Helenea


Classification: Mesozoic -> Podorhabdales -> Cretarhabdaceae -> Helenea
Sister taxa: Cretarhabdus, Cruciellipsis, Flabellites, Grantarhabdus, Helenea, Mattiolia, Miravetesina, Pickelhaube, Retecapsa, Speetonia,

Distinguishing features: Elliptical to subcircular cretarhabdid coccoliths with markedly bicylic rim - outer cycle darker in LM; central area typically narrow with axial or diagonal croos


Daughter taxa: (blue => in age window 0-800Ma)

Broadly elliptical to subcircular Helenea with a narrow central area spanned by an axial cross that bears a distinctly blocky (square) short spine. The long axis of the central area is less than one-half of the coccolith length (Bralower et al., 1989).


Subcircular to broadly elliptical cretarhabdid coccoliths with a central area spanned by a vaulted axial cross that bears a spine. The bars usually display a dark median line under XPL and flare at their ends. The vaulted nature of the central structure makes focusing on the cross bars difficult.
Small broadly elliptical cretarhabdid coccoliths with a central area spanned by diagonal cross bars. The bars may display median black lines under XPL.
Normally to broadly elliptical coccoliths with a narrow central area spanned by an axial cross. The long axis of the central area is equal to or greater than one-half of the coccolith length (Bralower et al., 1989).
Specimens not identified to species level

Taxonomy:

Citation: Helenea Worsley, 1971
Rank: Genus
Type species: Helenea staurolithina Worsley, 1971.
Synonyms: Microstaurus Black, 1971
Notes & discussion: Helenea and Microstaurus were described in the same year by Worsley (1971) and Black (1971), and after recombinations, they include one shared species name for different coccoliths. The genus Microstaurus is regarded here as a junior synonym of Helenea, and Microstaurus quadratus Black, 1971, a junior synonym of Helenea staurolithina. Helenea quadratus Worsley, 1971 is included in the genus here and is distinguished by coccoliths with diagonal cross bars.

Farinacci & Howe catalog pages: Helenea + * , Microstaurus *

Distinguishing features: Elliptical to subcircular cretarhabdid coccoliths with markedly bicylic rim - outer cycle darker in LM; central area typically narrow with axial or diagonal croos


Morphology remarks: Elliptical to subcircular cretarhabdid coccoliths that in LM show a change in birefringence toward the edge of the shield/rim, creating a diffuse bicyclic image; the outer cycle is narrowest. The central areas are typically narrow and spanned by structures dominated by primary cross bars rather than lateral bars or grills, although the former may be present. Both axial and diagonal cross structures occur. The bars often flare toward their ends.

Geological Range:
Last occurrence (top): at top of Cenomanian Stage (100% up, 93.9Ma, in Cenomanian stage). Data source: Total of range of species in this database
First occurrence (base): in mid part of Oxfordian Stage (54% up, 160.1Ma, in Oxfordian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Black, M. (1971a). Coccoliths of the Speeton Clay and Sutterby Marl. Proceedings of the Yorkshire Geological Society. 38: 381-424. gs

Bown, P. R. & Cooper, M. K. E. (1998). Jurassic. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 34-85. gs

Burnett, J. A. (1998). Upper Cretaceous. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 132-199. gs

Lambert, B. (1987). Nannofossiles calcaires de l'Albien supérieur et du Vraconnien du Cameroun méridional. Cahiers de Micropaléontologie. 2(2): 33-60. gs

Worsley, T. R. (1971). Calcareous nannofossil zonation of Upper Jurassic and Lower Cretaceous sediments from the Western Atlantic. In, Farinacci, A. (ed.) Proceedings of the Second Planktonic Conference Roma 1971. Edizioni Tecnoscienza, Rome 2: 1301-1321. gs


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Helenea compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 8-12-2019

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Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10499 Go to Archive.is to create a permanent copy of this page - citation notes



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