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Thanatocoenoses and biotopes of Lower Jurassic brachiopods in central and western Bulgaria
Palaeogeography, Palaeoelimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
THANATOCOENOSES AND BIOTOPES OF L O W E R J U R A S S I C BRACHIOPODS
IN CENTRAL
AND WESTERN
BULGARIA
PLATON TCHOUMATCHENCO
Geolog4calInstitute, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Accepted for publication March 6, 1972)
ABSTRACT Tchoumatchenco, P., 1972. Thanatocoenoses and biotopes of Lower Jurassic brachiopods in central and western Bulgaria. Palaeogeogr., Palaeoclimatol., PalaeoecoL, 12: 227- 242. Brachiopod biotopes are recognized in the Early Jurassic rocks of Bulgaria. The geographical distribution of these is plotted for each of the stratigraphical zones determined earlier on the basis of brachiopods. The biotopes are related in turn to bathymetry, energy level and bottom type. INTRODUCTION I have already been able to propose a standard zonation of the Lower Jurassic in Bulgaria, based on brachiopods (Tchoumatchenco, 1967). This consists of the following zones: (1) Spiriferina walcotti zone (stratigraphical range: Hettangian and Sinemurian up to the obtusum zone, not inclusive). (2) Tetrarhynchia dunrobinensis zone (stratigraphical range: from the obtusum zone to the davoei zone, not inclusive). (3) Zeilleria cornuta zone (stratigraphical range: from the davoei zone to the stokesi sub-zone o f the margaritatus zone, inclusive). (4) Homoeorhynchia acuta zone (stratigraphical range: from the subnodosus sub-zone to the tenuicostatum zone). (5) Homoeorhynchia cynocephala zone (stratigraphical range: from the falciferum zone to the opalinim zone). In studying the geographical distribution of the zones I have established that their specific composition varies in the different regions, always within the limits of a zone, and characterised by its index fossil. It is therefore necessary to discover the causes of these changes, and to reconstruct the palaeoecological picture. THANATOCOENOSES A thanatocoenosis is formed by all the fossils preserved in the sediments of a region in the course o f a certain interval of time. The thanatocoenosis is formed of fossils
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belonging to various groups of animals but I propose here to deal solely with the thanatocoenoses formed by the brachiopods, since the complete thanatocoenoses can only be studied by teams of palaeontologists, specialising in the different fossil groups. The thanatocoenoses reflect sufficiently well the biocoenoses from which they are derived (Johnson, 1960). In distinguishing the thanatocoenoses I have used the numerical classification of Merkline (1950) as follows: "predominant species" - the number of specimens of this species must exceed 50 % of the specimens of other species which form part of the thanatocoenosis; "characteristic species" - the number of specimens more than 25 %; "associated species" - the number of specimens more than 10 %; "rare species" - the number of specimens not more than 10 %. I add here a new class - "strongly predominating species", for species the number of specimens of which exceed 80 % of the other species forming part of the thanatocoenosis. In the Lower Jurassic sediments of central and western Bulgaria a number of different thanatocoenoses have been recognised which occur in a chronological order within the framework of the brachiopod zones.
Spiriferina walcotti zone During the time of this zone there have been recognised the following thanatocoenoses: (1) Thanatocoenosis of Gibbirhynchia sp. nov.: characteristic species - Gibbirhynchia sp. nov. (35 %); associated species - Spiriferina walcotti (Dav.) (20 %), S. tumida (v. Buch) (10 %), Lobothyris grestenensis (Radovanovic) (14 %), L. punctata (Sowerby) (12 %); rare species - Zeilleria indentata var. quiaoisensis Choffat, Quadratirhynchia sp. nov., Lobothyris subovoides (MOnster). (2) Thanatocoenoses of Lobothyris punctata - L. grestenensis: strongly predominating species - Lobothyris punctata (85 %); L. grestenensis (9.1%) - on the limit between associated species and rare species; rare species - Spiriferina tumida, Tetrarhynchia dunrobinensis (Rollier), Lobothyris subovoides (MOnster) etc. Certain thanatocoenoses are poorly defined because of the restricted number of brachiopods which have been found. Of this kind is the thanatocoenosis of Lobothyris grestenensis, of which the index species is the only one which has been found (26 examples). Such is also the thanatocoenosis of Spiriferina tumida (characteristic species: 33.6 %) and associated species: Spiriferina walcotti (22.1%), Lobothyris grestenensis (22.1%), L. punctata (22.1%).
Tetrarhynchia dunrobinensis zone During this time the following thanatocoenoses have been recognised: (1) Thanatocoenoses of Tetrarhynchia dunrobinensis - Lobothyris subovoides: predominant species: Tetrarhynchia dunrobinensis ( 6 0 - 7 0 %); associated species -
LOWER JURASSIC BRACHIOPODBIOTOPES
229
Lobothyris subovoides (20 %); rare species - Gibbirhynchia curviceps (Quenstedt) (5.4 %), Cirpa fronto (Quenstedt) (3 %), Zeilleria darwini E. Desl. (2 %).
(2) Thanatocoenosis of Tetrarhynchia dunrobinensis: strongly predominating species - T. dunrobinensis (85 %); rare species - L. subovoides (8 %), Z. darwini (6 %), etc Zeilleria cornuta zone
In the sediments of this zone different thanatocoenoses have been found: (1) Thanatocoenosis ofL. subpunctata - L. edwardsii: predominant species L. subpunctata (Dav.) ( 5 0 - 6 0 %); associated species - L. edwardsii (Day.) (20.2 %); rare species - Spiriferina aseendes E. Desl. (7.5 %), S. alpina falloti Corroy. (6.4 %); Zeilleria sarthacensis (d'Orb.) (6.5 %), Cincta subnumismalis (Day.) (6.5 %), Z. eornuta (Sow.) (1.6 %), etc. (2) Thanatocoenosis of Cirpa langi - S. oxyptera - S. alpina falloti: characteristic species - Cirpa langi Ager (31%), S. alpinafalloti (29 %); associated species - S. o x y p t e m (Buv.) (16 %); rare species - Homoeorhynchia sp. n. (7 %), L. subpunetata (5 %), S. ascendes (3 %), Z. cornuta (3 %), Cincta subnumismalis (1.6 %), etc. In spite of its relatively low percentage S. oxyptera is a very characteristic species for this thanatocoenosis. Around the village of Sarantzi, near Sofia, one can observe a variation in the composition of this thanatocoenosis: Cirpa langi Ager is replaced by Cincta subnumismalis, whilst the other components are in practically the same percentages. This variation could be called, following Merkline (1950) an "association" within the frame of the same thanatocoenosis. (3) Thanatocoenosis o f Homoeorhynehia sp. n. - L. subpunctata: predominant species - Homoeorhynchia sp. n. (52-56 %); characteristic species - L. subpunctata (28 %); rare species - Z. eornuta (8 %), C. subnumilmalis (6 %), L. edwardsii (4 %), Z. sarthacensis (2 %), etc. (4) Thanatocoenosis of C. subnumismalis - L. subpunetata: predominant species C. subnumismalis (70 %); associated species - L. subpunctata (15-18 %); rare species S. ascendes (8 %), S. alpina falloti (7 %), etc. (5) Thanatocoenosis of Tetrarhynehia tetrahedra: strongly predominating species T. tetrahedra (90 %); rare species - L. subpunetata (2 %), L. edwardsii (2 %), etc. Hornoeorhynchia acuta zone
Two thanatocoenoses have been recognised in the sediments of this zone comprising the same species but in different percentages. (1) Thanatocoenosis of Spiriferina villosa: predominant species - S. villosa (Qu.) (72 %); associated species - H. acuta (Sow.) (20 %); rare species - Quandratirhynchia quadrata (Buckman) (10 %).
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(2) Thanatocoenosis ofH. a c u t a - S. villosa: characteristic species - H . acuta (47 %), S. villosa (46 %); rare species - Q. quadrata (7 %). H o m o e o r h y n c h i a eynocephala zone
Two thanatocoenoses have been recognised in the sediments of this zone: (1) Thanatocoenosis of Zeilleria lycetti-Pseudogibbirhynchia moorei: predominant species - Z. lycetti (Day.) (70 %); characteristic species - P. moorei (Dav.) (22 %), rare species - H. cynocephala (Richard) (8 %). (2) Thanatocoenosis ofH. cynocephala: a strongly predominating species 11. cynocephala (90 %); rare species - Z. lycetti (7 %), P. m o o r e i (3 %). BIOTOPES Gekker (1957, p.20) defined biotope as a restricted area of the living habitat determined by definite physical and chemical characteristics, conditioning the possibility of existence of a particular biocoenosis. Because of the variability of the factors of the habitat, characterising the biotope, a biocoenosis can be subdivided into association of species, occupying different parts of the biotope. I call these different parts of a biotope "microbiotopes" as they have peculiarities which differentiate them from the biotope but which are not sufficient to separate them clearly from that biotope. The most frequent of these are the transitions from one biotope to another (in the horizontal sense) where the physical and chemical characteristics of a biotope pass gradually from one biotope to the other. Ager (1965) defined 7 biotopes (which he called "environments" or "habitats") inhabited by Mesozoic brachiopods with distinctive morphology. Some of these polarised biotopes have been established in the Lower Jurassic sediments of central and western Bulgaria. The most common are the transitional microbiotopes. This is to be expected because one cannot pigeon-hole the diversity of nature in the narrow framework of an equal classification. In Bulgaria the following biotopes have been recognised: (1) littoral biotope; (2) sublittoral biotope with a sandy floor and agitated water; (3) sublittoral biotope with a muddy bottom; (4) deeper water biotope (? bathyal); (5) pelagic biotope. Littoral biotopes
Sediments and brachiopods belonging to this biotope are only found very rarely. It is especially in the erosional pockets that they are preserved. The sediments are limestones, sandy to gravelly in grain size and made of well-rounded quartz. The brachiopods which
LOWER JURASSIC BRACHIOPODBIOTOPES
231
inhabit the littoral biotope have a typical morphology - strong ribs, thick shell, large foramen and a slightly developed sinus, The foramen insures a stable attachment and the wide and feebly developed sinus strengthens the shell. On the other hand a deep sinus is not necessary because the rapid movements of the environment carry away very quickly the exhaled material. The brachiopods which inhabited this biotope are: Caleirhynehia glevensis, S. walcotti, L. grestenensis. The last-named taxon, in order to strengthen its shell, had a rather strange mode of growth: the shell concentrated the growth lines after a temporary cessation of growth in length. A little later it produced further growth simultaneously in length and thickness. This alternation of stops and starts in growth happening many times gave the shell a staircase-like appearance.
