Sphenolithus avis Aljahdali et al., 2015
Medium size [4.0-8.0µm] species of Furcatolithus Martini, 1965 has a diminutive duolithic apical spine with the axial c-axis. The duolithic apical spine is a kite shape in the axial section and has an acute proximal angle. The proximal cycle is a semi-open umbrella shape with a convex periphery. The other side of the proximal cycle is better seen at 45° and display an axial c-axis.
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].
Sphenolithus avis differs from Furcatolithus ciperoensis (Bramlette & Wilcoxon, 1967) Howe, 2021 by having a semi-open umbrella shape proximal cycle and a diminutive non-bifurcated apical spine. The latter has a long, often bifurcating apical spine and relatively low cylindrical proximal cycle.
Aljahdali, M., Wise, S. W., Bergen, J. A., Pospichal, J. J., 2015. A new biostratigraphically significant Late Oligocene Sphenolithus species from the equatorial region. Micropaleontology 61(3), 193-197.
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.
Aljahdali et al., 2015
ODP Leg, 102, Hole 1237B, Southeast Pacific Ocean;