ON THE PALEONTOLOGY OF ANCIENT STRATA IN THE NORTHWESTERN PART OF THE EASTERN SAYAN

T. V. Yankauskas

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

This article is a brief summary of the results of a study of the morphology and phylogeny of a new group of archaeocyathid-like organisms, whose fossil remains were first discovered by K. V. Radugin in ancient deposits of the Krasnoyarsk Ridge, in the middle course of the Mana River. In preliminary descriptions [4, 6] they are regarded as primitive Precambrian ancestors of Cambrian archaeocyaths. The beds with the indicated fossils (the Akhor Formation) occur considerably below the group of strata characterized by trilobites of the Bulaiaspis group of the Lower Cambrian (the Leiba and Kolba formations), with a number of strata between them (!). Therefore the opinion of their discoverer, K. V. Radugin, concerning the Precambrian age of this new, most ancient skeletal fauna deserves attention, although one cannot fail to note the presence here of hyoliths, epiphyton-like and other calcareous algae, as well as small specimens of archaeocyaths, Tumuliolynthus sp., Ajacicyathus sp. Stratigraphically below lie the thick Ungut and Anastasino formations, also characterized by calcareous algae, hyoliths, and brachiopods. On this basis the author expressed the idea of an Early Cambrian age for the Akhor Formation, which we had previously regarded as the upper part of the Ungut [7].

We propose to call the new group of organisms pterocyathids (Pterocyathida ord. nov.) and to define it as a new order of archaeocyathid-like animals. The systematic position of this order has not yet been strictly established; however, the detailed study of this group of fossils carried out by the author makes it possible to conclude that it is very probably related to a new class of archaeocyaths—cribricyathids [2] (Cribricyathae Vologdin).

The morphological similarity of the two indicated groups of fossils is beyond doubt. The walls of the cups of cribricyathids, according to A. G. Vologdin (the monograph “Cribricyathids of the Cambrian of the USSR” is in press), are built of transversely oriented lamellar formations (peripteratae) and skeletal axial elements (baculae) associated with them, which led to the formation of porosity of a lattice type. The cups of pterocyathids differ in the absence of longitudinal rod-like elements and are formed by closely set peripteratae (peripteratae), which, unlike analogous formations in cribricyathids, are characterized by a more regular form and are joined not by their inner, but by their outer margins. They are distinguished as: conical or cy-

the cylindrical outer part of the peripterata and its inner part—the bottom, separated by a pointed or rounded upper edge (Fig. 1).

The inner wall of cribricyaths is either porous or ribbon-like in structure—striate; in pterocyathids it is monolithic or transversely annulate (striate).

The study of the fauna has shown its very considerable morphological diversity. The skeletal remains studied by us belong to no fewer than 38 species of animals. The skeletons of pterocyathids have the appearance of strongly elongated, elegantly constructed, two-walled and one-walled cups, whose length usually amounts to 15–25 mm, with a width near the aperture of 0.4 to 0.6 mm, more rarely up to 2 mm. The thickness of the skeletal elements is 0.005–0.01 mm, more rarely up to 0.02 mm. Most forms are characterized by the presence of bilateral symmetry. Conventionally, one may distinguish a ventral (flattened or rounded) and a dorsal (comb-like or also rounded) side.

Fig. 1. Structure of the peripterata of pterocyathid cups in transverse (I) and longitudinal (II) sections (using Malaella basaica as an example): a—height; б—width; в—length; г—outer part; д—upper edge; е—bottom.

