Distinguishing features: Parent taxon (Polycyclolithaceae): Nannoliths composed of two superposed wall cycles and a central-area which may be closed, open and vacant, or spanned by a diaphragm-like structure. Elements have tangential c-axis orientation. This taxon: Typically cubiform nannoliths usually formed from two superimposed and complexly-intergrown cycles of four pyramidal/blocky elements each
Daughter taxa (time control age-window is: 0-800Ma)
M. staurophora and variants - both layers similar
Micula staurophora Typical form; cube-shaped; all 8 elements of similar size
Micula adumbrata Like M. staurophora but with atypical structure, possible ancestral
Micula concava Like M. staurophora but with markedly concave faces
Micula swastica Like M. staurophora but with strong chirality
Maastrichtian species with extended elements in one layer
Micula murus Micula in which one cycle of four elements have protrusions extending tangentially from the corners of the cube
Micula prinsii 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
Micula clypeata 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.
Micula praemurus Flattened, circular, disk-like Micula composed of a single cycle of four elements joined along curving, S-shaped sutures.
Micula premolisilvae A near-square to cruciform, flat Micula, composed of a single cycle of four elements joined along distinct straight, or gently curving, sutures.
Micula sp. Specimens not identified to species level
Taxonomy:
Citation: Micula Vekshina, 1959taxonomic rank: GenusType species: Micula staurophora (Gardet, 1955) Stradner, 1963 = Micula decussata Vekshina, 1959.Synonyms: Nannotetraster Martini & Stradner 1960 (type species was Discoaster staurophorus Gardet 1955)Taxonomic 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.
Distinguishing features: Parent taxon (Polycyclolithaceae): Nannoliths composed of two superposed wall cycles and a central-area which may be closed, open and vacant, or spanned by a diaphragm-like structure. Elements have tangential c-axis orientation. This taxon: Typically cubiform nannoliths usually formed from two superimposed and complexly-intergrown cycles of four pyramidal/blocky elements each
Morphology: 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.
Phylogenetic relationsMicula 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.
Lith size: 3->9µm; Data source notes: size range of included species
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 Late Maastrichtian Substage (100% up, 66Ma, in Danian stage). Data source: Total of ranges of the species in this database First occurrence (base): within UC9c subzone (87.18-89.49Ma, base in Turonian stage). Data source: Total of ranges of species in this database
Plot of occurrence data:
Range-bar - range as quoted above, pink interval top occurs in, green interval base occurs in.
Triangles indicate an event for which a precise placement has been suggested
Burnett, J. A. (1998). Upper Cretaceous. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series . 132-199. gsO
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. gsO
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. 5: 477-550. gsO
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. In, Proceedings of the Sixth World Petroleum Congress. Section 1 Paper 4: 167-183. gs
Varol, O. (1992). 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. Trudyi Instituta Geologii i Geogiziki, Sibiriskoe Otlodelenie, Akademiya Nauk SSSR (Nauka) Moscow. 2: 56-81. gs
Missing or ambiguous references: ;
Micula compiled by Jeremy R. Young, Paul R. Bown, Jacqueline A. Lees viewed: 16-10-2024
