ON THE DISCOVERY OF MIDDLE CAMBRIAN TRILOBITES IN THE AREA OF THE VILLAGE OF SUKHAYA ERBA (BATENEVSKY RIDGE)

V. L. Tomashpolskaya, R. B. Karpinsky

(Presented by the scientific seminar of the Department of General Geology)

In 1958 the authors discovered a trilobite fauna of the family Oryctocephalidae Beecher in deposits of the polycyathid horizon in the area of the village of Sukhaya Erba.

The area of the village of Sukhaya Erba (Mt. Dolgii Mys) was first studied by Ya. S. Edelstein, who as early as 1926 discovered here, in limestones, a rich fauna of archaeocyaths and trilobites [7]. From the archaeocyaths A. G. Vologdin [2] established an entirely new family of their colonial representatives, which received the name Polycyathidae, or Erbocyathidae, and the “horizon” itself, containing such archaeocyaths, was called the polycyathid, and subsequently [4] the Obruchev horizon. A list of archaeocyaths from these deposits is given by A. G. Vologdin [2].

In 1940 the area of Mt. Dolgii Mys was visited by A. G. Vologdin and K. V. Lermontova [1]. They established that above the limestones of Mt. Dolgii Mys lie sandstones and shales. East of the limestones of Mt. Dolgii Mys, on Mt. Ostraya and Mt. Chastye Gorki, among these rocks they discovered two limestone lenses 10 and 30 meters thick. In the latter they found a trilobite and brachiopod fauna close to the fauna of the Obruchev horizon. In 1953 V. M. Yaroshevich discovered archaeocyaths of the Obruchev horizon in the limestones of Mt. Dolgii Mys and in limestone lenses in the area of Mt. Ostraya.

A. G. Vologdin and E. V. Lermontova determine the age of this horizon as the very base of the Middle Cambrian. N. S. Zaitsev and N. V. Pokrovskaya [3] assign it by age to the upper part of the Lower Cambrian.

As a result of the authors’ work in 1958 in the Batenevsky Ridge, data were obtained that make it possible to refine the age of the polycyathid horizon. We mapped the area of Mt. Dolgii Mys (elevation 583.0) and Mt. Ostraya (elev. 628.8) and collected a fauna of archaeocyaths and trilobites from all lenses and interbeds of limestone. On the geological map compiled, it is clearly evident that the massive white, light-gray, and light-beige organogenic limestones composing Mt. Dolgii Mys and its spurs represent an enormous massif, which to the east on Mt. Ostraya (elev. 623.8) and along its western and southern spurs is replaced by an effusive-sedimentary sequence composed chiefly of felsites, felsite-porphyries and their tuffs, siliceous shales, and more rarely sandstones and conglomerates with lenses of light massive limestones and interbeds of very thinly platy…

Schematic Geological Map
of the area of Mt. Dolgii Mys

Legend:
1. Massive white and coffee-colored light- and dark-brown limestones.
2. Green schists.
3. Felsites, felsite-porphyries, their tuffs, siliceous schists, more rarely sandstones and limestones.
4. a) Covers of Devonian diabase porphyrites; b) basalt conglomerate with polymictic pebbles.
5. Disjunctive fault.
6. Localities of finds of faunal remains: 1) archaeocyathids, trilobites, brachiopods; 2) oryctocephalids.

brownish-black and dark-gray limestones. The thickness of the interbeds ranges from 5 to 20 m. The limestone lenses are of various sizes: their length ranges from several meters to a hundred meters, and their thickness from units to tens of meters. In the lenses the limestones are light-colored, massive, and contain a fauna of archaeocyathids, trilobites, and brachiopods.

In the appended list, for the points indicated on the map, determinations are given for the trilobite fauna and for some forms of archaeocyathids¹.