Following A. G. Vologdin, we call the transverse elements of the outer walls peripterata. It should be noted, however, that in the previously published works of K. V. Radugin [4, 6] and of the author [7] these formations were called “vesicles”; at present, however, after a thorough study of the pterocyathids, we have considered it necessary to replace this term with another. The point is that the term “vesicles,” as well as “vesicular tissue,” has already been applied to designate a special type of skeletal elements in archaeocyaths and coelenterates [1]. Vesicular tissue consists of shapelessly swollen lamellar formations and has nothing in common with the transverse ring-like elements of pterocyathids, which are always characterized by a strictly definite form for each species. In addition, these two types of skeletal tissue differ in their biological functions. Vesicular tissue is a secondary formation in the skeletons of archaeocyaths and corals; it serves to separate the dead part of the soft tissue from the living, functioning part—that is, it marks the dying off of part of the organism—whereas the transverse elements of pterocyathids serve to support the soft tissue growing in the near-apertural part of the cup and are connected with the development of the organism, with its growth. To call such different types of skeletal tissue by one term is, of course, inadvisable; therefore, for pterocyathids we use another term, already used for analogous skeletal elements of cribricyaths.

Within the order Pterocyathidae (ord. nov.) we propose to distinguish four families: Leibaellidae fam. nov., Vologdinophyllidae Radugin, 1964, Pterocyathidae fam. nov., Achorocyathidae fam. nov.

The family Leibaellidae fam. nov. includes the most primitive forms of pterocyathids. Their one-walled cups are constructed by peripterata of simple funnel-shaped form, with extremely thin rudimentary bottoms or without bottoms.

Genus Ramifer gen. nov. Narrow-conical one-walled cups, composed—

...by simple, bottomless peritremes of a funnel-like form. The transverse section is oval (Table 1, figs. 3, 4; Fig. 2, calyx б).

Genus Leibaella Jankauskas, 1964. Narrow-conical, single-walled calyces formed by peritremes with thin and short rudimentary bottoms. The transverse section is oval or loaf-shaped (Table 1, figs. 1 and 2; Fig. 2, calyx в).

Family Vologdinophyllidae Radugin, 1964. In general, narrow-conical (cylindrical in the adult stage), single-walled calyces, constructed of peritremes with well-developed long bottoms, which, extending inward into the calyx and curving toward the initial stage, partly or completely merge inside into a single, smooth inner part of the wall. A doubled calyx wall is formed.

Genus Vologdinophyllum Radugin, 1964. Calyces with doubled walls and an oval transverse section. The upper edges of the peritremes are pointed or sharply rounded (Table 1, figs. 5 and 6; Fig. 2, calyx г).

Genus Manaella Jankauskas, 1964. Calyces with a doubled wall and a heart-shaped transverse section. The upper edges are pointed (Table 1, figs. 7 and 8).

Genus Crispus gen. nov. The peritremes are rounded on one side, and on the other have sharply pointed upper edges (Table 1, fig. 9).

Genus Longaevus gen. nov. The peritremes are arranged asymmetrically relative to the longitudinal axis of the calyx, as a result of which their upper edges project on the outer surface in the ventrally flattened part and are as if pressed inward into the calyx on its dorsally convex side (Table 1, fig. 10).

Family Pterocyathidae fam. nov. Calyces elongated, narrow-conical, double-walled. The outer wall is peritremal, doubled, generally indistinguishable from the calyces of the family Vologdinophyllidae. The inner wall is monolithic and has the appearance of a longitudinal continuous trough-shaped plate developed in the dorsal and lateral parts.

Genus Pterocyathus gen. nov. The peritremes of the outer wall are characterized by upper edges rounded in the dorsal part and pointed in the ventral part. The transverse section is oval, loaf-shaped. The inner wall is of the usual trough-shaped form (Table II, figs. 6–8; Fig. 2, calyx д).

Genus Akademiophyllum Radugin, 1964. The peritremes of the outer wall have pointed upper edges on both the ventral and dorsal sides. The transverse section is oval. The inner wall is monolithic, trough-shaped (Table II, figs. 3 and 4).

Genus Archaeobullatus gen. nov. differs from the genus Akademiophyllum by the heart-shaped form of the transverse section (Table II, figs. 1 and 2).