Sublittoral biotope with sandy floor and agitated water In the sediments formed in the conditions of this biotope we have collected the largest numbers of brachiopods. The sediments enclosing the fossils are sandy and detritic limestones, often oolitic and ferruginous. These are the limestones of the Ozirovo Formation (sensu Sapunov, 1967). The brachiopods inhabiting this biotope have a "normal" aspect. The rhynchonellids possess more or less well-developed ribs and a more or less wide and deep sinus. One finds here the following rhynchonellids: Tetrarhynch& dunrobinensis, T. subconcinna,
T. argotinensis, Pseudogibbirhynchia moorei, Q. quadrata, Rhynchonelloidea lineata tetewensis, etc., also the cynocephalous rhynchonellids - Homoeorhynchia acuta, Homoeorhynchia sp. nov. The representatives ofH. acuta coming from this biotope have thicker shells with a wider sinus compared with those which come from the sublittoral biotope in calm water and with a muddy sea floor. The tetebratulids are represented by species of Lobothyris: L. punctata, L. subpunctata, L. subovoides, L. edwardsii. The zeilleriids are represented by Zeilleria indentata, Z. indentata vat. quiaosensis Choffat, Z. cornuta, Z. sarthacens&, Z. lycetti, Keratothyris cot, Cincta subnumismalis, but the last-named taxon has its maximum development in the sublittoral biotope with deeper water and a muddy floor. The greater part of the spiriferinas have inhabited this biotope: S. tumida, S. alpina falloti, S. walcotti, etc. A large proportion of the brachiopods which inhabited the floor of this biotope lived in banks in order to resist the currents and we find them now in the form of lenses. This mode of life was followed by: 7]. dunrobinens& (this taxon forms lenses in the vicinity of the town of Teteven), Lobothyris subpunctata, Spiriferina ascendes, Z. sarthacensis. Examples of these have lived so tightly crowded together one against the other that their growth has been distorted and many of them have asymmetrical shells (Z. sarthacens& around the village of Ravna, near Sofia). In the sediments around the village of Berend~ izvor, near Sofia, are found shells with traces of attachment similar to those described by Boekschoten (1966) in Arthopyren& subfittoralis (Leighton), living, according to Sandeson (Boekschoten, 1966) in water of variable salinity from 15-20 % and in well-aerated waters rich in oxygen.
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Sublittoral biotope with muddy bottoms and calm waters The waters of this biotope are comparatively deeper and have a muddy bottom, not very suitable for the life of brachiopods. The typical sediments are marks of the Boukorovtzi member and the Ravna Member (sensu Sapunov, 1967). Of all the brachiopods, those which have been transported by palaeocurrents into the conditions of this biotope survive and give a single generation of those of small size and those which in the adult state possess adaptations for a life resting on mud. These are the following brachiopods: Gibbirhynchia sp. nov., S. villosa, Homoeorhynchia acuta. Spiriferina villosa possesses spines which held the creature above the mud. H. acuta has a deep sinus which facilitates the separation of inhalant and exhalant currents. The representatives of these taxa living in this biotope have thinner and lighter shells than those inhabiting the previous biotope. Only a limited number of species are adapted for a life in the conditions of the sublittoral biotope with a muddy bottom and calm water, but certain of them are developed in large numbers: S. villosa (north of the village of Komstitza, near Sofia), Gibbirhynchia sp. nov. (north of the villages of Komstitza, Ravna, etc. near Sofia).
Deep water biotope (bathyal?) A limited number of brachiopods are adapted to life in the conditions of this biotope. The sediments are of argillaceous limestones and marly clays. The characteristic features for this biotope are calm water and limited circulation of oxygen. In the conditions of this biotope the only brachiopods which can survive are those having the inhalant and exhalant currents separated as far as possible. It is difficult to speak of bathyal regions in the Lower Jurassic basin of central and western Bulgaria occupied by brachiopods. Rather there are bays of calm water with limited circulation of oxygen. A region with such conditions existed around the villages of Gintzi and Zimevitza, near Sofia. In this region the only species to be developed is S. oxyptera, which possesses a long and straight cardinal margin which provides a maximum separation of the inhalant and exhalant currents. The floor has been populated by Cirpa langi, the sinus of which - wide and relatively deep - plays the same role. The conditions of this biotope have been optimal for the development of Cincta subnumismalis. Its shell - wide and in the form of a coin - with its little foramen, has not been able to attach itself firmly to the floor and this is the reason for its relatively small size and rare occurrence in the conditions of biotopes other than that of the deeper waters. In the sediments of this biotope C. subnumismalis reaches its maximum development in size and in number in the argillites and marls around the town of Teteven. In order to preserve its position with the umbonal region downwards, C. subnumismalis deposits calus in this region to make this part of the shell heavier, and this serves to stabilise the shell.
LOWER JURASSIC BRACHIOPOD BIOTOPES
233
Pelagic biotope For certain brachiopods of small size and thin shell Ager (1965) suggested that they lived an epiplanktonic life. That is proved by our observations. During the time of the H. cynocephala zone the representatives of this group dwelt in the region of the villages of Beledi6 khan and Dragovistitza, near Sofia, the Ambaritza chalet, the village of Glojene, near Lovetch, of Gaganitza and Gorno Ozirovo, near Mihailovgrad. In some of these localities ferruginous limestones were deposited or even ironstones, whereas in the other parts of these outcrops the limestones are impoverished in iron. This demonstrates that in the first group of localities, during sedimentation, the water has had a strong concentration of ferric ions. In both groups of localities the following brachiopods are the same but in different percentages: Z. lycettg H. cynocephala and P. moorei. In the group of localities with sediments rich in iron H. cynocephala is represented by 9 1 % and Z. lycetti and P. moorei together by 9 %. In the sediments not containing iron H. cynocephala and P. moorei are represented by 92 % and Z. lycetti by8%. Around the village of Glojene, near Lovetch I have collected only Z. lycettL This great difference in the percentages of the same species in the two groups can be explained by the great resistance ofH. cynocephala to the high ferric ion content in the marine waters and an epiplanktonic mode of life. It was suited to this mode of life by its small size (even in the adult stages) and light shell. Representatives of this taxon are found in large numbers with good swimmers - ammonites and belemnites - to which they could have attached themselves and survived this poisoning of the water with ferric ions, which were probably in higher concentration near the sea floor. GEOGRAPHICALDISTRIBUTION OF THE BRACHIOPODBIOTOPES
Spiriferina walcotti zone During the deposition of the beds characterised by the representatives of the S. walcotti zone, uniform conditions did not exist in the Lower Jurassic basin of central and western Bulgaria (Fig.1 and 2). The transgressive sediments and the fossils which they contain are preserved in isolated localities. The thanatocoenosis S. walcotti was formed in this littoral biotope; these come from around the villages of Komstitza (near Sofia) and of Glojene (near Lovetch). Later, after the Lower Jurassic transgression, still in the S. walcotti zone in the region between the villages of Komstitza, Ravna and Kalotina there were established the conditions of the sublittoral biotope with muddy bottom. In the sediments accumulated the thanatocoenosis of Gibbirhynchia sp. nov. The sediments are the marls of the Ravna Member (sensu Sapunov, 1967). In the region between the villages of Slivnitza and Kremikovtzi (to the north of Sofia) were established the conditions of the sublittoral biotope of agitated waters - a
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P. TCHOUMATCHENCO
BOV
II/ t 1
SLIVNIT Z ~ ' ~ ~ t
1 ~ ~
~
~
'
SO~IA'~
~
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GABROVOo P
SLJ!L!!J
~
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i
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Fig.1. Geographical distribution o f brachiopod biotopes during Walcotti zone times. 1 = littoral biotope w i t h o u t conditions suitable for the growth o f brachiopods; 2 = littoral biotope; 3 = sediments eroded; 4 = sublittoral biotope o f m u d d y b o t t o m s ; 5 = sublittoral biotope o f agitated waters; 6 = sublittoral b i o t o p e - m i c r o b i o t o p e o f deeper water; 7 = Spiriferina tumida; 8 = Spiriferina walcotti; 9 = Gibbirhynchia sp. nov.; 10 = Lobothyris gresienensis; 11 = L. punctata; 12 = outcrops of Lower Jurassic; 13 = sublittoral biotope o f agitated w a t e r s - m i c r o b i o t o p e passing into the littoral biotope.
thanatocoenosis
W B
D
Jsker river I I i I
E
TETEVEN i C
E
A
i
I
C
biotope 20kin 1
2
3
2
1
3
Fig.2. Idealised section across the sea floor during Walcotti zone times. A = thanatocoenosis of
Spiriferina walcotti-Lobothyris punctata ; B = thanatocoenosis o f Gibbirhynchia sp. nov.; C = no brachiopods; D = thanatocoenosis ofLobothyris punctata; E = thanatocoenosis o f Lobothyris grestenensis; 1 = sublittoral biotope o f m u d d y b o t t o m s ; 2 = sublittoral biotope o f agitated waters; 3 = littoral biotope w i t h o u t conditions for brachiopod growth.
