PROCEEDINGS OF THE TOMSK ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV

Vol. 217　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1971

FIRST FINDS OF TRILOBITES IN THE SOLGON RIDGE

(AGATA RIVER BASIN)

L. M. MIKHAILOVA

(Presented by Professor A. G. Sivov)

The Lower Paleozoic deposits of the Solgon Ridge were studied by T. P. Osotskikh, M. A. Sherman, Ya. G. Kats, G. I. Rudakova, B. N. Krasilnikov, A. A. Mossakovskii, and others.

These researchers subdivided the above-mentioned Lower Paleozoic deposits into the Agata (carbonate) and Borsuk (volcanogenic) suites, in which paleontological remains were not known. Later (1964–1965), M. G. Ekhanin and L. M. Mikhailova (Western Comprehensive Geological-Prospecting Expedition of the Krasnoyarsk Geological Administration), while refining the stratigraphy of the oldest deposits of the Solgon Ridge, divided the Agata Suite into the Kulbyurstog, Tarzhul, Usin, and Karasuk suites. Among the latter, at the very base of the Usin Suite, which forms a basal unit up to 40 m thick and unconformably overlies the dolomites of the Tarzhul Suite, the presence was established of remains of trilobites, archaeocyaths, brachiopods, and pelecypods, collected from bedrock exposures (locality 711). On the basis of the archaeocyaths, I. T. Zhuravleva identified the Sanashtykgol assemblage.

The study of pelecypods, brachiopods, and trilobites collected from the eluvium of clayey greenish-gray limestones, which apparently represent a thin lens among the tuffaceous sandstones of the basal unit (locality 721), and from a bedrock exposure of tuffaceous sandstones (locality 711), was undertaken by the author of this article.

As a result of this study, the following trilobites were established: Kolbinella cf. prima Rep., Bagradia (?) Rep., Erbiopsidella cf. convexa Poc., Bonnaspis (?) salgonica sp. nov., Chondraulos sp., Eoptychoparia (?) sp., Bonnaspis (?) sp. nov., Corynexochus (?) sp. n., and others, as well as brachiopods and pelecypods.

Of the listed trilobites, recorded for the first time in the Lower Cambrian of the Solgon Ridge, this article provides descriptions only of those that are satisfactorily preserved. The characteris-

…istics of the trilobite, brachiopod, and pelecypod fauna is accompanied by photographs placed in Plate 1.

Description of the trilobites

Locality 721

Family Hicksiidae Hupe, 1953
Subfamily Bonnaspinae Suvorova, 1964.
Genus Bonnaspis (?) Resser, 1936.
Species Bonnaspis (?) salgonica sp. n.
Plate 1, figs. 13—16

Holotype. Illustrated in Plate 1, fig. 13. Kept in collection No. 721 of the Western Expedition of the Krasnoyarsk Administration.

Material. 4 cranidia, 1–1.5 mm in size, in good preservation.

Description. Cranidium of subtrapezoidal form, convex, with a glabella widening anteriorly. Occipital ring convex, ridge-like. Occipital furrow, posterior border furrow, and axial furrows broad, of moderate depth. Posterolateral lobes triangular, bent forward, angularly broken. Fixed cheeks weakly convex, triangular. Palpebral lobes small, shifted forward. Anterior branches of the facial sutures very short. Anterolateral lobes present. Anterior margin weakly curved. Border rim very narrow, slightly noticeably widening toward the margins. Limb absent. Surface of the carapace smooth.

Comparison. The species described is similar to Bonnaspis akaira Suv. It differs from the latter in the absence of sculpture on the carapace and in the forward-shifted position of the palpebral lobes. A similarity is also observed between Bonnaspis (?) salgonica sp. n. and Bonnaspis palaea Suv., from which it differs in the forward-shifted palpebral lobes and the glabella widening more sharply anteriorly. Somewhat longer palpebral lobes and the presence of an anterior border rim distinguish Bonnaspis (?) salgonica sp. n. from the similar Eocorynexochus convexus Korob.

Distribution and age. Solgon Ridge (right bank of the Agata River), lower part of the Usinsk Formation. Found together with single specimens of Corynexochus (?) sp. n. (fig. 17), Chondragraulos sp. (fig. 20), Gaphuraspis sp. (fig. 21), an indeterminate trilobite pygidium (fig. 18), and a brachiopod (fig. 19).

Locality 711

Family Anomocaridae Poulsen.
Genus Kolbinella Repina, 1960.
Species Kolbinella cf. prima Rep., 1960
Plate 1, figs. 1, 2.

Material. 5 cranidia and 5 of their impressions, in satisfactory preservation.

