Triquetrorhabdulus carinatus

Triquetrorhabdulus carinatus Martini, 1965

Elongate biconical nannolith with three birefringent blades having an axial c-axis [parallel to the length of the blades]. The identical blades joined each other at the same angle.

This form naturally settles with the median blade's keel facing outward, exposing the two blades' faces. The blades are extinct at 0° (parallel extinction) and displayed strong interference colours at 45°.

Triquetrorhabdulus carinatus differs from Velonalithus rioi by having an axial c-axis, whilst the latter has a tangential c-axis. Moreover, the blades joined each other at a different angle in Velonalithus rioi, whereas the blades' angles are equal in Triquetrorhabdulus carinatus.

Triquetrorhabdulus carinatus is distinguished from Triamutilaferrum donti in its plan view by having the biconical shape with an axial c-axis. The latter has blades truncated at one end with a vertical c-axis in plan view.

Orthorhabdus serratus, in its side view, is a hockey-stick shape nannolith displaying high birefringence with a tangential c-axis. In contrast, Triquetrorhabdulus carinatus is a biconical form in every profile.

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.

Martini, E., (1965): Mid-Tertiary calcareous nannoplankton from Pacific deep-sea cores. - Colston Papers. 17: 393-411.

Olafsson, G., (1989): Quantitative calcareous nannofossil biostratigraphy of upper Oligocene to middle Miocene sediment from ODP Hole 667A and middle Miocene sediment from DSDP Site 574. - Proceedings of the Ocean Drilling Program, Scientific Results. 108: 9-22.