S. predistentus group


Classification: Coccolithophores -> Discoasterales -> Sphenolithaceae -> Sphenolithus -> S. predistentus group
Sister taxa: S. capricornutus group, S. dissimilis group, S. moriformis group, S. heteromorphus group, S. predistentus group, S. furcatolithoides group, S. radians group, S. conspicuus group, S. sp.,

Daughter taxa: (blue => in age window 0-800Ma)
main species
Basal cycle relatively high. At 45° position isogyres separated and broadly curved (obtuse angle) 
Basal cycle low (w:h 1-7-2.5). At 45° position isogyres (nearly) meet and are tightly curved (acute angle) 
Basal cycle very low, separated from stem by flat isogyres.
additional species - extinction angle decreasing
Like S. ciperoensis but with flaring base
Like S. distentus but with bulged base and extinction lines slightly separated at 45° position
Like S. ciperoensis but isogyres orthogonal rather than curved.
Like S. predistentus but with very high spine
Small to medium sized sphenolith with low base that meets the spine along horizontal or near horizontal sutures but when at 45° to polarising direction a birefringent structure passes between the basal ‘feet’.
Medium to large sphenolith with a long, robust, tapering apical spine, sometimes bifurcated, and short laterally extending proximal elements forming two pointed feet. In XPL the spine is bright at 45º and weakly birefringent at 0º.
Like Sphenolithus predistentus but the basal part of the sphenolith is considerably broader than the spine, giving an inverted T-shaped outline.
angle independent
Like S. ciperoensis but extinction lines "linear to slightly curving"
Similar to S. ciperoensis but with minute spine and flaring base
Proximal shield only, formed of about ten rays with umbrella-like appearance
Triangular and wider than high
Eocene species
Small, only the spine is present, most likely due to preservation but probably also reflecting the small size and fragility of the base.
Medium to large (height 5-12 µm) with tall spine and low base comprising a single cycle of triangular quadrants. Spine is dark at 0° and brightest at 45°. Most spines appear monocrystalline, but some are duocrystalline; this may be orientation related.

Taxonomy:

Citation: Sphenolithus predistentus group sensu Bown & Dunkley Jones, 2012
Rank: species group

Distinguishing features: Middle Eocene and Oigocene sphenoliths with duocrystalline spines and monocyclic base

Farinacci & Howe catalog pages:


Morphology remarks: A plexus of late Eocene to Oligocene spinose sphenoliths that have been widely used in biostratigraphy. Variability, homeomorphic trends and the occurrence of intergradational forms has hindered their use as marker species and has been the focus of considerable discussion (e.g., Olafsson and Villa, 1992; Blaj et al., 2010). Their morphology is characterised by a base with two, low quadrants (or ‘feet’) that increase in size in younger forms. Spines are tall, duo- or monocrystalline, and taper, with terminal bifurcations that may be very long. Older representatives (S. predistentus, S. akropodus) have duocrystalline spines and low basal ‘feet’ and later forms have relatively larger feet that encroach up the spine, which may appear monocrystalline.
The sphenolith basal-width to proximal-cycle-height ratio was considered a useful diagnostic character by Blaj et al. (2010), but they did not include S. akropodus or S peartiae, which have ratios identical to S. distentus. Generally, S. predistentus has basal width/height (W/H) ratios of >2.5, S. distentus 1.7-2.5 and S. ciperoensis <1.7.

Search data:
TagsLITHS: N: 3D-radiate, conical, RIM: tiered,
CSPH: not known,
MetricsLith size: 3->18µm;
Sources:
The morphological data given here can be used on the advanced search page. See also these notes

Geological Range:
Last occurrence (top): in upper part of Chattian Stage (75% up, 24.3Ma, in Chattian stage). Data source: Total of range of species in this database
First occurrence (base): in upper part of Lutetian Stage (74% up, 42.9Ma, in Lutetian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Blaj, T., Henderiks, J., Young, J. R. & Rehnberg, E. (2010). The Oligocene nannolith Sphenolithus evolutionary lineage: morphometrical insights from the palaeo-equatorial Pacific Ocean. Journal of Micropalaeontology. 29(1): 17-36. gs :: view :: open

Bown, P. R. & Dunkley Jones, T. (2012). Calcareous nannofossils from the Paleogene equatorial Pacific (IODP Expedition 320 Sites U1331-1334). Journal of Nannoplankton Research. 32(2): 3-51. gs :: view :: open

Olafsson, G. & Villa, G. (1992). Reliability of sphenoliths as zonal markers in Oligocene sediments from Atlantic and Indian Oceans. In, Monechi, S. , Proto-Decima, F. & Rio, D. (eds) Proceedings of the International Nannoplankton Association Conference, Firenze, 2006. Memorie di Scienze Geologiche. 43: 261-275. gs :: ::


nannotaxlogo400px

S. predistentus group compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 8-7-2020

Taxon Search:
Advanced Search

Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=1216 Go to Archive.is to create a permanent copy of this page - citation notes



Comments (0)

No comments yet on this page. Please do add comments if you spot any problems, or have information to share

Add Comment

* Required information
1000
Captcha Image
Powered by Commentics