Cirravus novojilovi sp. n. from the Usyat Subformation of the Kuznetsk Basin

L. L. Khalfin

More than twenty years ago, B. I. Chernyshev thoroughly studied and described [4] the remains of crustaceans found in the coal-bearing deposits of the Kuznetsk and Donets basins and shown to belong to the order Cirripedia. B. I. Chernyshev established for them a new genus, Cirravus, and provided a reconstruction of the general appearance of the animal (Fig. 1), which, in outward form and in the arrangement of the plates composing its skeleton, very much resembled modern representatives of the genus Lepas Linné; in both genera the skeleton consists of five plates—two ventral (terga), two lateral (scuta), and one dorsal (carina). In the fossil state these plates occur, as a rule, isolated.

All the specimens of Cirravus from Kuzbass available to him were assigned by B. I. Chernyshev to a single species—Cirravus jaworskii Chernyshev. They were all found at one locality—on the Mungat River near the village of Krapivino, in deposits of the Alykaev Subformation. The dimensions of the plates forming the shell of this species are very small: by the largest measurement they did not exceed 4–5 mm.

Later, entirely analogous plates (belonging to the same species or to closely related species) were found at a number of additional localities—from the extreme northern (the town of Anzherka) to the extreme southern (the Korchakol River) districts of Kuzbass [2, p. 42]. Everywhere they are part of the brackish-water Alykaev fauna and occur only in deposits of the Alykaev Subformation. Only quite recently have entirely similar plates of Cirravus been found at one locality in the Kemerovo district in deposits assigned to the intermediate subformation, likewise in association with pelecypods of the Alykaev fauna [1].

Fig. 1. Reconstruction of the general appearance of a representative of the genus Cirravus Chern. Drawing by B. I. Chernyshev.

It may be considered established that Cirravus occurred everywhere in Kuzbass and disappeared together with this fauna as a result of the freshening of the basins situated in the Kuznetsk depression. Nowhere higher in the section have Cirravus been encountered, including in the deposits of the Usyat Subformation, whose fauna has been repeatedly and carefully collected in many districts. Nor did they appear later, when,

at the beginning of the formation of the deposits of the Kuznetsk Series, the descendants of the Alykayev fauna returned to Kuzbass.

There was every reason to believe that the last representatives of the genus Cirravus, which had found refuge in the last brackish-water basins in the northwest of Kuzbass, became extinct long before the beginning of the formation of the sediments of the Usyat Subformation. All the greater interest, therefore, attaches to the find made by R. N. Benediktova in the deposits of the Usyat Subformation of the Glushinsky district, which proved to be an impression of the left abdominal plate (tergum sinistrale) of a certain new, unusually large species of the genus Cirravus. Despite the not entirely perfect preservation of this as yet unique specimen, it merits description.

Cirravus novojilovi sp. n.

Table I; Fig. 2 in the text.

Holotype (tergum sinistrale): collection of the Tomsk Polytechnic Institute, museum number 476-sp. Upper Balakhon Formation, Usyat Subformation; vicinity of the village of Glushinka, Lomovskaya II area, borehole 476, 100 m above the Kemerovo bed.

Diagnosis. Tergum of strongly elongate-quadrangular outline, slightly widening anteriorly. The carinal¹ (upper) margin, very weakly and quite evenly convex, passes, bending steeply but without forming an angle, into the likewise convex basal (posterior) margin. The scutal (lower) margin in the posterior half is barely noticeably concave, almost straight; with the basal margin it forms a distinct angle, only with a slightly rounded apex. The occludent (anterior) margin is arcuately concave. The relief growth ridges run parallel to the scutal and basal margins. The intervals between them are broad, flat, and bear finer and lower concentric ridges. Greatest length of the plate about 1 cm.

Fig. 2. Cirravus novojilovi sp. n. Tergum;
copy from a photograph; ×7.

Description. There is one incomplete impression of the left tergum: the outlines of the carinal and basal margins, the junctions of the latter with the carinal and scutal margins, and the posterior part of the latter are well preserved; the outlines of the occludent margin are discernible; the anterior part of the scutal margin has not been preserved. The growth ridges are well preserved, except for the pri-umbonal part of the plate.

The carinal margin, evenly and very weakly convex, is directed as a whole at an angle of about 130° to the likewise convex basal margin; the angle between them is completely cut off by a broad rounding. By contrast, the angle between the basal and scutal margins is quite distinct, its apex only slightly rounded, and its magnitude is about 60°. The scutal margin in its posterior and middle part (approximately over 2/3 of its length)

Plate 1

Cirravus novojilovi sp. n. Tergum. ×14.

very slightly concave; the outlines of its anterior part have not been preserved, but, judging from the course of the growth marks, it was very slightly convex. The closing margin is outlined by a concavity of comparatively small radius and, on the whole, is parallel to the basal margin; with the scutal margin it forms a rounded obtuse angle (about \(130^\circ\)). The umbo is narrow and long; its very tip has not been preserved.

