Nannotax Enabling nannofossil research

Taxa included: Families Syracosphaeraceae, Rhabdosphaeraceae and Calciosoleniaceae, plus the genus incertae sedis Coronosphaera. The families are united here on the basis of V/R/T coccolith structure (see below). The order is also characterised by polymorphism, oceanic ecology, motility of the heterococcolith phase and holococcolith formation in the alternate phase, although none of these are unique to the group or universal within it.

Typical image	Family and main Neogene genera	Typical coccolith type
	Syracosphaeraceae Syracosphaera	Elliptical muroliths
	Calciosoleniaceae Calciosolenia (Scapholithus)	Rhombohdral muroliths
	Rhabdosphaeraceae Rhabdosphaera Algirosphaera Discosphaera	Circular or elliptical planoliths often with spine

Coccolith structure: Coccoliths are unusually complex consisting typically of three components

1. A rim showing normal V/R structure, but with the proto-coccolith ring embedded within the rim;

2. A radial lath cycle with openings between the laths, outer ends interdigitate with rim elements. In some species the laths are composite, formed of two or more elements;

3. An axial structure in the centre of the coccolith. This may be a low mound, flat plaque, elevated ridge or spine and may be formed from the radial lath elements and/or from additional, disjunct, elements.The radial lath cycle is especially distinctive, these elements interdigitate with the rim elements, in the case of S. pulchra the laths have tangential c-axis orientations (Young et al. 2004) and we hypothesise that this is a general pattern. Hence, it appears likely that the proto-coccolith ring consists of three repeating nuclei types (V/R/T), rather than the usual V/R alternation. This distinctive structure makes it likely that this grouping is monophyletic in origin. The lath cycle is absent in some Rhabdosphaeraceae, probably as a result of secondary loss.

Coccolith types: Typically members of this order show more than one coccolith type, the main types which can be developed are:

1. Body coccoliths (BCs), the main coccolith type forming inner layer (endotheca) of coccosphere, always present.

2. Circum-flagellar coccoliths (CFCs); modified coccoliths occurring around the flagellar opening, typically they are similar to the body coccoliths but modified by e.g. presence of a spine or smaller size. Seen on numerous species.

3. Antapical coccoliths (AACs); modified coccoliths occurring at opposite end of the coccosphere to the flagellar opening, typically similar to the body coccoliths but modified by e.g. presence of a spine or smaller size. Seen on limited number of species.

4. Exothecal coccoliths (XCs); modified coccoliths forming outer layer (exotheca) of coccosphere. Usually show the same basic structure as body coccoliths but morphology may be highly modified. Seen on most Syracosphaera species, but often missing on individual specimens.

Life-cycles and culture studies: The only species which have been cultured from this very diverse order are Syracosphaera pulchra (Inouye & Pienaar 1998; Geisen et al. 2002), Algirosphaera robusta (Probert et al. 2007) and Coronosphaera mediterranea (Geisen et al. 2002). Of these, single strains of C. mediterranea and S. pulchra have undergone phase changes, producing holococcoliths of respectively Zygosphaera hellenica and Calyptrosphaera pirus (Geisen et al. 2002, Houdan et al. 2004). Numerous other species of the Syracosphaeraceae and two species of Rhabdosphaeraceae (Acanthoica quattrospina, Algirosphaera robusta) have been observed to form combination coccospheres with holococcoliths (Cros et al. 2000, and refs. therein, Cros & Fortuño 2002, Triantaphyllou & Dimiza 2003). This suggests that heteromorphic life-cycles are a common feature of the order.

Fossil record: Whist this order is highly diverse in the modern nannoflora it is poorly represented in the fossil record. This is probably primarily because the coccoliths produced by it are predominantly small and delicate, so they have low preservation potential and when they are preserved they are difficult to observe or identify especially in the light microscope. In addition many species are very rare. This is discussed further in Young et al. (2005).