1. Archaeocyathus cf. subradiatus Vologd. (m. m. k—38).
2. Archaeocyathus cf. radiatus (Tayl.) m. 870.
3. Erbocyathus chakassiensis Jar. (m. m. 870, 875, k—38, 862).
4. Erbocyathus obrutschevi (Vologd.) m. 878.
5. Erbocyathus sp. (m. k—38, 870, 875, 878).
6. Ethmofyllum cf. katunicum Krasn. (m. 860).
7. Tegerocyathus cf. erbiensis Krasn. (m. 860).
8. Vologdinocyathus cf. erbiensis Jar. (m. 870).
9. Edelsteinaspis ornatus Lerm. (m. k—38, A—86, B—97, 862, 870).
10. Erbia sibirica (Schm.) m. m. k—38, B—97.
11. Erbia granulosa Lerm. m. B—84, 862.
12. Kootenia vologdini Lerm. (m. k—38, A—86, B—97).
13. Kootenia globosa sp. nov. (m. k—38, B—84, B—97, 320, 862, 870).
14. Kooteniella cellulifera Lerm. (m. k—38, A—86, 320).
15. Pagetia cf. ferox Lerm. (m. B—97).
16. Chondranomocare sp. (m. B—97).

The entire above list of archaeocyathids and trilobites is close to the Obruchev complex of the Sayan–Altai region [4].

In interbeds of thin-platy dark-coffee-colored limestones at various stratigraphic levels within the effusive–shale–carbonate sequence replacing the limestones of Mt. Dolgii Mys, numerous remains of trilobites from the family Oryctocephalidae were found (points 863, 876, 314). The fauna in them was discovered on bedding planes. Small cephalic shields and single entire dorsal shields, or their impressions, predominate. The fauna as a whole is uniform and is represented by only two forms: all the larger forms belong to the new genus Paraoryctocephalops plana gen. nov. sp. nov., and the small ones to the new genus Oryctocarella sibirica gen. et sp. nov. Judging from the brief description given by N. E. Chernysheva [11], representatives of two rare genera of the family Oryctocephalidae—Oryctocara and Tonkinella—were found under similar conditions in the deposits of the basin of the Yangud River (a left tributary of the Vitim River). This fauna there was collected in thin-platy dark-gray limestones occurring among massive and thin-platy gray limestones with algae; it was also found on weathered bedding surfaces and is very uniform.

According to the materials collected by us in the area of the village of Sukhaya Erba, representatives of the family Oryctocephalidae: Oryctocarella sibirica and Paraoryctocephalops plana, were found within that sequence which, on the basis of the polilithid and trilobite fauna, was assigned to the uppermost Lower Cambrian [3].

Representatives of the family Oryctocephalidae were not known in these deposits. With regard to the stratigraphic distribution of this family, N. E. Chernysheva writes: “The finding of remains belonging to the subfamily Oryctocephalinae² is, with a high degree of probability, connected—

indicates the Middle Cambrian age of the deposits,” and further: “Beds with the fauna of Oryctocephalinae constitute a more ancient horizon than the Paradoxides oelandicus zone of Scandinavia and are among the most ancient Middle Cambrian deposits of Siberia” [11].

As will be seen from the description of the forms found in the area of the village of Sukhaya Erba, the new genus established by us, Oryctocarella sibirica, differs in no essential way from Oryctocara geikiei Walcott, known in the literature from Middle Cambrian deposits of the state of Idaho, and Oryctocara lata N. Tchern., from the Middle Cambrian deposits of the Yanguda River, and is also similar to Oryctocara sp., identified by E. V. Lermontova [5] from the Cambrian of the Anabar massif. This genus Oryctocara was found in black bituminous shales, whose age she determines as the lower half of the Middle Cambrian (Spence shale of North America). Together with these forms, representatives of the genus Oryctocephalops were found in the same beds; the closest analogue of these in the deposits of the Dolgii Mys area is the new genus Paraoryctocephalops. Together with the forms Oryctocara and Oryctocephalops in the shales of the Anabar region, E. V. Lermontova established Oryctocephalina reticulata and Oryctocephalus reinoldisformis [5], complete analogues of which were found by us in unquestionably Middle Cambrian deposits of the area of the town of Sladkie Korni (Potekhin district).

Hupe (Hupé, 1955) indicates for this family a broader interval of stratigraphic distribution, allowing the appearance of its representatives in the Lower Cambrian. Such an interpretation is based on his doubt concerning the limits of the vertical distribution of the genus Oryctocephalina Lermontova. However, after the works of K. K. Demokidov, N. P. Lazarenko, and V. E. Savitskii [6], such doubt falls away, and one may confidently speak of the exclusive confinement of reliable representatives of the family Oryctocephalidae to Middle Cambrian deposits.