LOWER JURASSIC BRACHIOPOD BIOTOPES
235
passage microbiotope passing towards the littoral biotope. There were deposited sandy and detritic limestones of the Ozirovo Formation (sensu Sapunov, 1967) in which accumulated the poorly-defined thanatocoenosis ofL. punctata, L. grestenensis, S. walcotti, S. tumida. In the region between the villages of Gintzi, Boy, Zimevitza and Milanovo (near Sofia) and the Ledenika chalet (near Vratza) the conditions were not favourable for the growth of brachiopods - there were deposited continental and marine sandstones. Further east there was an island - the island of Vratza. Around the village of Gaganitza there existed the conditions of the sublittoral biotope with agitated water and there formed the thanatocoenosis of L. punetata. Around Teteven there was a microbiotope intermediate between the sublittoral biotope with agitated water and that of muddy bottoms. The brachiopods there are: S. walcotti and L. grestenensis (single specimens). Around the villages of Lessidren, Chipkovo, Tchernata reka and in the Trman Balkan conditions were very close to those of the littoral biotope without being suitable for the development of brachiopods. Around the village of Jabalka (near Gabrovo) there were established the conditions of an intermediate biotope between the sublittoral biotope of agitated water and the littoral biotope. In the sediments accumulated the thanatocoenosis consisting almost entirely of L. grestenensis. Around the village of Etropol~ there are no brachiopods from this time; there was a biotope of deeper water without suitable conditions for brachiopod growth. Tetrarhynchia dunrobinensis zone
During the times of this zone there was a levelling out of the living conditions of the basin. The sandy or detritic limestones of the Ozirovo Formation were deposited (Fig.3 and 4). The T. dunrobinensis zone is situated immediately above the S. walcotti zone in the area between the villages of Kalotina, Ravna, Komstitza and Gaganitza. In this region accumulated the thanatocoenosis of T. dunrobinensis - L. subovoides very rich in specimens. In the area between the villages of Gintzi, Zimevitza, Boy and Gorno Ozirovo this was the lowest brachiopod zone established; that is to say the living conditions have become favourable for the development of brachiopods only at the time of this zone. Here was formed the same thanatocoenosis as in the preceding region but very poor in specimens. The brachiopods are found in sandy limestones, even in sandy limestones intercalated between sandstones around the villages of Bov and Zimevitza. In this region there was established the sublittoral biotope of agitated water. In the region between the villages of Slivnitza, Kremikovtzi, and Sarantzi was a biotope intermediate between the sublittoral biotope of agitated water and the littoral biotope, with the formation of the thanatocoenosis of T. dunrobinensis. The same conditions were established in the region of Krai'ste where almost all the shells are separated valves.
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oGABROVO /TETEVEN
© SOFIA
ZEMEN Fig.3. Geographical distribution of brachiopod biotopes during Dunrobinensis zone times. 1 = island; 2 = littoral biotope (without suitable conditions for the growth of brachiopods); 3 = microbiotope transitional between the sublittoral biotope of agitated waters and the littoral biotope; 4 = sublittoral biotope of agitated waters; 5 = sublittoral biotope, microbiotope passing into deeper water; 6 = denuded sediments; 7 = Tetrarhynchia dunrobinensis; 8 = Gibbirhynchia curviceps; 9 = Cirpa f r o n t o ; 10 = L o b o t h y r i s subovoides; 11 = Zeilleria darwini 12 = Cincta subnumisrnalis; 13 = outcrops of Lower Jurassic.
SW
thanatocoenosis
biotope 0
NE: :=NW
SLIVNITZA ii
D
1',A
SE
GLOJENE
VRATZA
l
C
I
:i
B
B
C
f[/
3 10
20kin
Fig.4. Idealised section across the sea floor during Dunrobinensis zone times. A = thanatocoenosis of Tetrarhynchia dunrobinensis; B = thanatocoenosis of Tetrarhynchia d u n r o b i n e n s i s - L o b o t h y r i s subovoides; C = no brachiopods; D = brachiopods rare, mostly single valves; 1 = littoral biotope; 2 = sublittoral biotope of agitated waters; 3 = microbiotope transitional between the sublittoral biotope of agitated waters and the littoral biotope; 4 = island.
LOWER JURASSICBRACHIOPODBIOTOPES
237
The conditions of the sublittoral biotope of agitated water which were developed in the region around Teteven were optimal for the development of the thanatocoenosis of T. dunrobinensis-L, subovoides. The representatives of T. dunrobinensis were developed in masses and have retained their largest dimensions. To the north around the village this zone was not established. Around the area of Raventa between Glojene and Teteven the representatives of the T. dunrobinensis zone have been found in a condensed state. This demonstrates that in the northern part of the Teteven area an island existed at this time, or a shallow sea around which there did not exist conditions favourable for the growth of brachiopods. In the southern part of the Teteven region around the village of Chipkovo and in the Tcherna reka river valley was established the sublittoral biotope, a microbiotope intermediate with the littoral biotope. Brachiopods are rare, and the thanatocoenosis is not well characterised - T. dunrobinensis-L, subovoides in almost equal numbers. Farther south brachiopods have not been found in this zone - the littoral biotope has not been favourable for brachiopod growth. Around Etropol6 the representatives of this zone are found in isolated beds of limestone. It appears that in this region there existed conditions unfavourable for brachiopods - a deep sea and favourable conditions were established only from time to time and then the T. dunrobinensis thanatocoenosis was formed.
Zeilleria cornuta zone The basin had almost the same contours as in that of the times of the T. dunrobinensis zone (Fig.5 and 6). To the west, in the region between the villages of Kalotina, Ravna, Komstitza and Gaganitza the sublittoral biotope with agitated water still continued. This region at this time provided the richest collection of brachiopods. There was formed the thanatocoenosis of L. subpunctata-L, edwardsii. Further east around the villages of Gintzi and Zimevitza there were established the conditions of the deep water biotope (? bathyal). It appears that there existed here the waters of a calm bay with limited oxygen circulation. On the floor was formed the thanatocoenosis of C. langi-S, alpina
fallotti- S. oxyptera. Around the village of Gorno Ozirovo was formed the thanatocoenosis of T. tetrahedra reflecting a biotope very near littoral conditions. The microbiotope which occupied the region between the villages of Slivnitza and Kremikovtzi was near the biotope of agitated waters, differing only in having higher energy conditions in the sea. There was formed the thanatocoenosis ofHomoeorhynchia sp. nov. L. subpunctata. In the region of Kraiste the Z. cornuta zone is indicated by the rare examples of Homoeorhynehia sp. nov. This shows that intermediate conditions still existed there between the sublittoral biotope of agitated waters and the sublittoral biotope. Around Teteven was another microbiotope, intermediate between the sublittoral biotope of agitated water and that of muddy bottoms. In the sediments of this biotope
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P. TCHOUMATCHENCO
Fig.5. Geographical distribution of brachiopod biotopes during Cornuta zone times. 1 = region of clastic sedimentation without conditions for brachiopod growth; 2 = littoral conditions-around the island; 3 = littoral biotope; 4 = sublittoral biotope of agitated waters-microbiotope towards very agitated waters; 5 = sublittoral biotope of agitated waters; 6 = sublittoral biotope of deeper waters; 7 = sublittoral biotope intermediate between the sublittoral biotope of muddy bottoms and that of agitated waters; 8 = .9 bathyal biotope (sublittoral biotope of muddy bottoms); 9 = sublittoral biotope of very agitated waters; 10 = denuded sediments; 11 = littoral part of the sublittoral biotope of muddy bottoms; 12 = Spiriferina oxyptera; 13 = Spiriferina alpina falloti; 14 = Spiriferina haueri; 15 = Tetrarhynchia tetrahedra ; 16 = Homoeorhynchia sp. nov.; 17 = Lobothyris subpunctata ; 18 = Lobothyris edwardsii; 19 = Zeilleria cornuta; 20 = Zeilleria sarthacensis; 21 = Cincta subnumismalis; 22 = Cirpa langi; 23 langi; 23 = outcrops of Lower Jurassic.
was formed the thanatocoenosis o f C. s u b n u m i s m a l i s - L ,
s u b p u n c t a t a . In the area
between the villages o f Lessidren and Chipkovo there was the typical sublittoral biotope o f agitated waters, on the floor of which was formed the thanatocoenosis of L. s u b p u n c t a t a - L ,
edwardsii.
In the region o f the river Tcherna reka existed the microbiotope intermediate between the sublittoral biotope o f agitated waters and the littoral biotope. On the sea floor formed the thanatocoenosis o f H o m o e o r h y n c h i a
sp. n o v . - L , s u b p u n c t a t a . T h e marine
waters were very agitated; the evidence for this is that almost all the shells o f L. s u b p u n c t a t a are single valves piled up one on the other.
In the area o f Entropol6 the sea was always deep, lacking in conditions favourable for the growth o f brachiopods. During the Z. c o r n u t a zone there were deposited everywhere the detritic sandy, or crinoidal limestones o f the Ozirovo Formation.