Genus Laceratus gen. nov. The outer wall consists of peritremes arranged asymmetrically relative to the longitudinal axis of the calyx (as in Longaevus). The transverse section is oval. The inner wall is usual for this family (Table II, fig. 9).

Family Achorocyathidae fam. nov. Calyces double-walled, relatively large. The outer wall is simple (not doubled), peritremal, porous. The inner wall is transversely annular (striate).

Genus Achorocyathus gen. nov. The outer wall is peritremal, simple (not doubled). The peritremes of the outer wall have pointed upper edges and short, unconnected bottoms. Its transverse section is rounded. The outer parts of the peritremes bear small round and irregular pores. The inner wall is developed throughout the entire adult stage and consists of transverse funnel-shaped elements—striae. The striae have uneven edges, as a result of which, when they join one another...

pores—slits—are formed, oriented transversely (Table III, figs. 1–6; Fig. 2, cup e).

The genus Topolinocyathus gen. nov. differs from the preceding one in the local development of the inner rod-like wall, which has the form of a barrel, often situated asymmetrically (Table III, figs. 7–10).

Thus, pterocyathids of different systematic subdivisions differ significantly in the degree of complexity of their skeletons. However, all pterocyathids have identical initial stages in the form of monolithic single-walled hornlets with rounded tips, without any traces of attachment. The cross sections of the initial stages in all pterocyathids are round; the shape is most often irregularly sinuous, more rarely straight or horn-like curved, but always with smooth and irregular bends.

Pterocyathids, like ordinary archaeocyaths and coelenterates, passed through a larval stage of development. Free-swimming larvae soon acquired the ability to build a skeleton, the formation of which began with the appearance of a hemispherical calcareous capsule 0.03–0.04 mm in diameter. This capsule subsequently grew in the form of a sinuous single-walled hornlet (the initial stage of the cup). The length of the initial stage usually amounts to fractions of a millimeter; in the largest Achorocyathus it reaches a length of 1 mm. The wall thickness of the initial stage is equal to the first microns.

The further development of individual specimens proceeds depending on their systematic affiliation. In the most primitive Dubius uncatus sp. n., the adult stage differs little morphologically from the initial one. The cups of this species are usually characterized by thin, uneven, monolithic walls, very rarely showing an indistinct peripterate structure (Fig. 2, cup a).

The individual development of representatives of the primitive genus Ramifer also proceeds relatively simply. Upon reaching a diameter of 0.1–0.2 mm, transverse rounded riblets of very small height appear on the outer surface of their cups; these subsequently elongate and are transformed into the upper edges of bottomless peripterates (Fig. 2, cup b).

In the individual development of the genus Leibaella, three age stages are clearly distinguished. The smooth-walled and monolithic initial stage is replaced by a juvenile stage, which is morphologically indistinguishable from the adult stage of the genus Ramifer—it is composed of bottomless peripterates, generally funnel-shaped, with edges sharply projecting above the outer surface (Fig. 2, cup b). The length of the juvenile stage reaches 0.5–1 mm, more rarely more. From a diameter of 0.2–0.3 mm the adult stage begins, which is characterized by the appearance of very thin and short bottoms in the peripterates. The bottoms, at angles close to 45° to the longitudinal axis of the cup, extend inward and taper out sharply. As the adult stage grows, the bottoms gradually lengthen, but their thickness is usually less than the thickness of their outer parts (Fig. 2, cup v).

The most widely distributed single-walled forms with a doubled wall (fam. Vologdinophyllidae) begin their individual development in the same way as representatives of the genus Leibaella, passing through a horn-shaped, unsegmented initial stage and a stage of bottomless peripterates. However, the following stage, with thin rudimentary bottoms, is not yet the adult one. This is the second juvenile stage, which at a diameter of 0.3–0.4 mm passes into the adult stage, characterized by considerably developed bottoms that merge into a single smooth inner part of the wall (Fig. 2, cup g).

