Catalog - Spiroplecta clarae

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Other pages this level: S. americana, S. clarae

Spiroplecta clarae

Citation: Spiroplecta clarae Georgescu 2013

Type age (chronostrat): Late Santonian–Campanian.

Diagnosis: Spiroplecta with holohelicid test and occasionally laterally pinched last–formed chambers.

Description: Test holohelicid; proloculus is spherical, situated in the central portion of the test early stage and with a diameter of 0.0090–0.0140 mm. The early stage consists of four to five subglobular chambers with gradual increase in size and arranged in one complete whorl around the proloculus. Adult stage consists of five to ten chambers with gradual size increase, which are alternately added with respect to the test growth axis; chambers of the adult stage are subglobular in shape and occasionally the last–formed one is laterally–pinched. Chambers overlap at various rates. Sutures in the adult portion of the test are distinct and depressed, straight or slightly curved and perpendicular to the test growth axis; sutures are less distinct and frequently hardly visible in the earlier test portion. Test compressed and symmetrical in edge view, with rounded and simple periphery. Aperture is a medium to high rounded arch and is situated at the base of the last–formed chamber; it is bordered by two symmetrical narrow subtriangular metaflanges, one on each side of the test.
Chamber surface ornamentation consists of longitudinal and discontinuous leptocostae, which are 2.5–3.1 µm in width, and parallel to the test growth axis and periphery. A pustulose periapertural area consisting of scattered dome–like and elongate pustules with a diameter of 2–5 µm is developed over the chamber anterior face. Test wall is calcitic, hyaline, simple and perforate; pores are circular in shape, with a diameter of 0.9–1.3 µm and are situated in the space between the leptocostae, rarely interrupting them.

Species new name is given after Clara Ehrenberg.

Comments on nomenclature: Spiroplecta clarae is the new name proposed for S. americana Ehrenberg 1844; the species originally named Textilaria americana Ehrenberg 1841 and S. americana belong to the same lineage, the former as ancestor and the latter as descendant, therefore it is a case of homonymy; the species named at an earlier date is preserved.

There is significant morphologic variability in S. clarae and it is most evident in the early planispiral coil and occasional occurrence of pinched chambers in the adult portion of the test. Test early portion has the proloculus completely enclosed by one complete whorl; specimens in which the proloculus is in eccentric position and tangential to the test margin are known and they represent around 10% of the total number of specimens of S. clarae. Specimens with pinched last–formed chambers occur occasionally in Sample 1595b and Bailey Collection slides, which were collected from the Fort Hays Limestone Member of the Niobrara Formation. This lithostratigraphic unit is situated in the lowermost stratigraphic position among the samples in the Ehrenberg Collection. Only specimens with holohelicid test and last–formed chambers without pinched shape occur in the samples above the Santonian/Campanian boundary. Therefore it is possible to recognize two varieties of S. clarae, namely a primitive variety (with hysterohelicid test and pinched last–formed chambers) and a morphologically advanced variety (test holohelicid and without pinched last–formed chambers). Specimens with mixed features between the two varieties (i.e., test holohelicid and with pinched last–formed chambers) occur in the Fort Hays Limestone Member samples but are absent at higher stratigraphic levels. These data suggest that the sample collected by J.N. Nicollet from the Santonian rocks along the Missouri River includes the evolutionary transition between S. americana and S. clarae. It is evident from the paragraph in which Ehrenberg (1841) mentions the provisional name Heterohelix that he refers to specimens from the same sample, which were assigned to Spiroplecta americana (renamed herein S. clarae) in Ehrenberg (1844).

Remarks: Spiroplecta clarae differs from Hartella harti Georgescu and Abramovich 2009 from the Maastrichtian mainly by having the test ornamented with leptocostae rather than aligned pustules. There is no overlap between the stratigraphic ranges of the two species.

Geographic distribution: USA (Mississippi, Nebraska, New Jersey, South Dakota, Texas).

References:

Georgescu, M. D. (2013c). Revised evolutionary systematics of the Cretaceous planktic foraminifera described by C.G. Ehrenberg. Micropaleontology. 59: 1-48. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Spiroplecta clarae Georgescu 2013

Spiroplecta clarae

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