Symmetrikiexafanisi magnifica Varol, 2025
From the Greek Symmetrikiexafanisi, symmetrical extinction – refers to the symmetrical extinction of its apical spine.
Calcareous nannofossils in the Symmetrikiexafanisi genus comprise segments spreading from a common origin, arranged in three cycles: proximal, lateral, and apical. The enlarged twin apical spines can be joined along their entire length (e.g., Symmetrikiexafanisi magnifica) or divergent (e.g., Symmetrikiexafanisi furcatolithoides) from the origin or after varying height of an initially joined section. In well-preserved specimens, a short supplementary segment is present between the joined sections of the apical spines, visible only when the specimen length is at 45° under cross-polarised light (XPL).
Optical Properties: From a side view, apical spines and the segments of the proximal cycle exhibit symmetrical extinction. The lateral cycle displays parallel extinction and length-fast (-) elongation. Under crossed polarised light (XPL), when the specimen is oriented at 45°, the lateral cycle, supplementary segment, and a segment of the proximal cycle create a cross in well-preserved specimens (i.e., Plate 17, Figs. 3-4 in Varol, 2025).
Symmetrikiexafanisi differs from Sphenolithus in the following ways:
• The apical spines and the segments of the proximal cycle (from their side view) exhibit symmetrical extinctions in the side view.
• All the segments of Sphenolithus display parallel extinctions
and length-fast (-) elongations.
Varol, O. 2025. A practical guide to optical studies of calcareous nannofossils. Grzybowski Foundation Special Publication, 29, 230 pp.