All the geological and paleontological data presented make it possible, with a high degree of confidence, to assign the deposits of the Dolgii Mys area, containing in the light-colored limestones a fauna of trilobites and archaeocyaths of the Obruchev complex, and in interbeds of dark-brown limestones representatives of the family Oryctocephalidae, to the very lowermost Middle Cambrian.

Description of the fauna

Fam. Oryctocephalidae Beecher, 1897, emend. Raymond, 1913.

Diagnosis. “Glabella subquadrate to suboval, reaching the marginal ridge. Glabellar furrows replaced by pits. Ocular ridges distinct; width of fixed cheeks usually great. Thorax of 5—7—11 segments. Pygidium large; interpleural and pleural furrows well expressed; margin entire or with spines. Georgian and Acadian.” (Hupé, 1955).

Composition of the family according to Hupé

1. Genus Oryctocephalus Walcott, 1886. Cm₂ lower part and middle. In the west of North America, in Australia, East Asia, Siberia, India.
2. Genus Oryctocephalina Lerm., 1940, Cm₂ (?)
3. Genus Oryctocephalites Resser, 1939, Cm₂, North America
4. Genus Oryctocara Walc., 1908, Cm₂, North America.
5. Genus Tonkinnella Mansuy, 1916, Cm₂, North America, East Asia, Kashmir.

The vertical distribution of the family according to Hupé was discussed above in the stratigraphic part of the essay.

Genus Oryctocarella gen. nov.

Genotype. Oryctocarella sibirica gen. et sp. nov. Batenevsky Ridge of the Kuznetsk Alatau. Lower Middle Cambrian.

Diagnosis. Oryctocephalidae with a broad, flat dorsal shield, a large cephalic and pygidial shield, and a short thorax consisting of eight segments. The glabella is long, cylindrical, and has four pairs of furrows. In the first pair, the furrows join the dorsal furrows and are short; all the remaining furrows have the form of paired adjacent pits; the pygidial shield is entire-margined, with a fan-shaped arrangement of segments around the axis.

Comparisons. The new genus is very close to the genus Oryctocara and differs from it in the following characters. In Oryctocarella: a) there are four pairs of furrows on the glabella; b) the thorax in Oryctocarella is comparatively broader and shorter; instead of the 11 segments indicated for the genus Oryctocara, our forms have 8 segments, and therefore the length of the thorax is only 1.5 times greater than the length of the head, whereas in species of Oryctocara the length of the thorax is 2 times greater than the length of the head.

Distribution. Batenevsky Ridge of the Kuznetsk Alatau range. Area of Mount Dolgii Mys. Lower Middle Cambrian.

Oryctocarella sibirica gen. et sp. nov. Plate I, figs. 1—6.

Material. More than 10 specimens of dorsal shields, complete or incomplete.

Holotype is illustrated in Plate I, fig. 2. Specimen 863 (1) 58, kept in the Museum of Geology of TPI, Tomsk.

Description. The dorsal shield is elliptical in outline and flattened. The length of the cephalic shield is slightly greater than the length of the tail, and the thorax is only 1.5 times longer than the head. The cranidium is subrectangular. The glabella is flat, long, cylindrical. The length of the glabella is twice its width. The dorsal furrows are narrow and deep. The lateral furrows of the glabella are expressed as four pairs. The very anterior pair of furrows is visible only on the sides of the glabella near its anterior margin; these furrows are very short, deep, and join the dorsal furrows. The remaining three pairs have the character of distinct round pits, not in contact with the dorsal furrows and connected pairwise by narrow transverse furrows. The occipital furrow is narrow and straight. The occipital ring is of uniform width, narrow, convex. The fixed cheeks are almost flat and broad. The posterolateral lobes are short. The posterior border furrow is very deep and narrow; the posterior border from the dorsal furrows widens noticeably and is bent downward. The anterior border has not been preserved. The free cheeks have not been preserved.

The thorax is flat and broad, its length being 1.5 times greater than the length of the head. The greatest width of the thorax is in the middle; it is twice its length. The number of thoracic segments is 8. The rachis is rounded in cross section, projecting above the pleurae, and very narrow. Its width amounts to 2/5 of the greatest width of the thorax. The rings of the rachis are convex, straight, and separated by deep furrows. The pleurae are flat and straight; the pleural and interpleural furrows are parallel and almost equal in width and depth. The ends of the pleurae are blunt.

