Sphenolithales
Calcareous nannofossils are characterised by a concave base and numerous segments spreading from a common origin. These segments produce hexagonal or rhombohedral patterns arranged in two (i.e., Furcatolithus) or three (i.e., Sphenolithus) cycles. The apical spines can be a single segment (i.e., Sphenolithus heteromorphus), double segments (i.e., Symmetrikiexafanisi furcatolithoides, Furcatolithus ciperoensis and Furcatolithus? obtusus) and multi-segments (i.e., Sphenolithus radians).
Optical Properties: From a side view, the apical cycles exhibit parallel (straight) extinction as in Sphenolithus heteromorphus and Sphenolithus didymikoryfi, and inclined (oblique) extinction as in Furcatolithus? obtusus or symmetrical extinction, as in Symmetrikiexafanisi furcatolithoides.
Varol, O. 2025a. A practical guide to optical studies of calcareous nannofossils. Grzybowski Foundation Special Publication. 29: 1-222