LOWER JURASSIC BRACHIOPOD BIOTOPES S
239
N SW
ZEMEN
NE NW SE
~ VRATZA
KALOTINA 1
TETEVEN
I i
i
; 1
B
D
B
E
; 0
2
1
3
',
iF F .
! c
thanatocoenosis
biotope
~
Z i
1 O
m
2Okra
2
5
I
12
2
I, I
Fig.6. Idealised section across the sea floor during Cornuta zone times. A = thanatocoenosis of Homoeorhynchia sp. nov.-Lobothyris subpunctata ; B = thanatocoenosis of Lobothyris subpunctata L. edwardsii; C = no brachiopods; D = thanatocoenosis of Cirpa langi-Spiriferina alpina falloti S. oxyptera: E = thanatocoenosis of Tetrarhynehia tetrahedra; F = thanatocoenosis of Cincta subnumismalis-Lobothyris subpunctata; F 1 = the same thanatocoenosis partly condensed; H = brachiopods abundant: lobothyris punetata predominating and less Tetrarhynchia tetrahedra; 1 = sublittoral biotope of agitated waters; 2 = microbiotope of very agitated waters; 3 = littoral biotope; 4 = island; 5 = bathyal biotope; 12 = microbiotope intermediate between the sublittoral biotope of agitated waters and the sublittoral biotope of muddy bottoms.
Homoeorhynchia acuta zone
One may collect brachiopods belonging to this zone only in some localities - around the villages of Komstitza, Kalotina, Ravna, Beledie khan, Dragovistitza and Gaganitza (Fig.7 and 8). Around the villages of Komstitza and Kalotina there existed the sublittoral biotope of m u d d y b o t t o m s on which was formed the thanatocoenosis of S. villosa, which can be collected from the marls of the Boukorovtzi member (sensu Sapunov, 1967). The sublittoral biotope o f agitated waters prevailed around the villages of Beledie khan and Dragovistitza (there were deposited detritic, sandy or crinoidal limestones o f the Ozirovo Formation). On the floor was accumulated the thanatocoenosis of H. acuta S. villosa. The shells of the representatives of these taxa are thicker, stronger and relatively larger than the shells of the same taxa, inhabiting the sublittoral biotope with m u d d y bottoms. Elsewhere in central and western Bulgaria conditions favourable for brachiopods did not exist, either because the sea was too deep (in the region o f Teteven and Etropol~) or because the sea was very shallow and the water very agitated (in the region of Kraiste, the Troian Balkan and the southern parts of Teteven).
Homoeorhynchia cynocephala zone During this time conditions were only favourable for brachiopod growth in isolated places (Fig.9 and 10 A, B). Around the village of Gaganitza was established the micro-
I
3
240
P. TCHOUMATCHENCO
biotope intermediate between the sublittoral biotope of agitated water and the biotope o f m u d d y b o t t o m s . On t h e f l o o r was f o r m e d t h e t h a n a t o c o e n o s i s o f Z. l y c e t t i - P , moorei. In this place was d e p o s i t e d t h e B o u c o r o v t z i M e m b e r (marls). T h e same t h a n a t o c o e n o s i s was f o r m e d a r o u n d t h e villages o f G o r n o Ozirovo a n d G l o j e n e . A t this t i m e in t h e region n o r t h o f Sofia a n d in t h e T r o i a n Balkan, b e c a u s e o f t h e p o i s o n i n g o f the sea b y ferric
GABROVO?~ ~, JABALKA. - ?~?
SKOBELEVO
[ff~]a ff-2q~
Fig.7. Geographical distribution of brachiopod biotopes during Acuta zone times. 1 = littoral conditions around island; 2 = region of clastic sedimentation without conditions for brachiopod growth; 3 = littoral biotope; 4 = sublittoral biotope of agitated waters; 5 = sublittoral biotope of muddy bottoms; 6 = deeper marine region without conditions suitable for brachiopod growth; 7 = denuded sediments; 8 = Spiriferina villosa; 9 = Hornoeorhynchia acuta ; 10 = Quadratirhynchia quadrata ; 11 = outcrops of Lower Jurassic. NE VRATZA
SW ZEMEN
!
C
A
B
A
C
thanatocoenosis
biotope 0
-
1
10
20 km
~
2
-~a~
3
p
2
1
1
Fig.8. Idealised section across the sea floor during Acuta zone times. A = thanatocoenosis of Homoeorhynchia acuta-Spiriferina villosa; B = thanatocoenosis of Spiriferina villosa; C = no brachiopods; 1 = littoral biotope; 2 = sublittoral biotope of agitated waters; 3 = sublittoral biotope of muddy bottoms.
241
LOWER JURASSIC BRACHIOPOD BIOTOPES
ions (there were deposited ferruginous limestones rich in leptochlorites) suitable conditions for the development of benthic life have not existed. This is why in these places are found especially pelagic organisms or forms which have an epiplanktonic mode of life. It appears that H. cynocephalalived in this way which can explain its relative abundance and the formation of the H. cynocephalathanatocoenosis. It is the pelagic biotope of Ager (1965).
GABROV0 o ?
? ?
JASALKA~',~
1o
38
2pkr~
F~-]g
Fig.9. Geographical distribution of brachiopod biotopes during Cynocephala zone times. 1 = sublittoral biotope of agitated waters; 2 = deeper water region without conditions suitable for brachiopod growth; 3 = littoral biotope without conditions for brachiopod growth; 4 = ? pelagic biotope with conditions suitable for the development of epiplanktonic brachiopods; 5 = Zeilleria lvcetti; 6 = Pseudogibbirhynchia moorei; 7 = Homoeorhynchia cynocephala; 8 = eroded sediments; 9 = outcrops of Lower Jurassic.
SW ,
thanclt°c°en°sis
KHAN
VRATZA I
A
C
B
i
~C
AMBARIT2~A
C
~
A
,
biotope I
A
SE
GLOJENE 4 thanatocoenosis IB
i
biotope
L
NW
NE
8ELEDIE i
19
2
3
3
&
0 t
2o~m
10
20kin
2
1
B
Fig.10 A, B. Idealised sections across the sea floor during Cynocephala zone times. A = thanatocoenosis of Homoeorhynchia cynocephala; B = thanatocoenosis of Zeilleria lycetti Pseudogibbirhynchia moorei; C = no brachiopods; 1 = ? pelagic biotope with conditions for the growth of epiplanktonic brachiopods; 2 = regions of deeper water; 3 = sublittoral biotope of agitated waters; 4 = littoral biotope.
242
P. TCHOUMATCHENCO
REFERENCES Ager, D. V., 1954. The genus Gibbirhynchia in the British Domerian. Proc. Geol. Assoc. (Engl.), 65: 21. Ager, D. V., 1963. Principles o f Paleoecology. McGraw-Hill, New York, N.Y., 371 pp. Ager, D. V., 1965. The adaption of Mesozoic brachiopods to different environments. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1: 143-172. Ager, D. V., 1967. Brachiopod palaeoecology. Earth-Sci. Rev., 3 (3): 157-179. Boekschoten, G. J., 1966. Shell borings of sessile epibiontic organisms as palaeo.ecological guides (with examples from the Dutch coast). Palaeogeogr., Palaeoclimatol., Palaeoecol., 2 (4): 333-379. Gekker, R. F., 1957. Vvedenie v paleoecologiu. Gosgeoltekhizdat, Moscow, 125 pp. (in Russian). Gekker, R. F., 1966. O prijiznenikh sviazakh organizmov v geologitsheskom prochlom. In: Organizm i sreda v geologitsheskom proshlom. Nauka, Moscow, pp. 14-30 (in Russian). Gekker, R. F., Ossipova, A. I. and Belskaia, T. N., 1962. Fergansldi zaliv paleogenovogo moria Srednei Assii,.Ego istoria, ossadki, fauna, flora, usslovia ich obitania i razvitie. Vol. 1 and 2. Akad. Nauk. S.S.S.R., Moscow, 330 and 332 pp. (in Russian). Gekker, R. F. and Kamicheva-Elpatskaia, V. G., 1966. O rabote i recheniakh pervikh dvukh paleoecologitsheskikh sessii. In: Organizm i sreda v geologitsheskom prochlom. Nauka, Moscow, pp. 263-266 (in Russian). lvanova, E. A., 1949. Uslovia sushtestvovania, obraz jizni i istoria nekotorikh brachiopod srednego i verkhnogo karbona Podmoskovnoi kotlovini. Tr. P I N A N U.S.S.R., 21:152 pp. (in Russian). Johnson, R. G., 1960. Models and methods for analysis of the mode of formation of fossil assemblages. Geol, Soc. Am. Bull., 71: 1075-1086. Merklin, R. L., 1950. Plastintshojabernie spirialissovikh glin, ikh sreda i jizni. Tr. PIN A N U.S.S.R., 28:93 pp. (in Russian). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1967. Biostratigraphy of the Lower Jurassic rocks near the village of Komstitza (distr. of Sofia) - West Balkan range. Bull, Geol. Inst., Ser. Stratigr. Lithol., 1 6 : 1 2 5 - 1 4 3 (in Bulgarian with English and Russian summary). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1971. Concerning certain features of the paleogeography of the Teteven area in the Early Jurassic. Bull. Geol. Inst., Ser. Stratigr. Lithol., 2 0 : 3 3 - 6 2 (in Bulgarian with English and Russian summary). Tchoumatchenco, P. V., 1967. Note sur la r~partion stratigraphique des Brachiopodes du Jurassique inf~rieur dans les Balkanides centrales et occidentales (Bulgarie). Coll. Jurassique, Luxembourg et Lorraine, 1967 (sous presse).
