Chiastozygaceae


Classification: Mesozoic -> Eiffellithales -> Chiastozygaceae
Sister taxa: Chiastozygaceae, Eiffellithaceae, Rhagodiscaceae,

Distinguishing features: Murolith coccoliths with rim formed of outer cycle of imbricate V-units and variably developed inner cycle of R-units. Disjunct central bar or cross, usually supporting a spine.


Daughter taxa: : Genera arranged by main structure in central area, then alphabetically (blue => in age window 0-800Ma) Granddaughter taxa
Transverse bar in central area
Muroliths with bicyclic rim and tranverse bar formed from laths which continue around the inner edge of the rim, delineating the two circular perforations.

Bicyclic (LM) murolith with a birefringent, spine-bearing bar. Distal/outer-cycle displays anticlockwise imbrication, unlike all other Mesozoic loxoliths. Triassic genus.

Bicyclic (LM) muroliths with broad, bright inner-cycle and robust, birefringent, transverse bar.

Murolith with distal/outer-cycle that shows little or no imbrication and central area spanned by transverse bars; rim exhibits spiral interference pattern.

Unicyclic muroliths with very broad rim and narrow central-area spanned or filled by a tranverse bar.

Murolith (loxolith) coccoliths with central area spanned by complex transverse structures principally constructed from two or four projections. Delicate lateral bars may also be present but are generally only seen in exceptional preservation.

Murolith (loxolith) coccoliths with central area spanned by variably constructed transverse bar. The bar is usually spine bearing. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic. Delicate lateral bars may also be present but are generally only seen in exceptional preservation.

Diagonal cross in central area
Murolith (loxolith) coccoliths with relatively narrow rim and wide central area spanned by diagonal bars. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic.

Simple axial cross in central area
Loxolith muroliths with relatively narrow rim and wide central area spanned by axial cross bars. The cross is usually spine bearing. Inner/proximal rim cycle is variably developed and so the LM image may be unicyclic or bicyclic.

Square-shaped loxolith cococoliths with cross bars that run parallel to the coccolith sides.

Spinose forms with minute basal loxolith coccolith with axial cross bars. The spines are long, parallel sided (?or tapering) and cruciform in section.



Complex axial cross in central area - &/or cross fills central area
Muroliths with complex axial cross structure or plate, typically incorporating 8 near-axial or radial bars or suture lines.

Loxolith coccoliths with narrow unicyclic rim and central area spanned by a plate incorporating broad, fibrous, tapering axial-cross-bars. Dark overall image in XPL.

Elliptical loxoliths with a rim that appears tricyclic, composed of inner and outer rim cycles and a proximal plate. The outer rim cycle and proximal plate are only weakly birefringent, while the blocky inner rim cycle is brightly birefringent. Central structures are variable but include cross bars, lateral bars and curving concentric bars.

Bicyclic muroliths with axial cross and additional small bars in the central-area.

Muroliths with narrow, bicyclic rim and wide central-area spanned by axial cross bars and a perforate plate.

Muroliths with narrow, unicyclic rim and wide central-area spanned by axial, concentric and lateral bars.

Muroliths with axial, lateral and concentric bars spanning the central-area.

Others
Loxoliths with central area filled with a tall, columnar to bulbous boss borne on a plate. Seen in side view. Spine has a thick wall, narrow axial canal and is formed from two crystallographically distinct units.

Small loxolith coccoliths with multiple central area radial bars supporting a central spine.

Murolith (loxoliths) with a robust, broad, high rim and central area that may be vacant, closed or spanned by bars or grill.

Loxolith coccoliths with broad, open central-area, probably spanned by a rarely preserved, finely perforate net

Murolith (loxolith) coccoliths with broad, high rim and narrow or closed central-area, which may be spanned by bars.

Narrowly elliptical loxolith coccoliths with a central-area structure that supports a broad, hollow, flaring spine.

Taxonomy:

Citation: Chiastozygaceae Rood et al., 1973
Rank: Family
Notes & discussion: The orientation of the cross-bars has previously been used as a familial level criterion, e.g., Chiastozygaceae Rood, Hay and Barnard, 1973; Ahmuellerellaceae Reinhardt, 1965; and Zygodiscaceae Hay and Mohler, 1967; respectively.

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

Distinguishing features: Murolith coccoliths with rim formed of outer cycle of imbricate V-units and variably developed inner cycle of R-units. Disjunct central bar or cross, usually supporting a spine.


Morphology remarks: The proximal/inner-wall cycle of the rim is often low and/or narrow, and the rim may thus appear unicyclic (ie. of rather uniform brightness) in the LM. In other cases it is well-developed and forms a distinct bright inner cycle - giving a bicyclic appearance.  This is a useful feature in the LM, but does not reflect any very profound strcutural change and so it is now primarily used as species-level character.

The family is long-ranging (Late Triassic-early Eocene) and in the Mesozoic is largely represented by relatively simply-constructed coccoliths in which iterative homology appears to have been common. Apart from a few distinctive forms, many species (often small) differ in very subtle ways and many species names are poorly constrained and applied in different senses by different authors.

Search data:
TagsLITHS: murolith,
MetricsLith size: 0->0µm;
Sources:
The morphological data given here can be used on the advanced search page. See also these notes

Geological Range:
Last occurrence (top): at base of Ypresian Stage (1% up, 55.9Ma, in Ypresian stage). Data source: Total of range of species in this database
First occurrence (base): in mid part of Norian Stage (50% up, 218.9Ma, in Norian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

Bown, P. R. (1998b). Triassic. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 29-33. gs

Hay, W. W. & Mohler, H. P. (1967). Calcareous nannoplankton from Early Tertiary rocks at Point Labau, France and Paleocene-Early Eocene correlations. Journal of Paleontology. 41(6): 1505-1541. gs

Reinhardt, P. (1965). Neue Familien für fossile Kalkflagellaten (Coccolithophoriden, Coccolithineen). Monatsberichte der Deutschen Akademie der Wissenschaften zu Berlin. 7: 30-40. gs

Rood, A. P., Hay, W. W. & Barnard, T. (1973). Electron microscope studies of Lower and Middle Jurassic coccoliths. Eclogae Geologicae Helvetiae. 66: 365-382. gs


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Chiastozygaceae compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 21-2-2020

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Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10245 Go to Archive.is to create a permanent copy of this page - citation notes



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