Sphenolithus arthurii Bown, 2005b
This large [>7.0µm] species is an Early Eocene variation of Sphenolithus conicus Bukry, 1971b with a larger and more distinct apical cycle.
It has a biconical clustered apical spine and large lateral and proximal cycles with a proximal cavity.
In the cross-polarised light, the apical cycle shows (parallel extinction) maximum birefringent at 45° but extinct (dim) at 0° [axial suture is visible at this orientation].
The large proximal and lateral cycles are always birefringent in polarised light.
Sphenolithus arthurii is distinguished from Sphenolithus conicus Bukry, 1971b by having a larger and more distinct apical cycle. Moreover, these two species do not co-occur and their stratigraphic ranges are widely separated.
Sphenolithus arthurii differs from Sphenolithus grandis Haq & Berggren, 1978 and Sphenolithus moriformis (Brönnimann & Stradner, 1960) Bramlette & Wilcoxon, 1967 by having a biconical clustered apical cycle whereas later species have a bouquet of flower shape clustered apical cycle. It is further distinguished from Sphenolithus moriformis by being large.
Bown, P. R., 2005b. Cenozoic calcareous nannofossil biostratigraphy, ODP Leg 198 Site 1208 (Shatsky Rise, northwest Pacific Ocean). Proceedings of the Ocean Drilling Program, Scientific Results 198, 1-44.
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.
Brönnimann, P., Stradner, H., 1960. Die Foraminiferen- und Discoasteriden-zonen von Kuba und ihre interkontinentale Korrelation. Erdoel-Zeitschrift 76(10), 364-369.
Bukry, D., 1971b. Cenozoic calcareous nannofossils from the Pacific Ocean. - San Diego Society of Natural History Transactions 16, 303-327.
Haq, B. U., Berggren, W. A., 1978. Late Neogene calcareous plankton biochronology of the Rio Grande Rise (South Atlantic Ocean). Journal of Paleontology 52, 1167-1194.
ODP Leg 198, Site 1208, Shatsky Rise, northwest Pacific Ocean