THANATOCOENOSES AND BIOTOPES OF L O W E R J U R A S S I C BRACHIOPODS
IN CENTRAL
AND WESTERN
BULGARIA
PLATON TCHOUMATCHENCO
Geolog4calInstitute, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Accepted for publication March 6, 1972)
ABSTRACT Tchoumatchenco, P., 1972. Thanatocoenoses and biotopes of Lower Jurassic brachiopods in central and western Bulgaria. Palaeogeogr., Palaeoclimatol., PalaeoecoL, 12: 227- 242. Brachiopod biotopes are recognized in the Early Jurassic rocks of Bulgaria. The geographical distribution of these is plotted for each of the stratigraphical zones determined earlier on the basis of brachiopods. The biotopes are related in turn to bathymetry, energy level and bottom type. INTRODUCTION I have already been able to propose a standard zonation of the Lower Jurassic in Bulgaria, based on brachiopods (Tchoumatchenco, 1967). This consists of the following zones: (1) Spiriferina walcotti zone (stratigraphical range: Hettangian and Sinemurian up to the obtusum zone, not inclusive). (2) Tetrarhynchia dunrobinensis zone (stratigraphical range: from the obtusum zone to the davoei zone, not inclusive). (3) Zeilleria cornuta zone (stratigraphical range: from the davoei zone to the stokesi sub-zone o f the margaritatus zone, inclusive). (4) Homoeorhynchia acuta zone (stratigraphical range: from the subnodosus sub-zone to the tenuicostatum zone). (5) Homoeorhynchia cynocephala zone (stratigraphical range: from the falciferum zone to the opalinim zone). In studying the geographical distribution of the zones I have established that their specific composition varies in the different regions, always within the limits of a zone, and characterised by its index fossil. It is therefore necessary to discover the causes of these changes, and to reconstruct the palaeoecological picture. THANATOCOENOSES A thanatocoenosis is formed by all the fossils preserved in the sediments of a region in the course o f a certain interval of time. The thanatocoenosis is formed of fossils
228
P. TCHOUMATCHENCO
belonging to various groups of animals but I propose here to deal solely with the thanatocoenoses formed by the brachiopods, since the complete thanatocoenoses can only be studied by teams of palaeontologists, specialising in the different fossil groups. The thanatocoenoses reflect sufficiently well the biocoenoses from which they are derived (Johnson, 1960). In distinguishing the thanatocoenoses I have used the numerical classification of Merkline (1950) as follows: "predominant species" - the number of specimens of this species must exceed 50 % of the specimens of other species which form part of the thanatocoenosis; "characteristic species" - the number of specimens more than 25 %; "associated species" - the number of specimens more than 10 %; "rare species" - the number of specimens not more than 10 %. I add here a new class - "strongly predominating species", for species the number of specimens of which exceed 80 % of the other species forming part of the thanatocoenosis. In the Lower Jurassic sediments of central and western Bulgaria a number of different thanatocoenoses have been recognised which occur in a chronological order within the framework of the brachiopod zones.
Spiriferina walcotti zone During the time of this zone there have been recognised the following thanatocoenoses: (1) Thanatocoenosis of Gibbirhynchia sp. nov.: characteristic species - Gibbirhynchia sp. nov. (35 %); associated species - Spiriferina walcotti (Dav.) (20 %), S. tumida (v. Buch) (10 %), Lobothyris grestenensis (Radovanovic) (14 %), L. punctata (Sowerby) (12 %); rare species - Zeilleria indentata var. quiaoisensis Choffat, Quadratirhynchia sp. nov., Lobothyris subovoides (MOnster). (2) Thanatocoenoses of Lobothyris punctata - L. grestenensis: strongly predominating species - Lobothyris punctata (85 %); L. grestenensis (9.1%) - on the limit between associated species and rare species; rare species - Spiriferina tumida, Tetrarhynchia dunrobinensis (Rollier), Lobothyris subovoides (MOnster) etc. Certain thanatocoenoses are poorly defined because of the restricted number of brachiopods which have been found. Of this kind is the thanatocoenosis of Lobothyris grestenensis, of which the index species is the only one which has been found (26 examples). Such is also the thanatocoenosis of Spiriferina tumida (characteristic species: 33.6 %) and associated species: Spiriferina walcotti (22.1%), Lobothyris grestenensis (22.1%), L. punctata (22.1%).
Tetrarhynchia dunrobinensis zone During this time the following thanatocoenoses have been recognised: (1) Thanatocoenoses of Tetrarhynchia dunrobinensis - Lobothyris subovoides: predominant species: Tetrarhynchia dunrobinensis ( 6 0 - 7 0 %); associated species -
LOWER JURASSIC BRACHIOPODBIOTOPES
229
Lobothyris subovoides (20 %); rare species - Gibbirhynchia curviceps (Quenstedt) (5.4 %), Cirpa fronto (Quenstedt) (3 %), Zeilleria darwini E. Desl. (2 %).
(2) Thanatocoenosis of Tetrarhynchia dunrobinensis: strongly predominating species - T. dunrobinensis (85 %); rare species - L. subovoides (8 %), Z. darwini (6 %), etc Zeilleria cornuta zone
In the sediments of this zone different thanatocoenoses have been found: (1) Thanatocoenosis ofL. subpunctata - L. edwardsii: predominant species L. subpunctata (Dav.) ( 5 0 - 6 0 %); associated species - L. edwardsii (Day.) (20.2 %); rare species - Spiriferina aseendes E. Desl. (7.5 %), S. alpina falloti Corroy. (6.4 %); Zeilleria sarthacensis (d'Orb.) (6.5 %), Cincta subnumismalis (Day.) (6.5 %), Z. eornuta (Sow.) (1.6 %), etc. (2) Thanatocoenosis of Cirpa langi - S. oxyptera - S. alpina falloti: characteristic species - Cirpa langi Ager (31%), S. alpinafalloti (29 %); associated species - S. o x y p t e m (Buv.) (16 %); rare species - Homoeorhynchia sp. n. (7 %), L. subpunetata (5 %), S. ascendes (3 %), Z. cornuta (3 %), Cincta subnumismalis (1.6 %), etc. In spite of its relatively low percentage S. oxyptera is a very characteristic species for this thanatocoenosis. Around the village of Sarantzi, near Sofia, one can observe a variation in the composition of this thanatocoenosis: Cirpa langi Ager is replaced by Cincta subnumismalis, whilst the other components are in practically the same percentages. This variation could be called, following Merkline (1950) an "association" within the frame of the same thanatocoenosis. (3) Thanatocoenosis o f Homoeorhynehia sp. n. - L. subpunctata: predominant species - Homoeorhynchia sp. n. (52-56 %); characteristic species - L. subpunctata (28 %); rare species - Z. eornuta (8 %), C. subnumilmalis (6 %), L. edwardsii (4 %), Z. sarthacensis (2 %), etc. (4) Thanatocoenosis of C. subnumismalis - L. subpunetata: predominant species C. subnumismalis (70 %); associated species - L. subpunctata (15-18 %); rare species S. ascendes (8 %), S. alpina falloti (7 %), etc. (5) Thanatocoenosis of Tetrarhynehia tetrahedra: strongly predominating species T. tetrahedra (90 %); rare species - L. subpunetata (2 %), L. edwardsii (2 %), etc. Hornoeorhynchia acuta zone
Two thanatocoenoses have been recognised in the sediments of this zone comprising the same species but in different percentages. (1) Thanatocoenosis of Spiriferina villosa: predominant species - S. villosa (Qu.) (72 %); associated species - H. acuta (Sow.) (20 %); rare species - Quandratirhynchia quadrata (Buckman) (10 %).
230
P. TCHOUMATCHENCO
(2) Thanatocoenosis ofH. a c u t a - S. villosa: characteristic species - H . acuta (47 %), S. villosa (46 %); rare species - Q. quadrata (7 %). H o m o e o r h y n c h i a eynocephala zone
Two thanatocoenoses have been recognised in the sediments of this zone: (1) Thanatocoenosis of Zeilleria lycetti-Pseudogibbirhynchia moorei: predominant species - Z. lycetti (Day.) (70 %); characteristic species - P. moorei (Dav.) (22 %), rare species - H. cynocephala (Richard) (8 %). (2) Thanatocoenosis ofH. cynocephala: a strongly predominating species 11. cynocephala (90 %); rare species - Z. lycetti (7 %), P. m o o r e i (3 %). BIOTOPES Gekker (1957, p.20) defined biotope as a restricted area of the living habitat determined by definite physical and chemical characteristics, conditioning the possibility of existence of a particular biocoenosis. Because of the variability of the factors of the habitat, characterising the biotope, a biocoenosis can be subdivided into association of species, occupying different parts of the biotope. I call these different parts of a biotope "microbiotopes" as they have peculiarities which differentiate them from the biotope but which are not sufficient to separate them clearly from that biotope. The most frequent of these are the transitions from one biotope to another (in the horizontal sense) where the physical and chemical characteristics of a biotope pass gradually from one biotope to the other. Ager (1965) defined 7 biotopes (which he called "environments" or "habitats") inhabited by Mesozoic brachiopods with distinctive morphology. Some of these polarised biotopes have been established in the Lower Jurassic sediments of central and western Bulgaria. The most common are the transitional microbiotopes. This is to be expected because one cannot pigeon-hole the diversity of nature in the narrow framework of an equal classification. In Bulgaria the following biotopes have been recognised: (1) littoral biotope; (2) sublittoral biotope with a sandy floor and agitated water; (3) sublittoral biotope with a muddy bottom; (4) deeper water biotope (? bathyal); (5) pelagic biotope. Littoral biotopes
Sediments and brachiopods belonging to this biotope are only found very rarely. It is especially in the erosional pockets that they are preserved. The sediments are limestones, sandy to gravelly in grain size and made of well-rounded quartz. The brachiopods which
LOWER JURASSIC BRACHIOPODBIOTOPES
231
inhabit the littoral biotope have a typical morphology - strong ribs, thick shell, large foramen and a slightly developed sinus, The foramen insures a stable attachment and the wide and feebly developed sinus strengthens the shell. On the other hand a deep sinus is not necessary because the rapid movements of the environment carry away very quickly the exhaled material. The brachiopods which inhabited this biotope are: Caleirhynehia glevensis, S. walcotti, L. grestenensis. The last-named taxon, in order to strengthen its shell, had a rather strange mode of growth: the shell concentrated the growth lines after a temporary cessation of growth in length. A little later it produced further growth simultaneously in length and thickness. This alternation of stops and starts in growth happening many times gave the shell a staircase-like appearance.
