Biscutum


Classification: Mesozoic -> Podorhabdales -> Biscutaceae -> Biscutum
Sister taxa: Biscutum, Discorhabdus, Similiscutum, Boletuvelum, Chiastella, Crucibiscutum, Gaarderella, Gephyrobiscutum, Helicorhabdus, Jimenezberrocosoia, Seribiscutum, 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
long ranging generalised species
Small to medium (ca 3-8 µm), elliptical; central area narrow with discrete bright tube cycle and dark core.

Similar to B. constans.

Late Cretaceous - elliptical
Large (>8µm) Biscutum with moderately narrow central area with clear non-birefringent axial structure and well-developed, bright, inner tube cycle. 

Medium (4-6 µm), Central area spanned by a granular cross with hollow central knob

Large (8-10 µm) with vacant central area

Medium to large (7-10 µm), broadly elliptical, with a wide dark shield and a small, highly birefringent closed central area

Late Cretaceous - asymmetric
Small (3-4 µm), asymmetric rim; central area wide, largely filled by 4 plates.

Small (3-4 µm), asymmetric species, with strongly asymmetric proximal shield and weakly asymmetric distal shield. Central area closed by small plates

Medium (5-7 µm), weakly asymmetric with very reduced proximal shield

Large (8-11 µm), irregular formed/asymmetric, central area very small, closed, proximal shield reduced.

Medium sized (5-6 µm), weakly asymmetric, inner tube cycle forms protruding
collar.

Late Cretaceous - elongate
Medium sized (5-6 µm), lenticular with a wide central area, floored by a plate with a central spine. 

Small (3-5 µm), lenticular central area well-developed, almost closed.

Small (4-5 µm), parallel-sided, closed central area.

Early Cretaceous species
Small (4-5 µm), with well-developed central area with birefringent axial structure.

Small (3-5 µm), with a small central area lined with blocks and two central-area perforations separated by a birefringent transverse bar aligned with the short ellipse axis.

Jurassic species
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.

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.



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.





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.

Search data:
TagsLITHS: placolith, asymmetric, elliptical, CA: cross-axial, process, bar, closed, V-UNITS: V-prominent, R-UNITS: R-prominent, DETAILS: ca_disjunct, rim-bicyclic,
MetricsLith size: 3->11µm;
Sources: size range of included species
The morphological data given here can be used on the advanced search page. See also these notes

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(5): 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: 9-7-2020

Taxon Search:
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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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