The individual development of double-walled pterocyathids of the family Pterocyathidae proceeds in an analogous manner. The difference consists in the presence in the latter of an internal, monolithic, grooved inner wall. It appears after the calicle has reached the stage of doubled walls (diameter 0.3–0.5 mm). The inner wall separates smoothly from the dorsal and lateral parts of the inner surface of the calicle wall. In other cases it appears “suddenly,” at once at a considerable distance from the dorsal part of the calicle. The double-walled stage is considered adult. Thus, the double-walled pterocyathids of this family pass through five age stages; however, it should be noted that the second and third juvenile stages are not always clearly manifested (Fig. 2, calicle d), being often composed of bottomless peripterates of irregular or disorderly form, while the appearance of the doubled wall is accompanied by a sharp increase in the diameter of the calicle.

The development of skeletal tissue proceeds somewhat specially in representatives of the family Achorocyathidae (Fig. 2, calicle e). The initial stage is usually straight and thin-walled, up to 2 mm in length. The thickness of its walls is 0.01–

Fig. 2. Evolution of pterocyathids (explanations in the text).

0.015 mm. From a diameter of 0.1–0.2 mm, peripterate wall structure appears. The annular inner wall appears from a diameter of 0.2–0.3 mm, sometimes later. Pores appear only in the adult (double-walled) stage.

Thus, pterocyathids are distinguished by a certain commonality of structure. The similarity of the early stages of their individual development, as well as the morphological repetition in the early stages of some forms of the adult stages of others, indicates their close genetic relationship.

To study genetic relationships and to trace the evolution of pterocyathids, bed-by-bed collections of them were made in the excellently exposed stratotype section of the Akhor Formation, on the right side of the valley of the Elovka River, 6500 m from its mouth (the Elovka flows into the Mana River within the southeastern outskirts of the settlement of Bolshoi Ungut). More than five thousand specimens were collected here.

The study of the collected material makes it possible to conclude that pterocyathids underwent vigorous evolution in Akhor and post-Akhor time, from primitive single-walled, bottomless forms to complexly constructed double-walled ones.

In Fig. 2 are shown, arranged side by side, the cups of pterocyathids of different stratigraphic positions and different evolutionary levels. Cups a, b, c come from the lower member of massive black limestones. These are representatives of the most primitive family, Leibaellidae fam. nov. They occur together, even within a single polished slab. However, the features of their individual development allowed us to place them in the figure in a strictly definite sequence corresponding to the direction of their evolution. Cup g is a longitudinal section of the genus Vologdinophyllum, the most widespread in the middle part of the formation (the typical genus of the family Vologdinophyllidae). Cup d comes from the upper beds of the formation (Pterocyathus). This is the typical genus of the family Pterocyathidae, widely distributed here. Cup e is a drawing of a longitudinal section of Achorocyathus, which was found in the deposits of the Krol Formation, sharply unconformably overlying the Akhor Formation.

It is evident from the diagram that the primitive forms are situated stratigraphically below, and the more highly organized ones above. In addition, the diagram shows (dotted lines) how, in the course of evolution, the adult stages of the more simply constructed and more ancient forms become incorporated into the early stages of the younger and more complex morphological forms. In their individual development, the relatively younger forms pass through a series of age stages, each of which, in its morphological features, resembles or is indistinguishable from the adult stages of previously existing forms collected from more ancient deposits.

These facts indicate a vivid manifestation in the development of pterocyathids of one of the basic biological laws—the law of the repetition of the phylogeny of animals in their ontogeny (the Müller-Haeckel law), consideration of which made it possible to clarify the genetic relationships of primitive forms from the lower member of the Akhor Formation (for which, because of poor exposure and the massive character of the rocks, it proved impossible to make bed-by-bed collections), and also to verify phylogenetic schemes constructed for younger groups of pterocyathids.

Thus, the evolution of pterocyathids proceeded in a definite direction—an increase in size and complication of the skeleton. Each major leap in the continuous-discontinuous process of pterocyathid evolution is accompanied by the appearance of a new family (Fig. 3), while the more ancient forms either continue to exist or become completely extinct.

