ON AN INTERESTING TRILOBITE FROM THE LOWER CAMBRIAN OF THE KRASNOYARSK RANGE

I. I. KOPTEV

(Presented by Professor Doctor K. V. Radugin)

The Lower Cambrian of the northwestern part of the Eastern Sayan (the Kolbinskaya Formation of the Mana River basin) is characterized by a fauna of trilobites, archaeocyaths, brachiopods, pteropods, etc., and by algae—stromatolites and oncolites. In the variegated unit of the Kolbinskaya Formation (the unit of Vyezzhii Log), L. N. Repina (1956) identified trilobites of the genus Bulaiaspis, which are widespread in the Tolbachan beds of the Lena stage of the Lower Cambrian of the Siberian Platform. On the basis of this fauna L. N. Repina also regards the age of the variegated beds at Vyezzhii Log as Tolbachan.

However, from these same beds, as early as 1948, O. K. Poletaeva described a trilobite lacking facial sutures. This form was described by her as incertae sedis, and the description was not published. O. K. Poletaeva noted the close similarity of this form to the Olenellidae. A reexamination of O. K. Poletaeva’s materials confirms the correctness of her conclusion regarding the similarity of the form described below to the family Olenellidae. Hence there arises the question of the possibly more ancient age of the variegated horizons of the Kolbinskaya Formation than the Tolbachan beds of the Lena stage. On the basis of the presence of Olenellidae, and also of the general similarity of facies, the Vyezzhii Log unit may be correlated with the upper parts of the Aldan stage of the platform. Moreover, the fact of the first discovery of olenellids in Western Siberia is in itself very important and worthy of publication.

Eobulaiaspis saianicus gen. n. sp. n.
Trilobite incertae sedis Poletaeva 1948
(figs. 1, 2)

The material consists of three impressions of the dorsal shield, which have not preserved their posterior ends, and one incomplete cephalic shield. The systematic position of this genus, given the incomplete and imperfect state of preservation, is established somewhat provisionally.

Description. The dorsal shield is of medium size, ovoid in shape, but apparently strongly tapering posteriorly.

The cephalic shield is broad, semielliptical in form, with a straight posterior margin; the glabella, together with the occipital ring, forms a bluntly rounded cone anteriorly. On the best-preserved specimen, two pairs of short glabellar furrows are visible on the sides of the glabella, not connecting—

Table 1

...ending on the middle glabella. The furrows are parallel to one another and slightly inclined backward. The occipital ring, widened in the middle, imperceptibly merges with the glabella.

The dorsal furrows are shallow; the fixed cheeks are broad, flat. In front of the glabella they join one another, forming a narrow preglabellar field. The anterior ends of the narrow ocular ridges extend almost horizontally from the very anterior end of the glabella, and then, after curving arcuately, pass into the palpebral lobes, likewise narrow, running almost vertically backward and reaching the posterior marginal ridge.

Fig. 1,2 — Eobulajaspis sajanicus gen. n. sp. n.
Dorsal shield without tail, left side of the valley of the Mana River
near the settlement Vyezzhiy Log. Lower Cambrian.

The marginal border is not wide, flat, passing on the sides into equally flat genal spines, extending from the cephalic shield somewhat above its posterolateral angles and reaching the 3rd trunk segment.

The anterior end of the frontal lobe of the glabella is connected with the marginal border by a longitudinal, weakly noticeable ridge (possibly this is the hypostomal stalk impressed during fossilization).

Facial sutures are absent. It is true that on one side of the cephalic shield of two specimens, in the place of a facial suture, there are faintly noticeable furrows that may be taken for facial sutures, but these furrows cut across the ocular ridges and are more likely the result of mechanical damage to the cranidium.

The trunk consists of no fewer than 17 segments. Its axial part occupies one third of the width and gradually narrows posteriorly. On the 11th and 12th segments a powerful spine directed backward is indicated. On the axial parts of the trunk segments, now clearly, now indistinctly, tubercles are visible, which stand out in higher relief nearer the tail in the larger specimen. The outer ends of the axial part of the segment are blunt. The pleural parts bear very broad, poorly delimited pleural furrows running parallel to the interpleural furrows. The pleural furrows become sharper and deeper nearer the tail and, together with the pleurae, more strongly inclined backward.

The outer ends of the pleurae bear spines, appearing already from the 3rd segment, at first in the form of a point; later this point rapidly increases into a spine, claw-like bent backward. On the eighth segment this spine is more powerful.

The absence of facial sutures gives grounds for assigning the given form to the Olenellidae; however, it differs so much from the known representatives of this family that, for the present, until additional materials are collected, this assignment must be regarded as provisional. It stands closest to the genus Paedumias, having in common with it such features as a broad, large cephalic shield without facial sutures, a large number of trunk segments with a powerful spine on one of them close to the tail, a longitudinal ridge in front of the glabella, a narrow marginal border slightly widening toward the genal angles, and, finally, tubercles or spinules on the axial part of the trunk segment.

Eobulaiaspis has a great similarity in the structure of the shield and especially of the trunk with the Baltic representative of this family—Olenellus mickwitzii Schm.

The form described differs from the Olenellidae in the following characters:

1. The anterior ends of the ocular ridges depart from the very anterior end of the glabella, while the posterior ends of the ocular lobes are rather far removed from it. In the Olenellidae the posterior ends of the eyes again adjoin the glabella, and the ocular lobes are broader and more convex; the fixed cheeks, as a result of this arrangement of the eyes, are very narrowed.

2. The glabella has a subtrapezoidal form, whereas in the Olenellidae it is cylindrical.

In the structure of the trunk Eobulaiaspis closely resembles the Redlichidae, in particular Bulaiasis (1956), described by L. N. Repina. Common features for them are: a many-segmented trunk with tubercles on the axial parts of the segments and a powerful dorsal spine, claw-like flat terminations of the pleurae, widely spaced ridge-like ocular lobes, and a weakly segmented glabella narrowing anteriorly.

The combination of characters characteristic of the Olenellidae on the one hand and of Bulaiaspis on the other gives grounds for considering Eobulaiaspis a transitional form between these groups; however, its greater similarity to the Olenellidae compels one to assign it precisely to this family. Most probably the form described is a branch separated from early olenellids, which subsequently developed as far as Bulaiaspis.

It occurs together with Bulaiaspis in the multicolored suite of Vyezzhii Log on the left side of the valley of the Mana River, below the settlement of Vyezzhii Log by 200 m. Lower Cambrian.

Literature

1. Repina L. N. — Paleontological substantiation of the age of the Cambrian deposits of the western part of the Eastern Sayan (Reports of the Academy of Sciences of the USSR, vol. 110, No. 2, 1956).

2. Repina L. N. — New trilobites of the genus Bulaiaspis and their stratigraphic significance (Materials on Paleontology, collection of VSEGEI, new series, issue 12, 1956).

Corrections and Noted Misprints

In the article by S. A. Stroitelev, “Study of the Crystallization of Epsomite and Mirabilite,” for technical reasons braces have been replaced by square brackets, for example: [100] instead of {100}, etc.