Sublittoral biotope with sandy floor and agitated water In the sediments formed in the conditions of this biotope we have collected the largest numbers of brachiopods. The sediments enclosing the fossils are sandy and detritic limestones, often oolitic and ferruginous. These are the limestones of the Ozirovo Formation (sensu Sapunov, 1967). The brachiopods inhabiting this biotope have a "normal" aspect. The rhynchonellids possess more or less well-developed ribs and a more or less wide and deep sinus. One finds here the following rhynchonellids: Tetrarhynch& dunrobinensis, T. subconcinna,
T. argotinensis, Pseudogibbirhynchia moorei, Q. quadrata, Rhynchonelloidea lineata tetewensis, etc., also the cynocephalous rhynchonellids - Homoeorhynchia acuta, Homoeorhynchia sp. nov. The representatives ofH. acuta coming from this biotope have thicker shells with a wider sinus compared with those which come from the sublittoral biotope in calm water and with a muddy sea floor. The tetebratulids are represented by species of Lobothyris: L. punctata, L. subpunctata, L. subovoides, L. edwardsii. The zeilleriids are represented by Zeilleria indentata, Z. indentata vat. quiaosensis Choffat, Z. cornuta, Z. sarthacens&, Z. lycetti, Keratothyris cot, Cincta subnumismalis, but the last-named taxon has its maximum development in the sublittoral biotope with deeper water and a muddy floor. The greater part of the spiriferinas have inhabited this biotope: S. tumida, S. alpina falloti, S. walcotti, etc. A large proportion of the brachiopods which inhabited the floor of this biotope lived in banks in order to resist the currents and we find them now in the form of lenses. This mode of life was followed by: 7]. dunrobinens& (this taxon forms lenses in the vicinity of the town of Teteven), Lobothyris subpunctata, Spiriferina ascendes, Z. sarthacensis. Examples of these have lived so tightly crowded together one against the other that their growth has been distorted and many of them have asymmetrical shells (Z. sarthacens& around the village of Ravna, near Sofia). In the sediments around the village of Berend~ izvor, near Sofia, are found shells with traces of attachment similar to those described by Boekschoten (1966) in Arthopyren& subfittoralis (Leighton), living, according to Sandeson (Boekschoten, 1966) in water of variable salinity from 15-20 % and in well-aerated waters rich in oxygen.
232
P. TCHOUMATCHENCO
Sublittoral biotope with muddy bottoms and calm waters The waters of this biotope are comparatively deeper and have a muddy bottom, not very suitable for the life of brachiopods. The typical sediments are marks of the Boukorovtzi member and the Ravna Member (sensu Sapunov, 1967). Of all the brachiopods, those which have been transported by palaeocurrents into the conditions of this biotope survive and give a single generation of those of small size and those which in the adult state possess adaptations for a life resting on mud. These are the following brachiopods: Gibbirhynchia sp. nov., S. villosa, Homoeorhynchia acuta. Spiriferina villosa possesses spines which held the creature above the mud. H. acuta has a deep sinus which facilitates the separation of inhalant and exhalant currents. The representatives of these taxa living in this biotope have thinner and lighter shells than those inhabiting the previous biotope. Only a limited number of species are adapted for a life in the conditions of the sublittoral biotope with a muddy bottom and calm water, but certain of them are developed in large numbers: S. villosa (north of the village of Komstitza, near Sofia), Gibbirhynchia sp. nov. (north of the villages of Komstitza, Ravna, etc. near Sofia).
Deep water biotope (bathyal?) A limited number of brachiopods are adapted to life in the conditions of this biotope. The sediments are of argillaceous limestones and marly clays. The characteristic features for this biotope are calm water and limited circulation of oxygen. In the conditions of this biotope the only brachiopods which can survive are those having the inhalant and exhalant currents separated as far as possible. It is difficult to speak of bathyal regions in the Lower Jurassic basin of central and western Bulgaria occupied by brachiopods. Rather there are bays of calm water with limited circulation of oxygen. A region with such conditions existed around the villages of Gintzi and Zimevitza, near Sofia. In this region the only species to be developed is S. oxyptera, which possesses a long and straight cardinal margin which provides a maximum separation of the inhalant and exhalant currents. The floor has been populated by Cirpa langi, the sinus of which - wide and relatively deep - plays the same role. The conditions of this biotope have been optimal for the development of Cincta subnumismalis. Its shell - wide and in the form of a coin - with its little foramen, has not been able to attach itself firmly to the floor and this is the reason for its relatively small size and rare occurrence in the conditions of biotopes other than that of the deeper waters. In the sediments of this biotope C. subnumismalis reaches its maximum development in size and in number in the argillites and marls around the town of Teteven. In order to preserve its position with the umbonal region downwards, C. subnumismalis deposits calus in this region to make this part of the shell heavier, and this serves to stabilise the shell.
LOWER JURASSIC BRACHIOPOD BIOTOPES
233
Pelagic biotope For certain brachiopods of small size and thin shell Ager (1965) suggested that they lived an epiplanktonic life. That is proved by our observations. During the time of the H. cynocephala zone the representatives of this group dwelt in the region of the villages of Beledi6 khan and Dragovistitza, near Sofia, the Ambaritza chalet, the village of Glojene, near Lovetch, of Gaganitza and Gorno Ozirovo, near Mihailovgrad. In some of these localities ferruginous limestones were deposited or even ironstones, whereas in the other parts of these outcrops the limestones are impoverished in iron. This demonstrates that in the first group of localities, during sedimentation, the water has had a strong concentration of ferric ions. In both groups of localities the following brachiopods are the same but in different percentages: Z. lycettg H. cynocephala and P. moorei. In the group of localities with sediments rich in iron H. cynocephala is represented by 9 1 % and Z. lycetti and P. moorei together by 9 %. In the sediments not containing iron H. cynocephala and P. moorei are represented by 92 % and Z. lycetti by8%. Around the village of Glojene, near Lovetch I have collected only Z. lycettL This great difference in the percentages of the same species in the two groups can be explained by the great resistance ofH. cynocephala to the high ferric ion content in the marine waters and an epiplanktonic mode of life. It was suited to this mode of life by its small size (even in the adult stages) and light shell. Representatives of this taxon are found in large numbers with good swimmers - ammonites and belemnites - to which they could have attached themselves and survived this poisoning of the water with ferric ions, which were probably in higher concentration near the sea floor. GEOGRAPHICALDISTRIBUTION OF THE BRACHIOPODBIOTOPES
Spiriferina walcotti zone During the deposition of the beds characterised by the representatives of the S. walcotti zone, uniform conditions did not exist in the Lower Jurassic basin of central and western Bulgaria (Fig.1 and 2). The transgressive sediments and the fossils which they contain are preserved in isolated localities. The thanatocoenosis S. walcotti was formed in this littoral biotope; these come from around the villages of Komstitza (near Sofia) and of Glojene (near Lovetch). Later, after the Lower Jurassic transgression, still in the S. walcotti zone in the region between the villages of Komstitza, Ravna and Kalotina there were established the conditions of the sublittoral biotope with muddy bottom. In the sediments accumulated the thanatocoenosis of Gibbirhynchia sp. nov. The sediments are the marls of the Ravna Member (sensu Sapunov, 1967). In the region between the villages of Slivnitza and Kremikovtzi (to the north of Sofia) were established the conditions of the sublittoral biotope of agitated waters - a
234
P. TCHOUMATCHENCO
BOV
II/ t 1
SLIVNIT Z ~ ' ~ ~ t
1 ~ ~
~
~
'
SO~IA'~
~
~
GABROVOo P
SLJ!L!!J
~
""
L
i
i
i
Fig.1. Geographical distribution o f brachiopod biotopes during Walcotti zone times. 1 = littoral biotope w i t h o u t conditions suitable for the growth o f brachiopods; 2 = littoral biotope; 3 = sediments eroded; 4 = sublittoral biotope o f m u d d y b o t t o m s ; 5 = sublittoral biotope o f agitated waters; 6 = sublittoral b i o t o p e - m i c r o b i o t o p e o f deeper water; 7 = Spiriferina tumida; 8 = Spiriferina walcotti; 9 = Gibbirhynchia sp. nov.; 10 = Lobothyris gresienensis; 11 = L. punctata; 12 = outcrops of Lower Jurassic; 13 = sublittoral biotope o f agitated w a t e r s - m i c r o b i o t o p e passing into the littoral biotope.
thanatocoenosis
W B
D
Jsker river I I i I
E
TETEVEN i C
E
A
i
I
C
biotope 20kin 1
2
3
2
1
3
Fig.2. Idealised section across the sea floor during Walcotti zone times. A = thanatocoenosis of
Spiriferina walcotti-Lobothyris punctata ; B = thanatocoenosis o f Gibbirhynchia sp. nov.; C = no brachiopods; D = thanatocoenosis ofLobothyris punctata; E = thanatocoenosis o f Lobothyris grestenensis; 1 = sublittoral biotope o f m u d d y b o t t o m s ; 2 = sublittoral biotope o f agitated waters; 3 = littoral biotope w i t h o u t conditions for brachiopod growth.
