Velonalithus rioi
Set number: 256
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10µm
in mobile
Set number: 252
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Set number: 255
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10µm
in mobile mount
Triquetrorhabdulus rioensis Olafsson, 1989
Orthorhabdus rioi (Olafsson, 1989) Young & Bown, 2014
Elongate needle-like nannolith with three birefringent blades having inclined tangential c-axis [perpendicular to the length of the blades]. The width of the blades is somewhat different. The blades joined each other at a different angle.
This form naturally settles with the median blade's keel facing outward, exposing the two blades' face. The faces of the blades have an inclined tangential c-axis. The c-axis appear oblique in the keel segments. The yellow appearance of the keel at -45° and +45° confirms the oblique c-axis in this profile for the medial keel.
Velonalithus rioi differs from Triquetrorhabdulus carinatus Martini (1965) by having a tangential c-axis whilst the latter has an axial c-axis. Moreover, the blades joined each other at a different angle in Velonalithus rioi, whereas the blades' angles are equal in Triquetrorhabdulus carinatus.
Velonalithus rioi is distinguished from Orthorhabdus serratus Bramlette & Wilcoxon (1967) in its plan view by having the biconical shape with a tangential c-axis. The latter has blades truncated at one end with a vertical c-axis.
Orthorhabdus serratus, in its side view, is a hockey-stick shape nannolith displaying high birefringence with a tangential c-axis. In contrast, Velonalithus rioi 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.
Young, J. R. & Bown, P. R., (2014): Some amendments to calcareous nannoplankton taxonomy. - Journal of Nannoplankton Research. 33(1): 39-46.
Holotype
Triquetrorhabdulus rioensis Olafsson, 1989
Middle Miocene
ODP Leg 108, Site 667A, Eastern Tropical Atlantic