M. Sarkisov

Materials toward the knowledge of the fauna of sea urchins from the Cretaceous deposits of the Kopet-Dagh Range, Transcaspian Region

Preface. The material for the present work was a small collection of remains of sea urchins from the Cretaceous deposits of the Kopet-Dagh Range of the Transcaspian Region, where in the summer of 1914 Prof. M. A. Usov, A. S. Danilovich, and I were sent at the expense of the Tomsk Technological Institute (250 rubles). Our trip pursued a twofold aim: 1) general geological investigations in the area of development of Mesozoic deposits of the Kopet-Dagh Range, and 2) the collection of paleontological material in a locality not yet geologically studied, for supplementing the Institute collections. In view of the understandable difficulty that is always encountered when determining fossils only from descriptions and drawings, often unsatisfactory ones, and also in view of the absence in Tomsk of all the necessary literature, I cannot consider my work on the study of the named collection to have been carried out irreproachably; nevertheless I hope that all available materials have been used by me in working up the collection. I consider it my duty to express my sincere gratitude to Prof. M. A. Usov for his varied and very valuable guidance to me during the treatment of this material, and to the Tomsk Technological Institute for the funds allocated for the conduct of the geological field practice.

Preparation of the fossils. Before beginning the study of the remains of sea urchins, it was necessary to prepare most of them in the most thorough manner, since the surface of the fossils, coming from layers of marl, was usually covered with firmly adhering particles of rock, which concealed the structure of the ambulacral areas; and the latter are, in general, characteristic systematic features of sea urchins. This preparation took quite a long time; at first it proceeded especially slowly, until Prof. M. A. Usov acquired for the Paleontological Cabinet a drill machine, which made it possible finally to clean the fossils and bring them into a more or less definite condition. The preparation of the remains of sea urchins was for the most part carried out in the following sequence. First, rough preparation was applied, the purpose of which was to separate large pieces of extraneous mass from the fossil. The usual tools in this operation are hammers of various shapes and sizes and a chisel. The chisel is placed at the junction of the rock and the fossil, and by a light blow of the hammer on the chisel the undesirable element is broken off in such a way as not to disturb the integrity of the specimen; this last circumstance had to be taken especially into account, since almost all specimens are internal casts with a thin crust, and at the slightest careless blow the fossil may lose its paleontological value. When the fossil is almost freed from coarse incrustations, it is necessary to wash it; but better results may be achieved if the specimens are boiled for 2–3 hours, after which the mass adhering to the fossil loosens and is easily removed with a steel needle, chisels, and a knife. When a layer of rock of 1–1 1/2 mm remains on the shell of the urchin, we turn to the drill machine; the results of using the latter depend exclusively on the skill of the worker. The grinding itself must be done with a free movement of the hand, while avoiding strong pressure on the fossil. During grinding it is necessary to change the numbers of the burs several times, depending on the place and the character of the coating. And after the layer has been removed to the greatest possible thinness, without risk of damaging the fossil, one proceeds—

...then to the last agent—etching with hydrochloric acid. Dilute hydrochloric acid makes it possible to remove the remnants of adhering marly rock without causing a reaction on the fossil itself; after moistening with HCl, it is necessary each time to wipe the specimen with a cloth or rinse it with water. Work with hydrochloric acid is rather painstaking, but it already gives reliable results.

Of course, the collection contains quite a number of remains of sea urchins that already represent nuclei; their preparation may be rough, since such forms are determined only approximately.

Description of the fossils.

Regularia.

Fam. Diadematidae. Genus Pseudodiadema Desor 1856.