The pygidial shield is small, semicircular, somewhat stretched in width. The rachis is narrow, sharply tapering, not reaching the outer margin; the width of the rachis equals 1/4 of the total width of the pygidial shield. In length the rachis occupies about 4/5 of the length of the pygidial shield. On the rachis 6 rings are distinctly established, of which the first three are clearly delimited and inseparable from the thoracic axial rings; the last three rings are less distinct. The pleural ribs in the anterior part of the pygidial shield almost do not differ from the last thoracic pleurae and have transverse-

Oryctocarella sibirica gen. nov., sp. nov.

Fig. 1 ×3  Fig. 2 ×4  Fig. 3 ×3

Fig. 4 ×3  Fig. 5 ×2  Fig. 6 ×3

Paraoryctocephalops plana gen. nov., sp. nov.

Fig. 7 ×3  Fig. 8 ×2  Fig. 9 ×2

direction, and in general the ribs fan out and evenly surround the posterior end of the rachis. The posterior ribs are indistinctly delimited.

Comparisons. Oryctocarella sibirica has characters similar to species of Oryctocara. Thus, for example, in Oryctocarella sibirica and in Oryctocara lat the glabella is barely noticeably widened toward the anterior margin, or else has parallel sides. But the glabella in Oryctocara is almost isometric, whereas in Oryctocarella sibirica the glabella is narrow and long.

The dorsal shield of Oryctocarella sibirica differs little in width from that of O. lata. The ratio of width to length of the dorsal shield in Oryctocarella sibirica is somewhat less than 4/5; in Oryctocara lata it is equal to 4/5, and in Oryctocara geikiei it is almost equal to one-half.

In the dimensions of the pygidium, Oryctocarella sibirica is more similar to Oryctocara geikiei Walc.

Occurrence. Western Siberia—Batenevsky Ridge of the Kuznetsk Alatau. Area of Mount Dolgii Mys, specimen 863. Interbeds of sandy dark limestones in the clayey-tuffaceous unit. Lower part of the Middle Cambrian.

PARAORYCTOCEPHALOPS gen. nov.

Genotype. Paraoryctocephalaps plana gen. et sp. nov.

Description. Dorsal shield of small and medium size, with a weakly rounded anterior margin and an almost straight posterior margin. Cranidium subrectangular, flat or very weakly convex; glabella flat, somewhat elongated, widening toward the anterior rounded end and noticeably projecting into the area of the limb. The length of the glabella is almost twice its width at the base, and its width at the posterior margin amounts to 2/3 of its greatest width at the anterior margin.

The dorsal furrows are shallow, narrow, and diverge anteriorly.

On the glabella, instead of the pits usual for the family Oryctocephalidae, there are distinctly expressed four pairs of pit-like transverse furrows. All the furrows, except the anterior first pair, do not reach the dorsal furrows and do not join in the middle. An exception occurs only for the posterior pair of furrows, which in individual specimens is connected by a transverse furrow. The anterior pair of furrows, unlike all the others, reaches the dorsal furrows and is situated near the very anterior margin of the glabella; opposite it the ocular ridges approach the glabella. By all four pairs of furrows the glabella is divided into equal parts.

The occipital furrow is deeper laterally, in the middle shallower and somewhat curved backward. The occipital ring is very narrow and lies at the level of the posterior part of the glabella.

The fixed cheeks are very broad, subtriangular in shape. Their width in the middle is almost equal to the width of the glabella at the same level; in the di-

posteriorly they widen very slightly. The posterior marginal furrow is narrow, deep, and straight. The posterior marginal border is very narrow. The ocular ridges are slightly oblique and distinct. The anterior branches of the facial sutures are parallel; the posterior branches are very short and strongly divergent.

The frontal limb is slightly concave, broader at the sides and markedly narrower in front of the glabella because the glabella projects forward.

The marginal border is barely noticeable, in the form of a very narrow ridge. The surface of the carapace is smooth. The free cheeks have not been preserved.

In the fragments of the thorax accompanying the cranidia described above, and most probably belonging to them, the ends of the pleurae bend backward and at the same time become separated and probably terminate in spines (the latter have not been preserved). The pleural furrows are broad and deep and tend to become oblique near the rachis. The interpleural furrows are narrow. The rachis of the thorax narrows noticeably posteriorly.