We see the causes of such vigorous evolution in the extremely restless tectonic regime at the time of deposition of the Ungut, Akhor, and a number of subsequent sequences, as indicated by the large number of local

and regional interruptions in sedimentation, accompanied by more or less significant erosion of the deposited strata. For such areas, a frequent change of facies in space and time is quite natural; this, undoubtedly, must have affected the state of the organic world, so inseparably linked with the conditions of existence, with the external environment.

Fig. 3.
Scheme of the phylogeny of the pterocyathids of the Eastern Sayan: 1 — Dubius unsatus (hypothetical common ancestor of the pterocyathids); 2 — family Leibaellidae fam. nov.; 3 — family Vologdinophyllidae fam. nov.; 4 — family Pterocyathidae fam. nov.; 5 — family Achorocyathidae fam. nov.

The evolution of the pterocyathids does not end here. In the overlying deposits of the Lower and Middle Cambrian, their “descendants”—cribricyathids [2]—are relatively widely distributed.

The rapid evolution of the pterocyathids makes it possible to distinguish, within the Lower Cambrian deposits of the Krasnoyarsk Range, a number of paleontological-

Plate I

Figs. 1 and 2 — Leibaella clovica Jankauskas, 1964: 1 — part of a longitudinal section, thin section No. 70, spec. 6, ×20; 2 — transverse section, thin section No. 39, spec. 4, ×30. Figs. 3 and 4 — Ramifer giratus sp. nov.: 3 — part of a longitudinal section, thin section 265, spec. 5, ×20; 4 — drawing of the same specimen. Figs. 5 and 6 — Volvgdinophyllum ciachlovi Radugin, 1962: 5 — oblique-longitudinal section, thin section 80, spec. 2, ×20; 6 — transverse section of another specimen, thin section No. 466, spec. 1, ×40. Figs. 7 and 8 — Manacella basica Jankauskas, 1964: 7 — oblique-longitudinal section, thin section No. 239, spec. 6, ×20; 8 — transverse section of another specimen, thin section No. 318, spec. 7, ×30. Fig. 9 — Crispus subdimidiatus sp. nov., holotype, oblique-longitudinal section, thin section No. 300, spec. 1, ×20. Fig. 10 — Longaevus vitalis sp. nov., holotype, part of a longitudinal section, thin section No. 460, spec. 8, ×20.

Plate II

Figs. 1 and 2 — Archaeobullatus cereiformis sp. nov.: 1 — longitudinal section, thin section No. 303, spec. 3, ×20; 2 — transverse section, thin section No. 205, spec. 1, ×20. Figs. 3–5 — Akademiophyllum rotundus sp. nov.: 3 — part of a longitudinal section, thin section No. 355, spec. 5, ×20; 4 — transverse section, thin section 332, spec. 1, ×40; 5 — obliquely longitudinal section, thin section No. 142, spec. 3, ×20; 7 — nearly obliquely longitudinal section, thin section No. 161, spec. 1, ×20; 8 — transverse section, thin section No. 316, spec. 6, ×30. Fig. 9 — Laceratus cuneatus sp. nov., obliquely longitudinal section, thin section No. 318, spec. 2, ×20.

Plate III

Figs. 1–6 — Achorocyathus perbellus sp. nov.: 1 — oblique longitudinal section, thin section no. 62—26—B, specimen 1, ×30; 2 — drawing of the same specimen; 3 — oblique longitudinal section, thin section no. 62—26—B, specimen 2, ×20; 4 — drawing of the same specimen; 6 — transverse section, thin section no. 62—26—B, specimen 3, ×30; 5 — drawing of the same specimen. Figs. 7–10 — Tyrolinocyathus gen. nov., two longitudinal sections of cups and drawings of these same specimens, thin section 62—26—B, specimens 5 and 6, ×30. Figs. 11 and 12 — photograph and drawing of a transverse section of a cup of Achorocyathus sp.; pores are visible, thin section 62—26—A, ×30.

