Zeugrhabdotus


Classification: Mesozoic -> Eiffellithales -> Chiastozygaceae -> Zeugrhabdotus
Sister taxa: Amphizygus, Archaeozygodiscus, Gorkaea, Placozygus, Reinhardtites, Tranolithus, Zeugrhabdotus, Staurolithites, Diadorhombus, Rhabdophidites, Ahmuellerella, Bukrylithus, Heteromarginatus, Misceomarginatus, Monomarginatus, Neocrepidolithus, Vacherauvillius, Chiastozygus, Braloweria, Clepsilithus, Crepidolithus, Loxolithus, Tubirhabdus,

Distinguishing features: 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.


Daughter taxa: (blue => in age window 0-800Ma)
Medium sized Zeugrhabdotus with cross bar flaring slightly as it mets the rim
Medium-large Zeugrhabdotus with oblique bridge formed of two parallel crossbars
Small Zeugrhabdotus with a narrow central area nearly filled by a broad bipartite transverse bar that is bright in XPL at the 0° position, darkest at 60°, but which is generally visible throughout rotation. Unicyclic rim image in XPL.

Small (holotype 3.5 µm), unicyclic, bar formed of two transverse units and goes into extinction parallel to polars.
Minute Zeugrhabdotus with spine bearing transverse bar and additional lateral bars.
Zeugrhabdotus with broad rim and broad tranverse bar. Unicyclic rim image in XPL.
Zeugrhabdotus with bicyclic rim and and a central area spanned by a single bar that tapers at both ends. The inner-rim cycle is prominent and bright in XPL. The two rim cycles are comparable in width but the inner cycle may be broader, and is usually complete around the inner edge of the rim. Strongly curved extinction lines cross the bright inner cycle.
Medium to large Zeugrhabdotus with broad, high rim and narrow central area spanned by a diamond-shaped transverse bar. Bicyclic rim image in XPL.
Zeugrhabdotus with a bipartite transverse bar which is bright in XPL at the 0° position, darkest at 60°, but which is generally visible throughout rotation. Unicyclic rim image in XPL.
Typical small to medium Zeugrhabdotus - used as default identification by many workers - a modern name would be Z. howei
Large bicyclic murolith (loxolith) coccoliths with broad, blocky rim and central area spanned by a broad, disjunct, birefringent transverse bar that bears a spine. The bar has two clear lateral parts. Rim image is clearly bicyclic.
Small Zeugrhabdotus with a relatively narrow rim and wide central area spanned by a birefringent transverse bar supporting a spine. Typically unicyclic rim image in XPL.
Zeugrhabdotus with two part bar that is dark at 0° and brighter at 45°. Unicyclic rim image.
5-8µm long Zeugrhabdotus species, otherwise similar to Z. erectus
Medium sized, unicyclic or diffusely bicyclic Zeugrhabdotus with a bar which has a characterstically birefringent, granular appearance when orientated parallel with the polarising directions. At 45° the bar becomes dark but bright protrusions (typically four) from the rim may be visible. In PC the darker, outer rim cycle dominates, but the bright protrusions are visible at the ends of the bar.
Like Zeugrhabdotus embergeri but with very broad spine. Often seen in side view.
Similar to Z. erectus but with a central extinction line along the bar and non-birefringent stem-base in the centre of the bar.
Small zeugrhabdotids (2-3µm) with small central openings and overall low birefringence (mid-grey).
Medium-sized unicyclic Zeugrhabdotus with a central area spanned by a broad, disjunct, transverse bar. The bar is usually brighter than the rim in LM XPL and formed from two plain blocks divided by a median suture; the two blocks are offset at each end of the bar.
Like Z. sigmoides but smaller and with broader inner cycle
Medium sized Zeugrhabdotus with reticulate net on proximal surface
Medium-large (6-9µm) Zeugrhabdotus with relatively narrow unicyclic or diffusely bicyclic rim and wide central area spanned by a complexly constructed transverse bar. Four triangular projections support the two, distally-fused bars of the bridge and are conspicuous in the XPL. The narrow bridge consists of two optically separate bars that are fused and surmounted by a diamond-shaped distal process. The whole coccolith is moderately birefringent (grey-white), with the distal process yielding the brightest birefringence.
Zeugrhabdotus with relatively narrow rim and wide central area spanned by a spine-bearing bar that narrows at both ends. Narrow bright inner cycle in XPL that broadens where the bar meets the rim.
Large (holotype 8.8 µm) with narrow, bicyclic, rim and slender bar.

Large (c. 8.0 µm) Zeugrhabdotus with a broad, bicyclic or diffusely bicyclic rim and a broad, complexely constructed transverse bar. 
Medium-sized (5-6µm) Zeugrhabdotus with a well-developed inner/proximal rim cycle and, in XPL, the rim appears clearly bicyclic throughout its height, with equally well-developed inner and outer cycles; the inner cycle is brighter (white) than the outer (grey) and the extinction gyres are spirally-arranged. The bridge, which arises from the inner wall cycle, consists of two fused bars and bears a relatively small, solid, distal process.
Small Zeugrhabdotus with bicyclic rim and transverse bar with two diagnostic bright structures and a dark median part. The birefringent structures are brightest at 45° and appear as a X-shaped bridge.
Specimens not identified to species level

Taxonomy:

Citation: Zeugrhabdotus Reinhardt, 1965
Rank: Genus
Type species: Zygolithus erectus Deflandre in Deflandre and Fert, 1954, designated by Reinhardt, 1965.
Synonyms:

Farinacci & Howe catalog pages: Zeugrhabdotus + + * , Glaukolithus + * , Microrhabdulinus * , Lordia * , Rectapontis *

Distinguishing features: 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.


Morphology remarks: Long-ranging group (late Triassic-early Eocene) and often common to dominant.

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): 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:

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

Deflandre, G. & Fert, C. (1954). Observations sur les coccolithophoridés actuels et fossiles en microscopie ordinaire et électronique. Annales de Paléontologie. 40: 115-176. gs

Deflandre, G. (1963). Sur les Microrhabdulidés, famille nouvelle de nannofossiles calcaires. Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences. Paris. 256: 3484-3487. gs

Reinhardt, P. (1964). Einige Kalkflagellaten-Gattungen (Coccolithophoriden, Coccolithineen) aus dem Mesozoikum Deutschlands. Monatsberichte der Deutschen Akademie der Wissenschaften zu Berlin. 6: 749-759. gs

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

Varol, O. & Girgis, M. (1994). New taxa and taxonomy of some Jurassic to Cretaceous calcareous nannofossils. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen. 192: 221-253. gs

Varol, O. & Jakubowski, M. (1989). Some new nannofossil taxa. INA Newsletter. 11: 24-29. gs


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Zeugrhabdotus compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 15-11-2019

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