1. Pseudodiadema Tschapani nova species.

There is only one specimen, rather strongly flattened, as a result of which it is impossible to obtain a complete idea of the original form of the test; separate portions of the ambulacral and interambulacral fields have been preserved more or less satisfactorily. At first glance at this specimen it is difficult to establish which of the polar openings of the radially constructed test belongs to the periproct or to the peristome, but by analogy with the figures of known forms of this family given in the literature one must suppose that the peristomial opening is located on the side with the full number of rows of the more or less well-preserved tubercles of the interambulacral fields. And the very affiliation of the form with the fam. Diadematidae could likewise be established only indirectly, since the structure of the plates of the ambulacral fields is indistinct.

The tubercles for the spines, present on all plates, are perforated and crenulated along the margins. The spines themselves, preserved here and there in incomplete form, are covered with fine longitudinal striae and noticeably thicken toward the upper end. By these characters the form can readily be assigned to the genus Pseudodiadema Desor.

The specific diagnosis of this genus is based chiefly on the arrangement of the tubercles of the interambulacral fields. Each meridional row of plates of these fields bears four straight rows of tubercles, the latter being arranged more or less concentrically with respect to the poles of the test. The first row, counting from the ambulacral opening, gradually wedges out; the second row is more developed, but also becomes attenuated toward the peristomial opening; the third row is most sharply expressed, which, connecting the periproctal and peristomial openings, alone is clearly visible on the dorsal side of the test; as for the fourth row, it is less developed and likewise becomes attenuated, not reaching the peristomial opening (fig. 1). The tubercles of both the ambulacral and the interambulacral fields are arranged more or less wave-like along the equator, having here almost identical dimensions.

Similarity and differences. The described form closely resembles Pseudodiadema Renevieri Cotteau ^1). The difference amounts essentially to the fact that the latter species possesses a more sharply developed fourth row of tubercles, reaching the peristomial opening, and the distances between the tubercles of this row are more considerable. It also resembles the species Pseudodiadema Brongniarti Agassiz ^2), but in this form the fourth row

^1) A. D’Orbigny, Paléontologie Française 1862—7. T. VII, Pl. 1108, fig. 7—9.
^2) Thomas Wright, Monograph on the British Fossils. 1864—1882. Echinodermata from the Cretaceous Formations. Pl. XX, 2a-c.

the tubercles are considerably less developed than in ours. In addition, it should be noted that in both of the indicated species the third paired rows of interambulacral tubercles diverge before the periproctal opening, whereas in our form they are arranged normally.

I am inclined to think that this specimen, being a form intermediate between those cited, belongs to a special species which, according to the literature available to me, has apparently not yet been described by anyone, and I give it a new specific name after its locality: Pseudodiadema Tschapani mihi.

Locality. The specimen was obtained by me in light-gray marl, lying almost horizontally on the north-eastern slope of Mount Chapan-Dag (9100 feet), which is situated on the border between Russia and Persia in the district of the city of Ashkhabad.

2. Pseudodiadema sp.

The remaining specimens of Pseudodiadema present in the collection are of poor preservation, in places broken, compressed, and eroded, and are for the most part nuclei, as a result of which their specific determination is impossible; judging by their external appearance, these fossils apparently belong to a single species. Their test is rounded in outline, with a hemispherical upper surface and a slightly concave lower surface, and with the peristomial and periproctal openings situated at the poles. The first zones of the moderately broad ambulacral fields are straight; in places it can be observed that near the peristomial opening and the apical disc they contain more than one row of pore-pairs.

These characters, together with the perforate nature of the tubercles preserved in places, make it possible to refer the forms in question to Pseudodiadema sp.

The originals were found in light-gray marl on Mount Chapan-Dag, Shah-Shah and Khano-Dag, which lie to the N.-E. of the village of Firuza.

Irregularia.

Fam. Echinoneidae. Spatangidae d’Orbigny.

The main mass of sea urchins in the collection studied consists of representatives of the family Spatangidae. In this family M. Agassiz distinguished a large number of genera on the basis of the following characters: 1) the form of the mouth, 2) the form and arrangement of the ambulacra, and 3) the thickness of the tubercles. The same characters were made the basis of the classification of the Spatangidae proposed by d’Orbigny and retained in its general outlines by Zittel. Our material also very conveniently divides according to this classification, corresponding to one part of d’Orbigny’s scheme.