Description. Cranidium elongate, slightly convex, with an arcuate anterior margin. Glabella subrectangular, convex, smooth, or with two pairs of faintly perceptible glabellar furrows. Axial furrows distinct, comparatively broad and deep. Occipital furrow broad and deep. Occipital ring broad, flattened. Fixed cheeks very narrow, constituting one-fifth of the width of the glabella. Palpebral lobes long, constituting three-fifths of the length of the glabella, angularly broken in the middle; their anterior and posterior ends are pushed up to the axial furrows. Anterior border furrow broad and deep. Anterior border rim very broad and approaches the glabella as a point. Anterior branches of the facial sutures long, divergent. Sculpture of the carapace smooth.

Comparison. The species described differs from the typical species of L. N. Repina by its smooth or weakly segmented glabella.

Distribution and age.

1. Kuznetsk Alatau (Mt. M. Koshkulak). Efremkino Formation.
2. Eastern Sayan (Mana River). Kolba Formation, Siner subformation.
3. Solgon Ridge (left bank of the Agata River). Lower part of the Usun Formation. Found together with isolated specimens of Bagradia (?) Rep., Erbiopsidella cf. convexa Pokr., Eoptychoparia (?) Rasetti, Kutorgina lenaica Lerm, Bonnaspis (?) salgonica sp. nov., Bonnaspis (?) sp. nov., and indeterminate remains of trilobites and pelecypods (Table 1, Figs. 3–12).

On the age of the trilobites of the basal member of the Usun Formation

The vertical distribution of the trilobites studied from the basal member of the Usun Formation of the Solgon Ridge is shown in Table 2.

The species Kolbinella prima Rep. is known from deposits of the Siner subformation of the Kolba Formation of the Eastern Sayan (according to I. I. Koptev, 1957–1958). L. N. Repina, V. V. Khomentovskii, and I. T. Zhuravleva correlate the above-mentioned deposits with the Olekma Horizon of the Siberian Platform. In the most recent stratigraphic scheme of the Lower Cambrian deposits of the Sayan–Altai region, the Olekma Horizon corresponds to the Sanashtykgol Horizon.

The genus Bagradia Rep. is widespread in the Lower Cambrian of the Sayan–Altai region.

The species Erbiopsidella convexa Pokr. is known from localities on Mt. Martyukhina and Mt. Aydachihe (Kuznetsk Alatau), in the Sanashtykgol and Karakol creeks (Western Sayan), and on the Shivelig-Khem River (Tuva), where this form is confined to the Sanashtykgol Horizon; in other areas of the Sayan–Altai region, however, it descends into the Kameshki Horizon.

The genus Bonnaspis Resser occurs in America in the Middle Cambrian; on the Siberian Platform it is distributed in the Atdabanian Horizon, that is, below the Sanashtykgol Horizon. On the Siberian Platform Bonnaspis Resser occurs together with Botomella ekaterinae Suv. Since the latter form in the Sayan–Altai region is confined to the Kameshki–Sanashtykgol level, we provisionally assign Bonnaspis Resser to the same level.

Chondragraulos sp. occurs in Siberia in the Lower Cambrian and in the lower part of the Middle Cambrian.

The genus Eoptychoparia Rasetti is distributed in America and in the Sayan–Altai region in the upper subdivision of the Lower Cambrian.

Gaphraspis sp. is known in the Kuznetsk Alatau and on the Siberian Platform in the lower part of the Middle Cambrian, and in Altai—in the upper part of the Lower Cambrian (Kayancha Formation). But since our specimen is small and has a poorly expressed segmentation of the glabella, that is, it is a young form, we accept its earlier existence.

Corvexochus sp. is known in many countries of the world, including in the Soviet Union, in the Middle Cambrian, more rarely in the lower part of the Upper Cambrian.

From the above analysis of the fauna it is evident (Table 2) that the faunal assemblage includes forms occurring only in the Sanashtykgol Horizon, “index” forms, forms “appearing for the first time,” and “transitional” forms.

On the basis of the foregoing, one may draw the conclusion that the basal member of the Usun Formation was formed in Sanashtykgol time.

Table 2

References

1. Atlas of guide forms of the fossil fauna and flora of Western Siberia. Vol. 1, 1955.

2. Biostratigraphy of the Paleozoic of the Sayan–Altai mountain region. Vol. 1, 1960.

3. Ya. G. Kats, B. N. Krasilnikov, A. A. Mossakovsky, E. D. Suleidi-Kondratyev, N. N. Kheraskov. Stratigraphy of Paleozoic deposits of the Minusinsk basin and its mountain frame. Materials on regional geology, All-Union Aerogeological Trust, issue 4, 1958.

4. L. N. Repina, V. V. Khomentovsky, I. T. Zhuravleva, A. Yu. Rozanov. Biostratigraphy of the Lower Cambrian of the Sayan–Altai folded region. Academy of Sciences of the USSR, Siberian Branch, Institute of Geology and Geophysics, 1960.

5. N. P. Suvorova. Trilobites of the Corynexochida and their historical development. Transactions of the Paleontological Institute, vol. 3, 1964.