Biscutaceae


Classification: Mesozoic -> Podorhabdales -> Biscutaceae
Sister taxa: Axopodorhabdaceae, Biscutaceae, Calyculaceae, Cretarhabdaceae, Mazaganellaceae, Prediscosphaeraceae, Tubodiscaceae,

Distinguishing features: Regular placoliths; rim broad, central area relatively narrow, with variable central structures.
The distal shield is formed from V-units and the proximal shield and tube cycle are formed from R-units.


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

Asymmetric base, with large, hollow, flaring spines, closed distally by a domed covering

Elliptical, minute with diagonal cross suporting a hollow spine

Elliptical with prominent, birefringent, axial cross.

Circular, shields broad, usually without a prominent inner tube cycle; central-area typically narrow, may contain a spine.

Elliptical, large with broad shields; central area wide, spanned by a granular plate.

Elliptical with bicyclic rim and narrow central-area spanned by oblique transverse bar.

Circular with proximal shield wider than distal; spine with helical structure

Elliptical with slits between shield elements and axial cross

Elliptical with central-area spanned by broad platelets.

Subcircular-elliptical with smooth, grey appearance in XPL and a narrow, bright inner-cycle; central-area narrow and may be empty or spanned by an axial cross.

Elliptical with relatively wide central-area spanned by complex structure

Taxonomy:

Citation: Biscutaceae Black, 1971
Rank: Family
Notes & discussion: The taxonomic significance of conspicuous tube cycles has led to a variety of classification schemes within this family. In a study of the oldest (Early Jurassic) biscutaceans, de Kaenel & Bergen (1993) proposed considerable revision to this family, distinguishing Palaeopontosphaera(arguably a junior synonym of Biscutum) from Biscutum by the presence of a birefringent, distal inner-cycle. The type species holotype (EM) of Biscutum is a proximal view and thus the presence or absence of a tube cycle cannot be determined. Moreover, they argue that the aforementioned holotype is very nearly circular and emend the genus in such a way as to render it equivalent to Bidiscus Bukry, 1969 (?=Discorhabdus Noël, 1965) (i.e., circular to subcircular, unicyclic placoliths). The interpretation of these problematic holotype images is questionable. Here we follow Bown & Young (1997) and Bown (1998) retain the traditional usage of Biscutum, i.e. subcircular to elliptical biscutacean coccoliths which typically have a distal, inner tube cycle.

Farinacci & Howe catalog pages: Biscutaceae [no catalog entry yet]

Distinguishing features: Regular placoliths; rim broad, central area relatively narrow, with variable central structures.
The distal shield is formed from V-units and the proximal shield and tube cycle are formed from R-units.


Morphology remarks: Placoliths with two broad shields that may or may not include a distal inner-cycle (tube cycle). The relatively narrow central areas are vacant, filled with granular calcite or spanned by variably oriented bars or axial cross-bars. 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. Coccospheres are well known and usually spherical or cylindrical.

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): in mid part of Sinemurian Stage (47% up, 195.3Ma, in Sinemurian 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

Bown, P. R. & Young, J. R. (1997). Mesozoic calcareous nannoplankton classification. Journal of Nannoplankton Research. 19(1): 21-36. gs

Bown, P. R. (1998a). Calcareous nannofossil biostratigraphy. British Micropalaeontological Society Publication Series. 328-. gs

Bukry, D. (1969). Upper Cretaceous coccoliths from Texas and Europe. The University of Kansas Paleontological Contributions. Article 51 (Protista 2): 1-79. gs

de Kaenel, E. & Bergen, J. A. (1993). New Early and Middle Jurassic coccolith taxa and biostratigraphy from the eastern proto-Atlantic (Morocco, Portugal and DSDP Site 547B). Eclogae Geologicae Helvetiae. 86(861-907): -. gs

Noël, D. (1965). Sur les Coccolithes du Jurassique Européen et d'Afrique du Nord. Éditions du Centre National de la Recherche Scientifique, Paris. -. gs


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Biscutaceae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 6-12-2019

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