narrower and less clearly visible in LM. In LM the individual elements are usually clearly visible in Calcidiscus species, especially in phase contrast illumination, whilst they are obscure in Umbilicosphaera. Umbilicosphaera species usually also show complex kinking of sutures in the distal shield, but this can occur in Calcidiscus (more complex in Umbilicosphaera).

Type species: U. mirabilis Lohmann 1902 (j. syn. of U. sibogae).

Synonym: Geminilithella Backman, 1980 - the type species U. rotula falls well within the range of variation shown by modern Umbilicosphaera species.