In general, the carinal and scutal margins are directed at an angle of about \(10^\circ\) to one another, as a result of which the plate broadens somewhat anteriorly.

On the surface of the plate there are very prominent rounded concentric growth ridges. They run parallel to the scutal and basal margins and disappear as they approach the carinal margin. The intervals between them are broad and flat, provided with finer and lower concentric ridges running parallel to the growth ridges. The fine ridges have not been preserved everywhere, but in places, under oblique illumination and at considerable magnifications, they are quite clearly visible.

Dimensions: greatest length of the plate—\(9\) mm; its height at the point where the carinal and basal margins meet—\(2\) mm.

Remarks. As the description given above shows, despite a certain defectiveness of the specimen available to us, almost all features of the structure of the tergum of our form can be observed.

Of particular importance to us seems the absence of any signs of that radial sculpture which is so characteristic of the cirravids from the Alykaevo fauna. It seems to us that this cannot be explained by the state of preservation of our impression, on which not only the growth ridges have been excellently preserved, but also, in places, considerably finer concentric ridges in the intervals between them. Apparently, the descendants of the Alykaevo cirravids had, by Usyat time, lost the radial (transverse) sculpture, and on this basis, perhaps, the form described should be separated into a distinct genus.

Comparisons. In the outlines of the tergum, the species described is extremely similar to those of Cirravus jaworskii; only the angle between the basal and scutal margins in the latter is sharper and more drawn out. A striking difference of our species is its twice larger size. This alone, in connection with the great difference in stratigraphic position, does not allow our specimen to be assigned to Cirravus jaworskii.

The absence of radial sculpture and the presence, in the intervals between the growth ridges, of concentric lines so greatly intensify the difference between our form and the known species of cirravids that it permits raising the question of separating it into a distinct genus.

Occurrence. Kuznetsk Basin, northwestern margin, environs of the village of Glushinka. Usyat Subformation of the Upper Balakhonka Series, \(100\) m above the Kemerovo seam, together with impressions of Tschernovia sp.

We shall make several concluding remarks concerning the find of a specimen of Cirripedia in deposits of the Usyat Subformation. This subformation is characterized by the so-called Late Balakhonka fauna [2, p. 66], in which any marine elements are entirely absent and which, for this reason, is regarded as freshwater.

Usunghi are almost exclusively marine organisms; in the coal-bearing deposits of Kuzbass, representatives of this order (the genus Cirravus) are widely distributed as part of the brackish-water Alykaevo fauna.

In connection with the foregoing, the question might arise of revising the facies characterization of the late Balakhonian fauna, but there is hardly any basis for this. First, there is no doubt that cirripedes constitute the rarest group in the late Balakhonian fauna (as a result of many years of careful collecting of this fauna, carried out by many persons, a unique specimen has been found). Second, there are no grounds for asserting that all Paleozoic barnacles necessarily lived under the same conditions as the later representatives of this order. Finally, third, among modern Cirripedia there are known forms (although by way of exception) that enter not only brackish but also fresh-water basins.

The migration of the descendants of brackish-water cirravi into freshwater basins entailed those significant changes which they underwent and which were indicated above. One cannot fail to note, in particular, the increase in their dimensions: the dimensions of the described abdominal plate exceed by two and three times the dimensions of analogous plates of the Alykaev cirravi.

This is of special interest in view of the fact that both the pelecypod fauna and the flora of the Usyat Subformation are characterized by the unusually large size of their representatives. This tendency toward gigantism, characteristic of various groups of the fauna and flora of the Usyat Subformation and expressed once again in the unusually large dimensions of Cirravus novojilovi, is one of the most interesting phenomena, still awaiting explanation, in the history of development of the Upper Paleozoic population of the Kuznetsk Basin.

REFERENCES CITED

1. Benediktova R. N. Lamellibranch mollusks of the intermediate subformation of Kuzbass (present collection).

2. Khalfin L. L. Lamellibranch mollusks of the coal-bearing deposits of Kuzbass. Transactions of the Mining-Geological Institute of the West Siberian Branch of the Academy of Sciences of the USSR, issue 9, 1950.

3. Darwin Ch. A monograph on the sub-class Cirripedia. London. 1851.

4. Tchernyshev B. Cirripedia aus den Donez und Kusnetskbecken. Yearbook of the All-Russian Paleontological Society, vol. X for 1931–1933, 1935.

Tomsk Polytechnic Institute
named after S. M. Kirov

1. The names of the margins of the tergum are given according to Ch. Darwin [3], and in parentheses according to B. I. Chernyshev [4]. ↩