Nannotax3 - ntax_main - Eiffellithales

Citation: Eiffellithales Rood et al., 1971

Taxonomic discussion: This is a fairly well-characterised group with a common structure of a rim formed of an outer/upper cycle of clockwise imbricate elements. These elements are typically thin and closely spaced and are inclined at about 30-40° from he horizontal. They have sub-vertical c-axes and so show very low birefringence in plan view. On the proximal surface the V-units are underlain by a cycle of R-units with radial sutures. The r-units also extend up the inside of the rim. This rim type may also be refered to as Eiffelithid or loxolith.

If the R-units are well-developed then they will be visible in distal view as a separate cycle in SEM imagesand the coccolith will show in cross-polars a birefringent inner cycle. Conversely if they are weakly developed the entire coccolith rim will show low birefringence - these are refered to as respectively bicyclic and unicyclic appearance.

Inside the rim almost all Eiffellithales have additional disjunct structures usually forming a crossbar or central cross, and a spine.

Most genera are placed in the Chiastozygaceae, with two sub-groups separated out, the Eiffelithaceae and Rhagodiscaceae. In the Eiffellithaceae the V-units form a narrow rim while the R-units extend inward forming a strongly bicyclic rim and often a biefrngent central plate. In the Rhagodiscaceae, by contrast, the R-units are weakly developed, the rim is unicyclic and the central structure is typically granular and shows low birefringence. Distinct crossbar or cross structures are lacking but there is often a, hollow, central spine. Within the Chiastozygaceae most genera include both unicyic and bicyclic forms.

The Cenozoic Zygodiscales are similar in being murolith coccolths with an outer rim of V-uints and an inner rim of R-=units. However, the imbrication directions are reversed in the two orders - in the Eiffeltihales the outer rim shows clockwise imbrication whilst the Zygodiscales show anti-clockwise imbrication. Also the Zygodiscales typically have more, finer elements. So, despite the superficial similarities, the orders are morphologically distinct and may be unrelated.

Distinguishing features:
Parent taxon (Mesozoic nannofossils): Predominantly Mesozoic nannofossils - except for Braarudosphaerales
This taxon: Murolith coccoliths with an upper/outer cycle of clockwise-imbricate V-units and a lower/inner cycle of R-units.

Geological Range:
Last occurrence (top): at top of NP11 zone (100% up, 53.7Ma, in Ypresian stage). Data source: Total of ranges of the species in this database
First occurrence (base): within NT2b subzone (194.83-218.41Ma, base in Norian stage). Data source: Total of ranges of species in this database

References:

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

Rood, A. P., Hay, W. W. & Barnard, T. (1971). Electron Microscope Studies of Oxford Clay Coccoliths. Eclogae Geologicae Helvetiae. 64: 245-272. gs O

Young, J. R. & Bown, P. R. (1997). Proposals for a revised classification system for calcareous nannoplankton. Journal of Nannoplankton Research. 19(1): 15-47. gs

Daughter taxa (time control age-window is: 0-800Ma)		Granddaughter taxa
	Chiastozygaceae Typical Eiffelithales - rim with outer/upper clockwise-imbricate V-units and lower/inner R-units. Central area open or spanned by disjunct cross or bar supporting a spine.	Amphizygus Archaeozygodiscus Gorkaea Placozygus Reinhardtites Tranolithus Zeugrhabdotus Chiastozygus Staurolithites Diadorhombus Rhabdophidites Ahmuellerella Bukrylithus Diloma Heteromarginatus Misceomarginatus Monomarginatus Vacherauvillius Braloweria Clepsilithus Crepidolithus Loxolithus Tubirhabdus Chiastozygaceae - unidentified
	Cenozoic Chiastozygaceae	Neocrepidolithus
	Eiffellithaceae Eiffelithales in which V-units form a narrow outer rim; R-units form broad inner rim or plate. Almost all with a disjunct central spine-bearing cross.	Eiffellithus Helicolithus Tegumentum
	Rhagodiscaceae Eiffelithales with V-units forming most of rim (R-units typically thin plates on proximal surface, so LM images are typically unicyclic). Central-areas typically filled by a plate of granular calcite. The central structure may be spine-bearing, perforate or massive.	Percivalia Rhagodiscus Calcicalathina Zebrashapka

Eiffellithales

References:

Add Comment

Comments