... zones, each of which corresponds to a definite stage in the development of the most ancient animals.

1. The first zone embraces the lower beds of the Akhor Formation, where only primitive forms of the family Leibaella occur. These beds, black argillaceous limestones, are widely distributed in the watershed area of the Bol’shoi Ungut and Leiba rivers, in the middle course of the Murtyk River, and in the watershed area of the Kolba and Zherzhul rivers. We propose to name this zone the Ramifer—Leibaella zone, after the names of the genera most widespread here, which do not occur higher (or lower) in the section.

2. The Vologdinophyllum—Pterocyathus zone embraces the entire Akhor Formation, with the exception of its lower member, and is divided, according to the composition of the pterocyathids, into two subzones:

a) the Crispus—Vologdinophyllum subzone includes the middle part of the formation, which lithologically is represented by black thin-bedded and thick-bedded limestones and calcareous conglomerate-breccias, passing along strike into white massive epiphyton limestones (these rocks are distributed in the basins of the B. Ungut, Korbik, Bazaikha, Kolba, Murtyk, Zherzhul, and Elovka rivers); the pterocyathids are represented exclusively by forms with a double wall (family Vologdinophyllidae);

b) the Pterocyathus subzone. It embraces the upper beds of the Akhor Formation, preserved from erosion in the area of its stratotypic section and on the watershed of the Bazaikha River and its left tributary, the Korbik, and is represented by black thin-bedded limestones, more rarely indistinctly bedded or massive limestones (Bazaikha River). Here, along with single-walled Vologdinophyllidae, two-walled forms (family Pterocyathidae) are widely (and predominantly) distributed.

1. The Achorocyathus zone includes the entire Krol Formation. The formation is distributed within the basins of the B. Ungut, Leiba, Murtyk, and Zherzhul rivers. Its paleontological content consists of pterocyathids of the family Achorocyathidae, as well as isolated and rare representatives of the genus Vologdinophyllum.

The paleontological zones (and subzones) that have been distinguished correspond to four major stages in the historical development of the pterocyathids. The presence of a large number of forms with restricted vertical distribution gives this group practical significance for purposes of stratigraphy and regional correlation. The pterocyathids are not purely endemic forms; their first finds have been made in the Ust’-Kundat Formation of the Kuznetsk Alatau, which provides grounds for speaking of the stratigraphic significance of the pterocyathids.

REFERENCES

1. A. G. Vologdin. Archaeocyaths. Atlas of index forms, Part 1, Cambrian. Gosgeolizdat, 1940.

2. A. G. Vologdin. Archaeocyaths and algae of the Cambrian of the Baikal Upland. Publishing House of the Academy of Sciences of the USSR, 1962.

3. I. T. Zhuravleva. Archaeocyaths of the Siberian Platform. Publishing House of the Academy of Sciences of the USSR, 1960.

4. K. V. Radugin. On early forms of archaeocyaths. Materials on the Geology of Western Siberia. Tomsk State University Publishing House, Tomsk, issue 63, 1962.

5. K. V. Radugin. Problems of the Proterozoic of Central and Western Siberia in the works of V. A. Obruchev. In: Ideas of Academician V. A. Obruchev on the geological structure of Northern and Central Asia and their further development. Publishing House of the Academy of Sciences of the USSR, Moscow, 1963.

6. K. V. Radugin. On a new group of the most ancient animals. Geology and Geophysics, No. 1, 1964.

7. T. V. Yankauskas. On some problematic organic remains from the Lower Cambrian of the Eastern Sayan. Proceedings of the conference dedicated to the 100th anniversary of the birth of Academician V. A. Obruchev and the 80th anniversary of the birth of Academician M. A. Usov. Tomsk State University Publishing House, Tomsk, 1964.