Nannotax3 - ntax_cenozoic - Sphenolithus preasii

Citation: Sphenolithus preasii Bergen & de Kaenel, in Bergen et al. 2017

Synonyms:

S. milanetti (elongate) - informal name in the BP Gulf of Mexico lexicon [Bergen et al. 2017]

Distinguishing features: Transitional to S. milanetti - base well-eveloped, convex lower surface, shouldered profile.

Morphology: Medium to large, conical sphenolith. The monocrystalline apical spine is moderate to long (~ ½ to ¾ the total length of the specimen), has a pointed to rounded apex, and becomes extinct when the specimen is aligned with the polarizing direction. In this orientation, the four basal quadrants each appear equant and the height of the upper quadrants is less than the height of the lower quadrants (0.67–0.76 ratio). In lateral profile, the proximal surface is concave and arcuate. The basal lateral periphery appears “bulged”, giving the impression of shoulders when contrasted to the width of the spine at its juncture with the base (most evident at 45° in XPL).

Sphenolithus preasii is somewhat transitional between S. conicus and S. milanetti. It differs from S. conicus by its “bulged” periphery, arcuate proximal surface, and lower upper quadrants

Sphenolithus milanetti also has a “bulged” base, but its base is hemispherical and constructed of much larger elements. The basal upper quadrants (lateral cycle) of S. milanetti is much more reduced than in S. preasii (both height and width) relative to the size of their lower quadrants (proximal cycle).
S. heteromorphus is also similar butshows uniform taper rather than a shouldered profile, has a linearly indented proximal surface, and the height of its upper and lower quadrants are equal. The lower quadrant elements of S. heteromorphus appear broad (width > height), whereas those of S. preasii are equant.

Size: L = 5.6–10.0 µm; W = 4.0–6.4 µm (holotype: 9.8mx6.4 µm). [Bergen et al. 2017]

Geological Range:
Last occurrence (top): within NN4 zone (14.91-17.95Ma, top in Langhian stage). Data source: Bergen et al 2017
First occurrence (base): within NN3 zone (17.95-19.00Ma, base in Burdigalian stage). Data source: Bergen et al 2017

References:

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. gs V O

Tags	LITHS: nannolith-radiate, circular, cylindrical, CA: process, CROSS-POLARS: R-prominent,
Metrics	Lith size: 5->10µm; Data source notes: Bergen et al 2017

Sphenolithus preasii

References:

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