Biscutum


Classification: Mesozoic -> Podorhabdales -> Biscutaceae -> Biscutum
Sister taxa: Biscutum, Boletuvelum, Chiastella, Crucibiscutum, Discorhabdus, Gaarderella, Gephyrobiscutum, Helicorhabdus, Jimenezberrocosoia, Seribiscutum, Similiscutum, Sollasites,

Distinguishing features: Elliptical with broad shields with or without a prominent inner tube cycle; the central-area may be imperforate or narrow and vacant or spanned by a simple structure (plate, cross or bar).


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


Asymmetric species, with strongly asymmetric proximal shield and weakly asymmetric distal shield.



Minute to medium biscutaceans without any discernible (LM) structure inside their birefringent central tube cycle. These forms show considerable variation in size and in the relative width of the central tube. The smallest forms (c.1.5µm) are barely discernible in the LM.



Biscutum with a narrow central area spanned by axial cross bars; the longitudinal bar is particularly broad and tapers towards the the ends of the central area. Bicyclic LM image in XPL.

Central area spanned by a granular cross with hollow central knob

Normally elliptical Biscutum with relatively narrow shields and open central area spanned by cross bars slightly rotated from axial. Unicyclic image in XPL.



Circular to subcircular bicyclic Biscutum with a narrow central area and tall spine. Bicyclic image in XPL.

Small, normally elliptical Biscutum with narrow central area. Bicyclic image in XPL.

Used in similar sense to Biscutum constans (Górka, 1957) Black in Black and Barnes, 1959





Large, normally elliptical Biscutum with relatively broad bright inner cycle and central area spanned by a transverse bar.



A broadly elliptical Biscutum with a narrow central area spanned by a broad, hollow-spine-bearing, oblique transverse bar. Weakly bicyclic image in XPL.



Medium to large, broadly elliptical Biscutum with a wide dark shield and a small, highly birefringent closed central area







Small Biscutum with a small central area lined with blocks and two central-area perforations separated by a transverse bar aligned with the short ellipse axis.







Specimens not identified to species level

Taxonomy:

Citation: Biscutum Black in Black & Barnes, 1959
Rank: Genus
Type species: Biscutum testudinariumBlack in Black & Barnes, 1959
Notes & discussion: See remarks for family Biscutaceae.

Farinacci & Howe catalog pages: Biscutum + + + * , ,

Distinguishing features: Elliptical with broad shields with or without a prominent inner tube cycle; the central-area may be imperforate or narrow and vacant or spanned by a simple structure (plate, cross or bar).


Morphology remarks: The distal shield is formed from V-units and the proximal shield and tube cycle are formed from R-units; the distal shield elements are joined along radial or near-radial sutures (+/- 'kinks') and show little or no imbrication. The LM image is generally dark, but bright inner-cycles are common.

Geological Range:
Last occurrence (top): at top of Thanetian Stage (100% up, 56Ma, in Thanetian stage). Data source: Total of range of species in this database
First occurrence (base): at base of Pliensbachian Stage (0% up, 190.8Ma, in Pliensbachian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Black, M. & Barnes, B. (1959). The structure of Coccoliths from the English Chalk. Geological Magazine. 96: 321-328. 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


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Biscutum compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 22-7-2019

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Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10066 Go to Archive.is to create a permanent copy of this page - citation notes



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