Furcatolithus humerus Varol, in prep.
Species of Furcatolithus (Martini (1965) have a duolithic biconical apical spine with a kite shape in the axial section and an open umbrella shape proximal cycle with the convex periphery.
From Latin humerus, shoulder - referring to its shoulder-like appearance of the proximal cycle.
Furcatolithus humerus has a duolithic biconical apical spine with or without short bifurcation distally and a moderately high open umbrella shape proximal cycle with the convex periphery. The apical spine is a kite shape in the axial section with an obtuse proximal angle.
In cross-polarised light, the apical spine shows maximum birefringence at 45° but extinct (dim) at 0° [the axial suture between the two helves is better seen at this orientation].
Furcatolithus humerus differs from Furcatolithus distentus Martini, 1965 by having an open umbrella shape proximal cycle, whereas the latter has a cylindrical proximal cycle.
Furcatolithus humerus and Furcatolithus bulbulus (Bergen & de Kaenel in Bergen et al., 2017) Howe, 2021 have open umbrella shape proximal cycles. They differ by the proximal angle of the apical spine. In the axial section, the Furcatolithus bulbulus have an acute angle at the proximal end. In contrast, the Furcatolithus humerus has an obtuse angle at the proximal end of the apical spine.
Bergen, J. A., de Kaenel, E., Blair, S. A., Boesiger, T. M., Browning, E., 2017. Oligocene-Pliocene taxonomy and stratigraphy of the genus Sphenolithus in the circum North Atlantic Basin: Gulf of Mexico and ODP Leg 154. Journal of Nannoplankton Research 37(2-3), 77-112.
Bramlette, M. N., Wilcoxon, J. A., 1967. Middle Tertiary calcareous nannoplankton of the Cipero section, Trinidad, W.I. Tulane Studies in Geology and Paleontology 5, 93-131.
Howe, R., 2021. Ultrastructure and taxonomy of the family Sphenolithaceae. Journal of Nannoplankton Research 39(1), 29-75.
Martini, E., 1965. Mid-Tertiary calcareous nannoplankton from Pacific deep-sea cores. Colston Papers 17, 393-411.
Varol, in prep.
DSDP Leg 25, Site 242, Mozambique Channel (Davie Ridge), Indian Ocean