Micula


Classification: Mesozoic -> Braarudosphaerales -> Polycyclolithaceae -> Micula
Sister taxa: Eprolithus, Radiolithus, Farhania, Lithastrinus, Uniplanarius, Quadrum, Micula, Palaeomicula,

Distinguishing features: Typically cubiform nannoliths usually formed from two superimposed and complexly-intergrown cycles of four pyramidal/blocky elements each


Daughter taxa: (blue => in age window 0-800Ma)
M. staurophora and variants - both layers similar
Typical form; cube-shaped; all 8 elements of similar size
Like M. staurophora but with atypical structure, possible ancestral
Like M. staurophora but with markedly concave faces 
Like M. staurophora but more cubic
Like M. staurophora but with strong chirality
Maastrichtian species with extended elements in one layer
Micula in which one cyle of four elements have protrusions extending tangentially from the corners of the cube
Micula with one cycle forming four rays in a cross shape and one cycle that is reduced/relict. The rays may terminate with small bifurcations.
single-layered species
Square, flat Micula that appears to be composed of a single cycle of four elements joined along uneven sutures that bend sharply at the their ends. Can appear circular at some focal depths.
Flattened, circular, disk-like Micula composed of a single cycle of four elements joined along curving, S-shaped sutures. 
A near-square to cruciform, flat Micula, composed of a single cycle of four elements joined along distinct straight, or gently curving, sutures.
Specimens not identified to species level

Taxonomy:

Citation: Micula Vekshina, 1959
Rank: Genus
Type species: Micula staurophora (Gardet, 1955) Stradner, 1963 = Micula decussata Vekshina, 1959.
Synonyms: Nannotetraster Martini & Stradner 1960 (type species was Discoaster staurophorus Gardet 1955)
Notes & discussion: M. staurophora is a universally used species, there is no consensus on whether it is useful to separate the variant forms such as M. concava and M. swastica (but in practice most people don't).
The forms with one reduced cycle M. murus and M. prinsii are important late Maastrictian markers.
The single-layered species are less well-documented but may prove useful.

Farinacci & Howe catalog pages: Micula * , Nannotetraster *

Distinguishing features: Typically cubiform nannoliths usually formed from two superimposed and complexly-intergrown cycles of four pyramidal/blocky elements each


Morphology remarks: Typically cubiform nannoliths usually formed from two superimposed and complexly-intergrown cycles of four pyramidal/blocky elements each; one of the cycles may be reduced (M. murus), relict (M. prinsii) or lost (M. clypeata, M. premolisilvae, M. praemurus). The elements are usually broadly triangular in shape, joined along sutures that go out to the points of the 'cube', and point/twist consistently in one direction on each surface; the elements may protrude slightly away from the edges of the cube.
When focussing through the structure in LM, the two cycles twist in opposite directions and at mid-focus, the bright, diagonal, bar-like structures, typical of Micula, are seen.  
All elements are bright in XPL when the sides of the cube/square are parallel with the polarising directions, but the whole structure goes into extinction when rotated around 45° from this position. 
No central opening or diaphragm.

Evolution & Phylogeny: Micula is thought to have evolved form Quadrum through intergrowth of elements (e.g. Crux 1982, Varol and so it i included in the Polycyclolithaceae, even though it has lost the simple tangential c-axis orientation characteristic of the other genera in the family.

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 Turonian Stage (99% up, 89.8Ma, in Turonian stage). Data source: Total of range of species in this database

Plot of occurrence data:

References:

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

Crux, J. A. (1982). Upper Cretaceous (Cenomanian to Campanian) calcareous nannofossils. In, Lord, A. R. (ed.) A Stratigraphical Index of Calcareous Nannofossils. British Micropalaeontological Society Series, Ellis Horwood, Chichester 81-135. gs

Gardet, M. (1955). Contribution à l'étude des coccolithes des terrains néogènes de l'Algérie. Publications du Service de la Carte Géologique de l'Algérie (Nouvelle Série). 5: 477-550. gs

Lees, J. A. & Bown, P. R. (2005). Upper Cretaceous calcareous nannofossil biostratigraphy, ODP Leg 198 (Shatsky Rise, Northwest Pacific Ocean). Proceedings of the Ocean Drilling Program. Scientific Results. 198: 1-60. gs

Stradner, H. (1963). New contributions to Mesozoic stratigraphy by means of nannofossils. Proceedings of the Sixth World Petroleum Congress. Section 1 Paper 4: 167-183. gs

Varol, O. (1992b). Taxonomic revision of the Polycyclolithaceae and its contribution to Cretaceous biostratigraphy. Newsletters on Stratigraphy. 27(93-127): -. gs

Vekshina, V. N. (1959). Coccolithophoridae of the Maastrichtian deposits of the West Siberian lowlands. Siberian Science Research Institute of Geology Geophysics Mineralogy and Raw Materials. 2: 56-81. gs


nannotaxlogo400px

Micula compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 17-10-2019

Taxon Search:
Advanced Search

Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10673 Go to Archive.is to create a permanent copy of this page - citation notes



Comments (0)

No comments yet. Be the first!

Add Comment

* Required information
1000
Captcha Image
Powered by Commentics