Calculites obscurus

Tetralithus obscurus Deflandre, 1959

Species of Calculites are constructed of a narrow outer wall and a central crystalline block that supports a solid distal process, elliptical in cross-section. The process is supported by six to seven struts, which produce depressions between adjacent struts. The scanning electron microscope image illustrated in Plate 1, Figure 7, of Farhan et al. (1994) provides the best example of this detailed structure.

In this holococcolith, in plan view under crossed polars, the outer wall is often absent but, when present, displays white interference colours. The inclined extinction lines within the central crystalline block are confined to the apex of the holococcolith as a result of the distal process and produce four optical blocks.

The detailed structure is visible only in well-preserved specimens. Under the light microscope, poorly preserved specimens reveal only four optical blocks in plan view.

The consistent base occurrence of Calculites obscurus is recorded at 85.04 Ma, within the Middle Santonian. Occurrences below this level are only sporadic.

Deflandre, G. 1959. Sur les nannofossiles calcaires et leur systématique. Revue de Micropaléontologie 2: 127-152.

Farhan, A. J., Burnett, J. A., Bown, P. R. & Lord, A. R. 1994. Holococcoliths from the Upper Cretaceous of Alabama and Mississippi (USA). Cahiers de Micropaléontologie. 9: 57-73.

Sissingh, W. 1977. Biostratigraphy of Cretaceous calcareous nannoplankton. Geologie en Mijnbouw. 65(1): 37-65.