Genus Eshinospatagus Bregnius 1732.

3. Eshinospatagus Collegnii d’Orbigny 1853.

1. Echinospatagus Collegnii, D’Orbigny. Paléontologie Française. T. VI, pl. 846; fig. 1—3.

Only one half of the test of a single specimen is present; in the prepared state it proved to be poorly preserved. The shell is of almost equal width and length, strongly expanded and with a heart-shaped indentation in front and truncated behind. The apical disc is situated almost in the middle, whereas the greatest height corresponds to the part of the test in the posterior interradius. The lower surface is almost flat, becoming slightly convex in the posterior half of the form. The anterior paired ambulacral fields, rather long, broad, and curved, lie in shallow depressions;

the narrow and elongated pores of these fields are arranged in pairs in two zones, the inner pores being shorter than the pores of the outer zones (fig. 2). The posterior pair of ambulacral fields is short and straight. The unpaired ambulacrum is situated in a broad and deep groove, running from the apex to the peristome and forming a large notch in the anterior margin of the test. The narrow pore-zones of this ambulacrum are identical, and each zone consists of two close rows, the pores of which are arranged in the form of a herringbone (fig. 3). Nothing can be said about the apical disc, since this part of the test is difficult to examine. In the middle of the ambulacra several tubercles may be observed. The small tubercles of the interambulacral fields are everywhere identical.

Similarities and differences. The paired ambulacra of this species, situated in a groove, bring it close in form to Echinospatagus Gibbus, but the latter differs in having a less high test and a narrower unpaired ambulacrum.

Locality. Light-gray marl of Mount Chapai-Dag.

4. Heteraster oblongus d’Orbigny 1853.

1. Heteraster oblongus D’Orbigny. Paléontologie Française T. IV, Pl. 847, fig. 1—5.

There are 8 specimens of this species in the collection. The state of preservation of the fossils is not entirely satisfactory; only separate portions of the ambulacral and interambulacral fields are accessible to observation. In the accompanying Table 1 are given (in mm) the dimensions of the following elements of the test: length—L, width—B, height—H, and distance from the posterior margin of the test to the center of the apical disc—P.

Table 1.

The test is elongated, broadened and heart-shapedly curved in front, truncated behind. The apical disc is situated in the second third of the test, at its highest point. The dorsal side is almost flat, while the lower side is convex everywhere. The unpaired ambulacrum is situated in a broad and deep groove, running from the apical disc to the peristomal opening. The diverging pore-zones of this ambulacrum are equal to one another, each zone consisting of unequal branches: a broad outer and a narrow inner one. The inner branch is represented by short, oblique, and equal pores; the broad outer branch consists of two kinds of pores: elongated, oblique pores, and more rare accessory pores irregularly alternating with them (fig. 4). The paired curved ambulacra are also formed by unequal zones. Thus, the anterior branch of the better-studied posterior ambulacra consists of slightly elongated oblique pores, whereas the posterior branch of the same ambulacra consists of rather strongly elongated oblique pores (fig. 5). The mouth opening is pentagonal, placed in the anterior quarter of the ventral side of the test. The anal opening is transversely oval, situated in the lower half of the height of the posterior wall of the test.

The tubercles on the dorsal side are small and identical; on the ventral side they are larger and also more or less identical.

Similarities and differences. This species differs from the following one essentially by its more elongated form, lesser height, and anal opening elongated in the transverse direction.

Locality. The originals in question were found in a bed of light-gray marl in the Zakonovan-Zov gorge, NW of the village of Firyuzy, and also in a marl bed 3 sazhens thick on the eastern slope of Mount Merkou, at an elevation of 50 sazhens above the Saram-Saly mine; both places lie SE of the town of Askhabad.

