Distinguishing features: Parent taxon (Podorhabdales): Placolith (or modified placolith) coccoliths showing little or no imbrication. Typically, V-units form distal shield, R-units form proximal shield. This taxon: 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 (time control age-window is: 0-800Ma)
Granddaughter taxa
Main genera
Biscutum 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).
Similiscutum 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.
Citation: Biscutaceae Black, 1971Rank: Family Taxonomic 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.
Distinguishing features: Parent taxon (Podorhabdales): Placolith (or modified placolith) coccoliths showing little or no imbrication. Typically, V-units form distal shield, R-units form proximal shield. This taxon: 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: 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.
Search data:
Tags
LITHS: placolith,
Metrics
Lith size: 0->0µm;
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): 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:
Range-bar - range as quoted above, pink interval top occurs in, green interval base occurs in.
Triangles indicate an event for which a precise placement has been suggested
Neptune data: this is a higher taxon page so Neptune data is not plotted. For the customisable plot option go to a genus page Parent: Podorhabdales
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. gsVO
Bown, P. R. (1998a). Calcareous nannofossil biostratigraphy. British Micropalaeontological Society Publication Series. 1-328. gsVO
Bukry, D. (1969). Upper Cretaceous coccoliths from Texas and Europe. University of Kansas Paleontological Contributions, Articles. 51 (Protista 2): 1-79. gsVO
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): -. gsVO
Noël, D. (1965b). Sur les Coccolithes du Jurassique Européen et d'Afrique du Nord. Essai de classification des coccolithes fossiles. Éditions du Centre National de la Recherche Scientifique, Paris. 1-209. gs
Biscutaceae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 18-10-2021
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