Rhagodiscus


Classification: Mesozoic -> Eiffellithales -> Rhagodiscaceae -> Rhagodiscus
Sister taxa: Calcicalathina, Percivalia, Rhagodiscus, Zebrashapka,

Distinguishing features: Loxolith coccoliths with low rim and central-area filled by a granular plate which may be perforate and spine-bearing. Rim image is typically unicyclic in XPL.


Daughter taxa: (blue => in age window 0-800Ma)
Small (<5µm) Rhagodiscus with relatively narrow central area spanned by a granular plate bearing large spine/spine base. The rim image is distinctly dark in XPL.
Medium-sized unicyclic loxolith coccoliths with a wide central area spanned by a butterfly-shaped bar with raised, birefringent edges.
Large (>8.0 µm) loxolith coccoliths with a wide central area (usually greater than twice as wide as the rim) that is spanned by a granular plate. A relatively large spine or spine base is typically present. The rim and plate usually show yellow interference colors in XPL.
Elongate, narrow Rhagodiscus with parallel or slightly concave sides and a spine/spine base that spans the central area.
Murolith (loxolith) coccoliths with a relatively broad central area spanned by a granular plate that normally bears a hollow spine or spine base; the rim appears unicyclic in LM.
Small (~4µm) loxolith coccoliths with granular basal plate and 'cross-bars' supporting a large, flaring spine with serrated edges. In LM, this species may be virtually indistinguishable from small specimens of R. asper.
Small Rhagodiscus with a birefringent spine/spine base that almost fills the central area.

Small (<5µm), elliptical Rhagodiscus with a length/width ratio of <2.5, straight or slightly convex longer sides, and a central area filled with a granular plate which supports a hollow spine base.
A species of Rhagodiscus with construction and light microscopic image similar to that of R. achlyostaurion, but which is larger (length c. 6.5-8.5 µm, width c. 5.0-6.5 µm), with a relatively narrower rim and broad central area, with no spine or a small spine base.
Medium to large elliptical rhagodiscid-like coccolith with moderately broad rim and narrow central area spanned by a spine (base). Rim is unicyclic in XPL and overall the coccolith exhibits low birefringence.
Medium to large Rhagodiscus with a wide central area spanned by a granular plate pierced by two longitudinally-aligned large holes and a central hollow spine. The plate extends completely around the central area with distinct circular pores surrounded by raised, birefringent edges.
Rhagodiscus with a granular central plate in the form of an axial cross.
Small (holotype 4 µm), with small slender spine with bright tip

Sub-rectangular murolith coccoliths with parallel sides, rounded ends and a narrow central area spanned by a large hollow spine base. The rim may be irregularly thickened. Often seen in side view.
Reniform Rhagodiscus with an irregularly thickened rim.
Large (>8.0 µm) loxolith coccoliths with a broad rim (usually as broad or broader than the central area) and a central area spanned by a granular plate; a relatively large spine or spine base is usually present. The broad and blocky rim shows yellow interference colors under XPL.
Loxolith coccoliths with a relatively broad central area spanned by a granular plate that incorporates a transverse, raised, bridge-like structure.
Large Rhagodiscus coccoliths with a very broad spine-base that is distinctly striated in LM XPL.
Specimens not identified to species level

Taxonomy:

Citation: Rhagodiscus Reinhardt, 1967
Rank: Genus
Type species: Rhagodiscus asper (Stradner, 1963) Reinhardt, 1967.
Synonyms:

Farinacci & Howe catalog pages: Rhagodiscus * , Alfordia * , Viminites * , Mitosia + * , Rhabdolithina *

Distinguishing features: Loxolith coccoliths with low rim and central-area filled by a granular plate which may be perforate and spine-bearing. Rim image is typically unicyclic in XPL.

Geological Range:
Last occurrence (top): at top of Maastrichtian Stage (100% up, 66Ma, in Maastrichtian stage). Data source: Total of range of species in this database
First occurrence (base): near top of Tithonian Stage (87% up, 145.9Ma, in Tithonian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Black, M. (1975). British Lower Cretaceous Coccoliths. I-Gault Clay (Part 3). Palaeontographical Society Monograph. 128: 113-142. gs

Bown, P. R., Rutledge, D. C., Crux, J. A. & Gallagher, L. T. (1998). Early Cretaceous. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 86-131. gs

Reinhardt, P. (1967). Fossile Coccolithen mit rhagoidem Zentralfeld (Fam. Ahmuellerellaceae, Subord. Coccolithineae). Neues Jahrbuch für Geologie und Paläontologie Monatshefte. 1967: 163-178. 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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Rhagodiscus compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 18-10-2019

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