LOWER JURASSIC BRACHIOPOD BIOTOPES
235
passage microbiotope passing towards the littoral biotope. There were deposited sandy and detritic limestones of the Ozirovo Formation (sensu Sapunov, 1967) in which accumulated the poorly-defined thanatocoenosis ofL. punctata, L. grestenensis, S. walcotti, S. tumida. In the region between the villages of Gintzi, Boy, Zimevitza and Milanovo (near Sofia) and the Ledenika chalet (near Vratza) the conditions were not favourable for the growth of brachiopods - there were deposited continental and marine sandstones. Further east there was an island - the island of Vratza. Around the village of Gaganitza there existed the conditions of the sublittoral biotope with agitated water and there formed the thanatocoenosis of L. punetata. Around Teteven there was a microbiotope intermediate between the sublittoral biotope with agitated water and that of muddy bottoms. The brachiopods there are: S. walcotti and L. grestenensis (single specimens). Around the villages of Lessidren, Chipkovo, Tchernata reka and in the Trman Balkan conditions were very close to those of the littoral biotope without being suitable for the development of brachiopods. Around the village of Jabalka (near Gabrovo) there were established the conditions of an intermediate biotope between the sublittoral biotope of agitated water and the littoral biotope. In the sediments accumulated the thanatocoenosis consisting almost entirely of L. grestenensis. Around the village of Etropol~ there are no brachiopods from this time; there was a biotope of deeper water without suitable conditions for brachiopod growth. Tetrarhynchia dunrobinensis zone
During the times of this zone there was a levelling out of the living conditions of the basin. The sandy or detritic limestones of the Ozirovo Formation were deposited (Fig.3 and 4). The T. dunrobinensis zone is situated immediately above the S. walcotti zone in the area between the villages of Kalotina, Ravna, Komstitza and Gaganitza. In this region accumulated the thanatocoenosis of T. dunrobinensis - L. subovoides very rich in specimens. In the area between the villages of Gintzi, Zimevitza, Boy and Gorno Ozirovo this was the lowest brachiopod zone established; that is to say the living conditions have become favourable for the development of brachiopods only at the time of this zone. Here was formed the same thanatocoenosis as in the preceding region but very poor in specimens. The brachiopods are found in sandy limestones, even in sandy limestones intercalated between sandstones around the villages of Bov and Zimevitza. In this region there was established the sublittoral biotope of agitated water. In the region between the villages of Slivnitza, Kremikovtzi, and Sarantzi was a biotope intermediate between the sublittoral biotope of agitated water and the littoral biotope, with the formation of the thanatocoenosis of T. dunrobinensis. The same conditions were established in the region of Krai'ste where almost all the shells are separated valves.
236
P. TCHOUMATCHENCO
oGABROVO /TETEVEN
© SOFIA
ZEMEN Fig.3. Geographical distribution of brachiopod biotopes during Dunrobinensis zone times. 1 = island; 2 = littoral biotope (without suitable conditions for the growth of brachiopods); 3 = microbiotope transitional between the sublittoral biotope of agitated waters and the littoral biotope; 4 = sublittoral biotope of agitated waters; 5 = sublittoral biotope, microbiotope passing into deeper water; 6 = denuded sediments; 7 = Tetrarhynchia dunrobinensis; 8 = Gibbirhynchia curviceps; 9 = Cirpa f r o n t o ; 10 = L o b o t h y r i s subovoides; 11 = Zeilleria darwini 12 = Cincta subnumisrnalis; 13 = outcrops of Lower Jurassic.
SW
thanatocoenosis
biotope 0
NE: :=NW
SLIVNITZA ii
D
1',A
SE
GLOJENE
VRATZA
l
C
I
:i
B
B
C
f[/
3 10
20kin
Fig.4. Idealised section across the sea floor during Dunrobinensis zone times. A = thanatocoenosis of Tetrarhynchia dunrobinensis; B = thanatocoenosis of Tetrarhynchia d u n r o b i n e n s i s - L o b o t h y r i s subovoides; C = no brachiopods; D = brachiopods rare, mostly single valves; 1 = littoral biotope; 2 = sublittoral biotope of agitated waters; 3 = microbiotope transitional between the sublittoral biotope of agitated waters and the littoral biotope; 4 = island.
LOWER JURASSICBRACHIOPODBIOTOPES
237
The conditions of the sublittoral biotope of agitated water which were developed in the region around Teteven were optimal for the development of the thanatocoenosis of T. dunrobinensis-L, subovoides. The representatives of T. dunrobinensis were developed in masses and have retained their largest dimensions. To the north around the village this zone was not established. Around the area of Raventa between Glojene and Teteven the representatives of the T. dunrobinensis zone have been found in a condensed state. This demonstrates that in the northern part of the Teteven area an island existed at this time, or a shallow sea around which there did not exist conditions favourable for the growth of brachiopods. In the southern part of the Teteven region around the village of Chipkovo and in the Tcherna reka river valley was established the sublittoral biotope, a microbiotope intermediate with the littoral biotope. Brachiopods are rare, and the thanatocoenosis is not well characterised - T. dunrobinensis-L, subovoides in almost equal numbers. Farther south brachiopods have not been found in this zone - the littoral biotope has not been favourable for brachiopod growth. Around Etropol6 the representatives of this zone are found in isolated beds of limestone. It appears that in this region there existed conditions unfavourable for brachiopods - a deep sea and favourable conditions were established only from time to time and then the T. dunrobinensis thanatocoenosis was formed.
Zeilleria cornuta zone The basin had almost the same contours as in that of the times of the T. dunrobinensis zone (Fig.5 and 6). To the west, in the region between the villages of Kalotina, Ravna, Komstitza and Gaganitza the sublittoral biotope with agitated water still continued. This region at this time provided the richest collection of brachiopods. There was formed the thanatocoenosis of L. subpunctata-L, edwardsii. Further east around the villages of Gintzi and Zimevitza there were established the conditions of the deep water biotope (? bathyal). It appears that there existed here the waters of a calm bay with limited oxygen circulation. On the floor was formed the thanatocoenosis of C. langi-S, alpina
fallotti- S. oxyptera. Around the village of Gorno Ozirovo was formed the thanatocoenosis of T. tetrahedra reflecting a biotope very near littoral conditions. The microbiotope which occupied the region between the villages of Slivnitza and Kremikovtzi was near the biotope of agitated waters, differing only in having higher energy conditions in the sea. There was formed the thanatocoenosis ofHomoeorhynchia sp. nov. L. subpunctata. In the region of Kraiste the Z. cornuta zone is indicated by the rare examples of Homoeorhynehia sp. nov. This shows that intermediate conditions still existed there between the sublittoral biotope of agitated waters and the sublittoral biotope. Around Teteven was another microbiotope, intermediate between the sublittoral biotope of agitated water and that of muddy bottoms. In the sediments of this biotope
238
P. TCHOUMATCHENCO
Fig.5. Geographical distribution of brachiopod biotopes during Cornuta zone times. 1 = region of clastic sedimentation without conditions for brachiopod growth; 2 = littoral conditions-around the island; 3 = littoral biotope; 4 = sublittoral biotope of agitated waters-microbiotope towards very agitated waters; 5 = sublittoral biotope of agitated waters; 6 = sublittoral biotope of deeper waters; 7 = sublittoral biotope intermediate between the sublittoral biotope of muddy bottoms and that of agitated waters; 8 = .9 bathyal biotope (sublittoral biotope of muddy bottoms); 9 = sublittoral biotope of very agitated waters; 10 = denuded sediments; 11 = littoral part of the sublittoral biotope of muddy bottoms; 12 = Spiriferina oxyptera; 13 = Spiriferina alpina falloti; 14 = Spiriferina haueri; 15 = Tetrarhynchia tetrahedra ; 16 = Homoeorhynchia sp. nov.; 17 = Lobothyris subpunctata ; 18 = Lobothyris edwardsii; 19 = Zeilleria cornuta; 20 = Zeilleria sarthacensis; 21 = Cincta subnumismalis; 22 = Cirpa langi; 23 langi; 23 = outcrops of Lower Jurassic.
was formed the thanatocoenosis o f C. s u b n u m i s m a l i s - L ,
s u b p u n c t a t a . In the area
between the villages o f Lessidren and Chipkovo there was the typical sublittoral biotope o f agitated waters, on the floor of which was formed the thanatocoenosis of L. s u b p u n c t a t a - L ,
edwardsii.
In the region o f the river Tcherna reka existed the microbiotope intermediate between the sublittoral biotope o f agitated waters and the littoral biotope. On the sea floor formed the thanatocoenosis o f H o m o e o r h y n c h i a
sp. n o v . - L , s u b p u n c t a t a . T h e marine
waters were very agitated; the evidence for this is that almost all the shells o f L. s u b p u n c t a t a are single valves piled up one on the other.
In the area o f Entropol6 the sea was always deep, lacking in conditions favourable for the growth o f brachiopods. During the Z. c o r n u t a zone there were deposited everywhere the detritic sandy, or crinoidal limestones o f the Ozirovo Formation.
LOWER JURASSIC BRACHIOPOD BIOTOPES S
239
N SW
ZEMEN
NE NW SE
~ VRATZA
KALOTINA 1
TETEVEN
I i
i
; 1
B
D
B
E
; 0
2
1
3
',
iF F .
