Nannotax

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 the distal shield.

Species:

images	species	characteristics
	U. sibogae Pliocene- Recent	circular, medium sized central opening, proximal shield larger than distal
	U. foliosa Pliocene- Recent	circular, small central opening, distal shield large than proximal, proximal shield bicyclic.
	U. jafari Miocene-Pliocene	circular, small with small central opening, shields similar in size
	U. rotula Miocene-Pliocene	circular, with large central opening, shields similar in size
	U. hulburtiana Pliocene- Recent

NB The modern species U. sibogae, U. foliosa and U. hulburtiana are all clearly recognisable at least in Late Pleistocene sediements and they almost certainly evolved from the Miocene species U. jafari and U. rotula. However, the detailed evolutionary history has not been studied.

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.

Description: Ring-shaped coccoliths with open central-area. No clear shield development, spines on distal side. Strongly birefringent.

Remarks:

Range: Palaeogene to NN6.

Synonyms:

• Coronocyclus serratus Hay, Mohler and Wade, 1966

Variant: Elliptical forms, with narrow rims, occur in NN5-6.