Cretarhabdaceae


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

Distinguishing features: Regular placoliths; central-area spanned by elevated distal structure typically with cross bars and central spine.
The distal shield usually weakly bicyclic, with narrow outer-cycle and broad inner-cycle; elements usually radial and non-imbricate. LM image is moderately birefringent. V/R structure unclear.


Daughter taxa: (blue => in age window 0-800Ma) Granddaughter taxa
Mattiolia
Cretarhabdids with a distinctly dark image in XPL. The type species has broadly elliptical shields and a narrow central area filled with a broad plate/axial cross; the plate is also dark under XPL but is crossed by at least four extinction lines.

Wisea


Lateral bar in central area
Speetonia
Cretarhabdid with wide central area spanned by a transverse bar.

Cross in central area
Cruciellipsis
Cretarhabdids with relatively broad shields; central area spanned by a broad axial cross, with strong medial extinction lines.

Flabellites
Cretarhabdids with relatively broad shields; central area spanned by blocky diagonal cross; outline may be asymmetric.

Grantarhabdus
Cretarhabdids with relatively narrow shields; central area wide with diagonal cross bars.

Helenea
Elliptical to subcircular cretarhabdids with bicylic rim - outer cycle darker in LM; central area typically narrow with axial or diagonal croos

Cross and lateral bars/grill in central area
Cretarhabdus
Cretarhabdids with central area net that often includes axial cross bars.

Retecapsa
Cretarhabdids with central area spanned by an axial cross with lateral bars.

Pickelhaube
Cretarhabdids with large, wide, strongly concavo-convex ?proximal shield, narrow ?distal shield and central-area spanned by an axial cross and lateral bars.

Miravetesina
Cretarhabdids with central area spanned by narrow axial cross and fine net.

Taxonomy:

Citation: Cretarhabdaceae Thierstein, 1973
Rank: Family
Type species: Cretarhabdus Bramlette and Martini, 1964.

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

Distinguishing features: Regular placoliths; central-area spanned by elevated distal structure typically with cross bars and central spine.
The distal shield usually weakly bicyclic, with narrow outer-cycle and broad inner-cycle; elements usually radial and non-imbricate. LM image is moderately birefringent. V/R structure unclear.


Morphology: Placoliths with two shields and a central-area spanned by a variety of structures, most commonly, fibrous axial cross-bars with subsidiary lateral bars and a solid central spine or process.
In SEM the distal shield typically appears bicyclic, with a narrow outer-cycle and a dominant, broad inner-cycle, overgrown specimens show however that this is a scissor structure with both elements formed from a single crystal unit with the outer element offset clockwise from the inner element. The proximal shield is unicycle and appears to formed of the same crystal units as the distal shield. The elements are usually radial or near radial and do not appear to imbricate. (our obs.)
In LM the coccolith is moderately birefringent, usually with diagonal isogyres.

The coccolith structure has not been fully worked out but the proimal and distal shield seem to be formed form  a single cycle of crystal units which show moderate birefringence and in plan view oblique isogyres - so they probably have an orientiton which is closest to radial but with significant rotation from truly radial.
In SEM the distal shield often shows a separate cycle around the outer edge but individual elements are often traceable from the main to the outer cycle so this is probably the results of suture kinking/ a scissor-structure rather than a discrete cycle of crystal units. In lm typically the whole coccolith shows a rather uniform moderate brightness, the outer part may be darker but this appears to be due to reduction of thickness at the outer edge, and is seen most obviously in species where the proxmial shield is smaller than the distal shield (e.g. Helenea chiastia). In proximal view the proximal shield is formed of a single cycle of large elements with dextrogyre curvature/dextral obliquity. This contrasts with the proximal side of the distal shield which shows sinistral obliquity. In side views the proximal and distal shields can often be seen to be formed from the same crystal units. The central structures are formed from a mass of small elements. [my interpretation - JRY 2020].

Search data:
TagsLITHS: placolith,
MetricsLith 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 Maastrichtian Stage (100% up, 66Ma, in Maastrichtian stage). Data source: Total of range of species in this database
First occurrence (base): at top of Toarcian Stage (95% up, 174.6Ma, in Toarcian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

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

Burnett, J. A. (1998). Upper Cretaceous. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 132-199. gs V O

Grün, W. & Allemann, F. (1975). The Lower Cretaceous of Caravaca (Spain): Berriasian Calcareous Nannoplankton of the Miravetes Section (Subbetic Zone, Prov. of Murcia). Eclogae Geologicae Helvetiae. 68: 147-211. gs V O

Thierstein, H. R. (1973). Lower cretaceous calcareous nannoplankton biostratigraphy. Abhandlungen der Geologischen Bundesanstalt. 29: 3-52. gs V O


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Cretarhabdaceae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 26-11-2020

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Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10298 Go to Archive.is to create a permanent copy of this page - citation notes



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