Description: Sphenoliths are more or less beehive-shaped nannoliths with a concave base, consisting of a mass of elements radiating from a common origin.

Neogene groups

	Sphenolithus abies group - species without prominent apical spines	S. abies S. moriformis S. conicus S. dissimilis S. disbelemnos
	Sphenolithus heteromorphosus group - species with prominent, apical spines formed from a single element.	S. heteromorphosus (NN4-5) S. belemnos (NN3) S. delphix (NN1) S. calyculus (NN1) S. capricornutus (NN1)

NB Species in the S. abies group are poorly defined and of very limited biostratigraphic value, by contrast S. heteromorphosus group species form invaluable marker species. 

Sphenolith structure: The individual elements of sphenoliths are elongated parallel to their c-axis (this is clear from light microscopy), and consist of three lath-like blades arranged in a Y-shaped form. This morphology is visible in SEMs of specimens from samples with very good preservation, or slight etching. With overgrowth the spaces between the segments become infilled and the elements develop a spinose form. The triple lath form is probably related to the trigonal symmetry of calcite, with laths developing parallel to the x-axes.

The elements radiate from a single origin, which gives the sphenoliths a compact form, and a clear extinction-cross, in polarized light. The proximal part of all sphenolith species is composed of a single cycle of 8 to 16 of these elements. The axes of the proximal elements slope down from the median plane. A concave base is formed by laths from adjacent elements meeting.

The upper half of sphenoliths is normally formed of two or three cycles of elements, radiating from the centre, at decreasing angles to the vertical. The details of the structure are, however, variable; particularly toward the apex, which gives rise to a large amount of species level variation.

Synonyms: Furcatolithus Martini, 1965; Sphenaster Wilcoxon, 1970.