Nannotax

Coccospheres: Motile or non-motile, typically with spine-bearing and non-spine-bearing coccoliths with similar shields, but polymorphic, varimorphic and monomorphic genera also occur. The spine-bearing coccoliths may be confined to the poles or distributed around the coccosphere, greatly increasing its outer diameter.

Coccoliths: Typically disc-shaped (planoliths) and formed of three components.

1. Rim: narrow; slightly elevated; formed of two cycles of elements. A. Upper/outer rim cycle of simple non-imbricate elements (Kleijne 1992), these are V-units (our LM obs.). B. Lower/inner rim cycle showing strong obliquity. Crystallographic orientation uncertain.

2. Radial cycle - joins rim to central lamellar cycle; radial laths, of equal number to rim units; slits often present between the laths (absent in Rhabdosphaera and Saturnulus).

3. Lamellar cycle(s) - lamellar elements showing clockwise imbrication, often multiple cycles with inner cycles more elongate, inclined and in helical arrangement forming spine or protrusion. May end in a “cuneate cycle” of a few well-formed elements.

References: Extant species are reviewed by Norris (1984), Kleijne (1992), Aubry (1999), Cros & Fortuño (2002), Young et al. (2003).

Fossil record: Most extant Rhabdosphaeraceae are very small and, apart from R. clavigera, are rarely recorded in the fossil record. There is, however, a diverse and abundant group of larger Rhabdosphaeraceae in the Eocene (see Perch-Nielsen 1985b; Varol 1989; Shafik 1989; Aubry 1999, Bown 2005).