! c
thanatocoenosis
biotope
~
Z i
1 O
m
2Okra
2
5
I
12
2
I, I
Fig.6. Idealised section across the sea floor during Cornuta zone times. A = thanatocoenosis of Homoeorhynchia sp. nov.-Lobothyris subpunctata ; B = thanatocoenosis of Lobothyris subpunctata L. edwardsii; C = no brachiopods; D = thanatocoenosis of Cirpa langi-Spiriferina alpina falloti S. oxyptera: E = thanatocoenosis of Tetrarhynehia tetrahedra; F = thanatocoenosis of Cincta subnumismalis-Lobothyris subpunctata; F 1 = the same thanatocoenosis partly condensed; H = brachiopods abundant: lobothyris punetata predominating and less Tetrarhynchia tetrahedra; 1 = sublittoral biotope of agitated waters; 2 = microbiotope of very agitated waters; 3 = littoral biotope; 4 = island; 5 = bathyal biotope; 12 = microbiotope intermediate between the sublittoral biotope of agitated waters and the sublittoral biotope of muddy bottoms.
Homoeorhynchia acuta zone
One may collect brachiopods belonging to this zone only in some localities - around the villages of Komstitza, Kalotina, Ravna, Beledie khan, Dragovistitza and Gaganitza (Fig.7 and 8). Around the villages of Komstitza and Kalotina there existed the sublittoral biotope of m u d d y b o t t o m s on which was formed the thanatocoenosis of S. villosa, which can be collected from the marls of the Boukorovtzi member (sensu Sapunov, 1967). The sublittoral biotope o f agitated waters prevailed around the villages of Beledie khan and Dragovistitza (there were deposited detritic, sandy or crinoidal limestones o f the Ozirovo Formation). On the floor was accumulated the thanatocoenosis of H. acuta S. villosa. The shells of the representatives of these taxa are thicker, stronger and relatively larger than the shells of the same taxa, inhabiting the sublittoral biotope with m u d d y bottoms. Elsewhere in central and western Bulgaria conditions favourable for brachiopods did not exist, either because the sea was too deep (in the region o f Teteven and Etropol~) or because the sea was very shallow and the water very agitated (in the region of Kraiste, the Troian Balkan and the southern parts of Teteven).
Homoeorhynchia cynocephala zone During this time conditions were only favourable for brachiopod growth in isolated places (Fig.9 and 10 A, B). Around the village of Gaganitza was established the micro-
I
3
240
P. TCHOUMATCHENCO
biotope intermediate between the sublittoral biotope of agitated water and the biotope o f m u d d y b o t t o m s . On t h e f l o o r was f o r m e d t h e t h a n a t o c o e n o s i s o f Z. l y c e t t i - P , moorei. In this place was d e p o s i t e d t h e B o u c o r o v t z i M e m b e r (marls). T h e same t h a n a t o c o e n o s i s was f o r m e d a r o u n d t h e villages o f G o r n o Ozirovo a n d G l o j e n e . A t this t i m e in t h e region n o r t h o f Sofia a n d in t h e T r o i a n Balkan, b e c a u s e o f t h e p o i s o n i n g o f the sea b y ferric
GABROVO?~ ~, JABALKA. - ?~?
SKOBELEVO
[ff~]a ff-2q~
Fig.7. Geographical distribution of brachiopod biotopes during Acuta zone times. 1 = littoral conditions around island; 2 = region of clastic sedimentation without conditions for brachiopod growth; 3 = littoral biotope; 4 = sublittoral biotope of agitated waters; 5 = sublittoral biotope of muddy bottoms; 6 = deeper marine region without conditions suitable for brachiopod growth; 7 = denuded sediments; 8 = Spiriferina villosa; 9 = Hornoeorhynchia acuta ; 10 = Quadratirhynchia quadrata ; 11 = outcrops of Lower Jurassic. NE VRATZA
SW ZEMEN
!
C
A
B
A
C
thanatocoenosis
biotope 0
-
1
10
20 km
~
2
-~a~
3
p
2
1
1
Fig.8. Idealised section across the sea floor during Acuta zone times. A = thanatocoenosis of Homoeorhynchia acuta-Spiriferina villosa; B = thanatocoenosis of Spiriferina villosa; C = no brachiopods; 1 = littoral biotope; 2 = sublittoral biotope of agitated waters; 3 = sublittoral biotope of muddy bottoms.
241
LOWER JURASSIC BRACHIOPOD BIOTOPES
ions (there were deposited ferruginous limestones rich in leptochlorites) suitable conditions for the development of benthic life have not existed. This is why in these places are found especially pelagic organisms or forms which have an epiplanktonic mode of life. It appears that H. cynocephalalived in this way which can explain its relative abundance and the formation of the H. cynocephalathanatocoenosis. It is the pelagic biotope of Ager (1965).
GABROV0 o ?
? ?
JASALKA~',~
1o
38
2pkr~
F~-]g
Fig.9. Geographical distribution of brachiopod biotopes during Cynocephala zone times. 1 = sublittoral biotope of agitated waters; 2 = deeper water region without conditions suitable for brachiopod growth; 3 = littoral biotope without conditions for brachiopod growth; 4 = ? pelagic biotope with conditions suitable for the development of epiplanktonic brachiopods; 5 = Zeilleria lvcetti; 6 = Pseudogibbirhynchia moorei; 7 = Homoeorhynchia cynocephala; 8 = eroded sediments; 9 = outcrops of Lower Jurassic.
SW ,
thanclt°c°en°sis
KHAN
VRATZA I
A
C
B
i
~C
AMBARIT2~A
C
~
A
,
biotope I
A
SE
GLOJENE 4 thanatocoenosis IB
i
biotope
L
NW
NE
8ELEDIE i
19
2
3
3
&
0 t
2o~m
10
20kin
2
1
B
Fig.10 A, B. Idealised sections across the sea floor during Cynocephala zone times. A = thanatocoenosis of Homoeorhynchia cynocephala; B = thanatocoenosis of Zeilleria lycetti Pseudogibbirhynchia moorei; C = no brachiopods; 1 = ? pelagic biotope with conditions for the growth of epiplanktonic brachiopods; 2 = regions of deeper water; 3 = sublittoral biotope of agitated waters; 4 = littoral biotope.
242
P. TCHOUMATCHENCO
REFERENCES Ager, D. V., 1954. The genus Gibbirhynchia in the British Domerian. Proc. Geol. Assoc. (Engl.), 65: 21. Ager, D. V., 1963. Principles o f Paleoecology. McGraw-Hill, New York, N.Y., 371 pp. Ager, D. V., 1965. The adaption of Mesozoic brachiopods to different environments. Palaeogeogr., Palaeoclimatol., Palaeoecol., 1: 143-172. Ager, D. V., 1967. Brachiopod palaeoecology. Earth-Sci. Rev., 3 (3): 157-179. Boekschoten, G. J., 1966. Shell borings of sessile epibiontic organisms as palaeo.ecological guides (with examples from the Dutch coast). Palaeogeogr., Palaeoclimatol., Palaeoecol., 2 (4): 333-379. Gekker, R. F., 1957. Vvedenie v paleoecologiu. Gosgeoltekhizdat, Moscow, 125 pp. (in Russian). Gekker, R. F., 1966. O prijiznenikh sviazakh organizmov v geologitsheskom prochlom. In: Organizm i sreda v geologitsheskom proshlom. Nauka, Moscow, pp. 14-30 (in Russian). Gekker, R. F., Ossipova, A. I. and Belskaia, T. N., 1962. Fergansldi zaliv paleogenovogo moria Srednei Assii,.Ego istoria, ossadki, fauna, flora, usslovia ich obitania i razvitie. Vol. 1 and 2. Akad. Nauk. S.S.S.R., Moscow, 330 and 332 pp. (in Russian). Gekker, R. F. and Kamicheva-Elpatskaia, V. G., 1966. O rabote i recheniakh pervikh dvukh paleoecologitsheskikh sessii. In: Organizm i sreda v geologitsheskom prochlom. Nauka, Moscow, pp. 263-266 (in Russian). lvanova, E. A., 1949. Uslovia sushtestvovania, obraz jizni i istoria nekotorikh brachiopod srednego i verkhnogo karbona Podmoskovnoi kotlovini. Tr. P I N A N U.S.S.R., 21:152 pp. (in Russian). Johnson, R. G., 1960. Models and methods for analysis of the mode of formation of fossil assemblages. Geol, Soc. Am. Bull., 71: 1075-1086. Merklin, R. L., 1950. Plastintshojabernie spirialissovikh glin, ikh sreda i jizni. Tr. PIN A N U.S.S.R., 28:93 pp. (in Russian). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1967. Biostratigraphy of the Lower Jurassic rocks near the village of Komstitza (distr. of Sofia) - West Balkan range. Bull, Geol. Inst., Ser. Stratigr. Lithol., 1 6 : 1 2 5 - 1 4 3 (in Bulgarian with English and Russian summary). Sapunov, I. G., Tchoumatchenco, P. V. and Shopov, V. L., 1971. Concerning certain features of the paleogeography of the Teteven area in the Early Jurassic. Bull. Geol. Inst., Ser. Stratigr. Lithol., 2 0 : 3 3 - 6 2 (in Bulgarian with English and Russian summary). Tchoumatchenco, P. V., 1967. Note sur la r~partion stratigraphique des Brachiopodes du Jurassique inf~rieur dans les Balkanides centrales et occidentales (Bulgarie). Coll. Jurassique, Luxembourg et Lorraine, 1967 (sous presse).