Distinguishing features: Placolith (or modified placolith) coccoliths showing little or no imbrication. Typically, V-units form distal shield, R-units form proximal shield.
|Daughter taxa: (blue => in age window 0-800Ma)||Granddaughter taxa|
Distal shield elements non-imbricate V-units; proximal shield and inner cycle formed from R-units. Rim often appears beaded in LM
The distal shield is formed from V-units and the proximal shield and tube cycle are formed from R-units.
The distal shield is formed from V-units and the proximal shield from R-units. Commonly observed in side view.
The distal shield usually bicyclic, with narrow outer-cycle and broad inner-cycle; elements usually radial and non-imbricate. LM image is moderately birefringent. V/R structure unclear.
The distal shield may be high and flaring, formed from non-imbricating elements, joined along radial sutures. LM image is generally dark, but birefringence increases when the distal shield is high.
The distal shield and proximal shield constructed from 16 large non-imbricating elements (V-units); R-units form restricted cycle on proximal shield and inner tube cycle, giving bicyclic LM image with bright inner cycle.
Farinacci & Howe catalog pages: Podorhabdales [no catalog entry yet], ,
Morphology remarks: Placolith (or modified placolith) coccoliths with shields formed from elements that display little or no imbrication and equal development of V and R crystal units, i.e. one shield (distal) formed from V units, the other from R units (proximal). LM images are typically low birefringence but high relief in phase contrast. Shield elements are typically joined along radial sutures, but these may curve or kink. Element curvature is consistently dextrogyre (veeing anticlockwise) and obliquity is broadly dextral (clockwise) in the distal shield, and the same in the proximal shield when viewed proximally.
This order includes the Biscutaceae, Axopodorhabdaceae and other closely related forms. The rim constructions of the Cretarhabdaceae, Tubodiscaceae and Mazaganellaceae are not well understood, and these families are only tentatively assigned to this order.
Last occurrence (top): at top of Thanetian Stage (100% up, 56Ma, in Thanetian stage). Data source: Total of range of species in this database
First occurrence (base): at base of Sinemurian Stage (0% up, 199.3Ma, in Sinemurian stage). Data source: Total of range of species in this database
Plot of occurrence data:
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 Bown, P. R. (1987a). Taxonomy, evolution, and biostratigraphy of Late Triassic-Early Jurassic calcareous nannofossils. Special Papers in Palaeontology. 38: 118-. gs Bown, P. R., Rutledge, D. C., Crux, J. A. & Gallagher, L. T. (1998). Early Cretaceous. In, Bown, P. R. (ed.) Calcareous Nannofossil Biostratigraphy. British Micropalaeontological Society Publication Series. 86-131. gs Rood, A. P., Hay, W. W. & Barnard, T. (1971). Electron Microscope Studies of Oxford Clay Coccoliths. Eclogae Geologicae Helvetiae. 64: 245-272. gs
Short stable page link: http://mikrotax.org/Nannotax3/index.php?id=10918 Go to Archive.is to create a permanent copy of this page - citation notes