Comparison. The genus described here, Paraoryctocephalops gen. nov., possesses all the basic characters of the family Oryctocephalidae and most closely resembles the genus Oryctocephalos Lerm. in the following principal features:

1) in the character of the segmentation of the glabella: the presence of pit-like transverse furrows that do not reach the dorsal furrows and are most often not joined in the middle;

2) in the broad and flat cheeks;

3) in the flat thorax with nearly straight or slightly oblique and deep pleural furrows;

4) in the narrow rachis of the thorax.

From all genera previously known in the literature that belong to the family Oryctocephalidae, and from the most similar of them, Oryctocephalops Lerm., the genus Paraoryctocephalops described by us differs sharply in the following features.

1) In P. Paraoryctocephalops there is a frontal limb, whereas in all known representatives of the family Oryctocephalidae there is no limb in front of the glabella. The presence of a frontal limb apparently indicates a somewhat older age of our genus in comparison with the genera of this family described in the literature.

2) The rachis of the thorax narrows posteriorly much more sharply.

3) The glabella is relatively more expanded at the anterior margin.

All these differences allow us to distinguish our forms as a new genus.

Distribution. Western Siberia, Kuznetsk Alatau, Batenevsky Ridge. Area of Mount Dolgii Mys. Lower Middle Cambrian.

PARAORYCTOCEPHALOPS PLANA gen. et sp. nov. pl. I, figs. 7—10.

Material. Ten cranidia and several fragments of the thorax.

Holotype. Illustrated on pl. I, fig. 8, specimen No. 863 (3)/58, housed in the Museum of Geology of TPI.

Description. Corresponds to the genus.

Dimensions in mm (spec. 863³)

Width of the limbus anteriorly — 0.7
Width of the limbus on the sides — 1.2

Locality. Western Siberia. Batenevsky Ridge, Kuznetsk Alatau. Area of Mount Dolgii Mys, outcrop 863. Interbed of impure dark limestones in a clayey tuffaceous sequence. Lower part of the Middle Cambrian.

LITERATURE

1. Vologdin A. G. On the stratigraphy of the eastern slope of the Kuznetsk Alatau. Soviet Geology, No. 27, 1947.

2. Vologdin A. G. Archaeocyathids. Atlas of Index Forms of the USSR, vol. 1, Cambrian. 1940.

3. Zaitsev N. S. and Pokrovskaya N. V. On the structure of the Kuznetsk Alatau. Soviet Geology, No. 6, 1958.

4. Krasnopeeva P. S. Archaeocyathid and archaeocyathid-trilobite horizons of the Cambrian of the Altai–Sayan region. Materials on the Geology of Western Siberia, issue 61, 1959.

5. Lermontova E. V. Class Trilobita. Atlas of Index Forms of the USSR, vol. 1, Cambrian, 1940.

6. Tkachenko B. V., editor; Demokidov K. K., Savitskii V. E., Lazarenko N. P. Stratigraphy of the Sinian and Cambrian deposits of the northeast of the Siberian Platform. Transactions of NIIGA, vol. 101, 1959.

7. Edelstein A. S. On some new data on the geology of Siberia. Proceedings of the Geographical Institute, issue 6, 1926.

8. Kobayashi T. The Cambro–Ordovician Formations and Faunas of South Chosen. Journ. Fac. Scien. Univ. Tokyo, vol. IX, pt. 2, 1935.

9. Resser Ch. Smithsonian Miscellaneous Collections, vol. 97, No. 10, 1938; vol. 97, No. 12, 1939a; vol. 93, No. 24, 1939b.

10. Walcott Ch. Cambrian Geology and Paleontology. Smiths. Misc. Coll., vol. 53, 1908; vol. 64, No. 3, 1916a; vol. 64, No. 5, 1916b.

11. Chernysheva N. E. On the discovery of Middle Cambrian trilobites in the Yangud River basin, 1952.

1. The archaeocyathid fauna was determined by A. G. Pospelov; the trilobite fauna by V. D. Tomashpolskaya. ↩

2. According to the latest classification (Fundamentals of Paleontology, collection of VSEGEI, 1959), the subfamily Oryctocephalinae is not distinguished. ↩