5. Heteraster Couloni d’Orbigny 1853.

1. Heteraster Couloni D’Orbigny. Paléontologie Française. T. VI. Pl. 848, fig. 1—6.

There are 23 specimens of this species. The degree of preservation is the same as in Heteraster oblongus. The dimensions of the above-mentioned elements of the test are presented in Table 2.

Table 2.

The test is oval, broadened and usually notched in front, truncated and rounded behind; the upper side is very convex, while the lower side is almost flat, somewhat convex in the middle region of the posterior part and in the lateral regions of the anterior part of the test. The structure of the pores and the form of the ambulacral fields are entirely identical with the corresponding elements of Heteraster oblongus. The posterior passage opening is oval, situated longitudinally in the middle of the height of the posterior wall of the test. In some places, between the zones of the ambulacral fields, tubercles are visible. The tubercles covering the main part of the test are small.

Similarities and differences. This species differs from the preceding one by its oval form, greater height, and the longitudinal position of the anal opening; in addition, as is seen from Table 2, in it the ratio P/L fluctuates between 0.461—0.514, whereas in Heteraster oblongus it is 0.427—0.437. D’Orbigny, in his description of the similarities and differences of Heter. Couloni, indicates that the zones of the ambulacra of Heteraster Couloni are less unequal in width than in Heteraster oblongus; this is not observed in my specimens and does not correspond to the drawings given by d’Orbigny himself.

Locality. Light-gray marl of the Zakonovag-Zova gorge and the eastern slope of Mount Merku.

Genus Pyrina Des Moulins 1837.

6. Pyrina orientalis Cotteau et Gauthiers, 1895.

1. Pyrina orientalis. Cotteau et Gauthiers. Mission scientifique en Perse par I. de Morgan. T. III. Pl. XI, fig. 1—3.

There are 6 specimens of this species in the collection. The state of preservation is satisfactory, since all essential characters are available for observation. The dimensions of the shell are as follows:

The shell is of comparatively small size, oval in outline; the upper side is convex, the lower more or less convex and only slightly flattened around the peristomial opening. The posterior part of the test is rounded and less elongated than the anterior (fig. 6). The apical shield occupies a central position. The ambulacral fields are all alike; they extend continuously from the shield to the peristomial opening. The simple straight narrow zones consist of paired pores of one and the same diameter. The mouth opening is oblique from right to left, situated in the middle. The anal opening is oval, pointed above, situated in the middle of the posterior part of the test, which is somewhat raised.

The whole shell is strewn with tubercles, the larger ones below being surrounded by smaller granules.

Locality. Light-gray marl of Mount Chapan-Dag.

Conclusion. In conclusion I briefly summarize the conclusions that may be drawn from the material described in the present work.

Table of distribution of the species considered.

The forms described in this work, as is evident from the table presented, belong in general to the Aptian. The beds of light-gray marl of Mount Mergou and the Zakopvan-Zova gorge, in which Heteraster oblongus and H. Couloni were found, must be regarded as a formation of Urgonian age. But Pseudodiadema sp., Pseudodiadema Tschapani, and Pyrina orientalis already belong to the Aptian stage—all the more so since Echinospatagus Collegnii, occurring together with these forms, according to M. Agassiz and Lory, approaches the Albian stage.

The combination of characters represented by the fauna of the Lower Cretaceous deposits of Kopet-Dagh permits them to be regarded as belonging to the neritic type of deposits, since, alongside cephalopods, brachiopods, gastropods, and sea urchins play a rather important role here.

For any definite generalizations concerning the attribution of the described sea urchins to one or another “province,” the objects examined are, of course, unsuitable because of their small number; only by studying the entire aggregate of forms from this region can we reconstruct the general picture of their relationships and thus find a point of support for judging the conditions under which this rock originated.

TO M. SARKISOV’S ARTICLE: TOWARD KNOWLEDGE OF THE FAUNA OF SEA URCHINS

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