Catalog - Globigerina moskvini

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Globigerina moskvini

Citation: Globigerina moskvini Shutskaya 1953

Type specimens: pl. I, figs. 1-9; pl. 2, figs. 15, 16; holotype (figs. l-3), no. 3525/35.

Type locality: Kheu River, near the city of Nal'chik, Northern Caucasus, USSR

Type repository: Moscow; Institute of Geological Sciences, Academy of Sciences of the USSR,

Test trochoidal with a low spire, chambers flattened. touching but not overlapping one another (internal structure), sutures weakly depressed, broadening towards the umbilicus. umbilicus broad and flat.

Description. - A massive number of specimens of this species, recovered from many rock samples, was studied closely. In studying all the material it turned out that most of the forms are characterized by the following features. The trochoid - as in all Globigerina - test is flattened and elongated in plan, less commonly rounded; consists of two whorls of a spiral. Dorsal side flat, ventral side convex. In the initial whorl here are five or six poorly discernible, weakly convex, rounded and small chambers which pass smoothly into the next whorl. This one consists of four or rarely of five chambers which evenly increase in size. Generally convex, as is characteristic for the globigerinids, a certain flattening in the chambers is also to be observed. The initial whorl does not surmount the next one, which together with the flatness of the chambers gives the dorsal side a quite flat surface. On the ventral side the chambers are pear-shaped. They are separated by straight, weakly depressed sutures, thin at the outer margin and becoming wider towards the umbilicus. Owing to this widening the sutures merge imperceptibly with the not very deep, broad umbilicus and give it its shape, with concave sides and strechecl-out corners, while the umbilical ends of the chambers are somewhat triangular in shape. On the dorsal side the sutures are straight, depressed, but as on the ventral side not very deep. Only in the region of the initial whorl at the ends of the sutures are there any low areas. The spiral suture is not sharply marked everywhere, as a result of which the initial whorl usually joins the first chamber of the final whorl and makes with it an integral whole. The outer margin of the test is broadly rounded and lobate. The last feature is extremely variable. The aperture is at the base of the last chamber, which is usually larger than the preceding one, and extends from the umbilicus almost to the outer margin. It is provided with a quite wide lip, often not preserved. The test wall is calcareous. Surface perforate, on the last chamber the perforations are barely discernible.

The internal structure was studied in thin sections. A thin section of the ventral side to the umbilicus showed (fig. 15) that the chambers are in close contact only and not lying one over the other. The rhombic structure of the broad umbilicus is clearly visible. Deeper sections passing through the initial whorl (fig. 16) emphasize the relatively weak lobation of the outer margin.
[Translaton from Ellis & Messina catalog]

Average dimensions. - Greatest diameter 0.53 mm., least diameter 0.45 mm., thickness 0.10 mm.

Variation. - The main features characteristic of this species are the degree of depression of the sutures, the lobulation of the outer margin, the structure of the umbilicus, and the nature of the initial whorl. In observing many forms it became noticeable that depressed, but not very deep, sutures always broaden to some degree towards the umbilicus and become part of it, so to speak, giving it a rhomboid shape (fig. 1). However, this feature, as do all the others, undergoes some variation. In individual cases it is expressed very sharply owing to the presence of sutures. which are barely depressed, flat, and widening for half the distance from the umbilicus to the outer margin (fig. 5), in other cases it is expressed less distinctly (fig. 7).

The depth of the sutures also conditions the nature of the lobation of the outer margin. For very little depressed sutures the lobation is seen to be weak, this is especially noticeable around the last and next-to-last chambers. The outline of the outer margin here forms a smooth, barely indented curve. In other forms the sutures are more depressed than in the typical forms, and the lobation becomes quite sharp (figs. 6, 7). Between such extreme deviations there are gradual transitions, so they can all be placed within a single species. The less variable features, from the number of principal ones, are the umbilicus and initial whorl. The umbilicus in its shape is almost always rhombic and shallow, but depending on the width of the adjacent suture ends it can be more or less broad. In the initial whorl a quite marked boundary between it and the first chamber of the second whorl is sometimes observed, but nonetheless it makes a single unit with this chamber.

Next some time should be spent on the secondary features of this species, which are: the size of the last chamber. the amount of flatness of the dorsal side of the test, and the number of chambers in the last whorl. All these features vary witltin known limits. The last chamber in typical forms is much larger than the preceding one, but very many specimens have chambers wltich are perfectly alike in size, and sometimes even the reverse - the last chamber is smaller than the preceding one. Usually the flat dorsal surface of the test in some specimens is slightly curved, although the height of the spire does not become greater, i.e. the initial whorl is no higher above the last one than it is in flat specimens. As a result of the increase in number of chambers in the last whorl to five (figs. 8, 9), as opposed to the usual four, the test shape becomes more round, although here the size of the last chamber is of equal importance.

Comparison. - The species described, especially its tendency toward very deep sutures and a lobate margin, is closest to Globigerina pseudobulloides Plummer from the overlying El'burgan deposits. However, there are qualitative differences between them, as shown in the following table.

Specific features	Globigerina moskvini Shutskaya	Globigerina pseudobulloides Plummer
Initial whorl	Weakly divided from the last whorl	Sharply divided from the last whorl
Chambers	Slightly flattened	Inflated
Sutures	Shallow, broader toward the umbilicus	Incised, thin
Umbilicus	Broad and shallow	Relatively narrow and deep
Outer margin	Weakly lobate	Sharply lobate
Nature of chamber positioning	Not overlapping one another	Overlapping one another

Some forms in our species having weakly depressed sutures and barely apparent lobation of the outer margin are closest in overall shape and in the course of chamber growth to Globigerina fringa Subbotina, but differ sharply from it nonetheless in having a broad umbilicus and in being larger.

References:

Shutskaya, E. K. (1953). Raschlenenie kubanskogo i elburganskogo gorizontov Severnogo Kavkaza po globigerinam:. Byulleten Moskovskogo Obshchestva Ispytateley Prirody Otdel Geologicheskiy. 28: 71-79. gs

CATALOG OF ORIGINAL DESCRIPTIONS: Globigerina moskvini Shutskaya 1953

Globigerina moskvini

References:

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