The Rhynchonellide Brachiopod Cyclothyris ? globata (Arnaud, 1877) from the Santonian-Campanian of Pannonides, Carpatho-Balkanides and Dinarides, South-Eastern Europe

The asymmetric rhynchonellide brachiopods Rhynchonella difformis Valenciennes in Lamarck and R. contorta d'Orbigny were often mentioned in the stratigraphic literature on Santonian-Campanian outcrops of the Pannonides, Carpatho-Balkanides and Dinarides (southeastern Europe). A recent taxonomic study including modern methods of serial sectioning and shell microstructure of brachiopods has resulted in revision of both species, identified now as Cyclothyris ? globata (Arnaud).


INTRODUCTION
Rhynchonellides with asymmetrical anterior commissure with a "step" on the right or left side frequently occur in several Santonian ‡Campanian outcrops in Pannonides, Carpatho ‡Balkanides and Dinarides of southeastern Europe (Fig. 1).The "twist" affects not only the anterior commissure, but also the overall morphology of the brachiopod.In the stratigraphic literature these asymmetric species were cited as Rhynchonella difformis Valenciennes in Lamarck or R. contorta d'Orbigny.In fact the last two species occur only in Cenomanian outcrops in Europe.Cyclothyris difformis is well known from Early and Middle Cenomanian of France, England, Belgium, Germany, Poland, Czech Republic and Bulgaria; while Cyclothyris contorta d'Orbigny is cited from Cenomanian of Belgium, Spain and Bulgaria (Owen, 1962(Owen, , 1988;;Nekvasilova, 1973;Popiel ‡Barczyk, 1977;Muñoz, 1985;Motchurova ‡Dekova, 1995, 1997).After careful examination of the external and internal morphology of material collected from different Santonian ‡Campanian localities of Pannonides, Carpatho ‡Balkanides and Dinarides it is concluded now that these asymmetric rhynchonellides can be referred to the species Cyclothyris ?globata (Arnaud).In this paper some details of the occurrences of Cyclothyris ?globata in the studied area are briefly presented and a revision of the previous determinations is proposed.The external and internal morphology of this peculiar rhynchonellide species and its shell microstructure are described.Mountains referred exclusively to Maastrichtian age, are well known with their extremely rich and well preserved macro ‡ and microfauna (Pethö, 1906;Pa{i} 1951b, Petkovi} et al., 1976, cum. lit.).The brachiopod fauna in these localities (6 species in Pethö's collection, and 7 in Pa{i}'s collection), originates from "two separated blocks" of compact green ("serpentine"?) sandstone, i. e. from Pethö's "horizon" 11 (block 7, sensu Petkovi} et al., 1976, p. 58 ‡62).Besides brachiopods, Pethö (1906) described a rich assemblage of cephalopods, gastropods and especially bivalves (the Cenomanian species Inoceramus crippsii Mantell among them!) from these beds.
Chaotic series (mélange) were found in this part of the Fru{ka Gora Mountains.These chaotic series differ in age from the chaotic series of Drenica ‡ West Serbia (Radoi~i}, 1994).The chaotic series of the Fru{ka Gora Mountains are post ‡Maastrichtian in age.
The list of the brachiopods and other fossils from the ^erevi}ki and Dobri stream contains species of different ages (Cenomanian, Santonian ‡Campanian, Campanian and Maastrichtian) and of different environments.Accordingly, it is not here possible to determine the original stratigraphic position of the beds with Cyclothyris ?globata.Further detailed studies of the outcrops in these localities should explain the position of the reworked older fossils and sediments, as those younger too.According to other occurrences in Serbia we suppose that the age of Cyclothyris ?globata in the Fru{ka Gora Mountains is most probably Campanian.
According to the data available, we consider that the beds with C. ?globata in Eastern Serbia are not younger than Campanian, while they are of Late Santonian age in the adjacent area in Bulgaria.
Macedonia.Sedimentary rocks west of Veles and village of Banjica are facially identical to those in Dupljaj stream (Western Serbia) and are probably of the same age, not older than Middle Santonian (Radoi~i} & Pejovi}, personal communication, 1999).They contain cephalopods (mostly nautilids), brachiopods (Cyclothyris ?globata) and bivalves.Based on the mentioned macrofossils, the beds with brachiopods are of Campanian age.Temkova (1962) considers these beds of Maastrichtian age.

Outer Dinarides
Croatia.In the island of Bra~ Rhynchonella difformis globata (= Cyclothyris ?globata) is known from the sedimentary rocks of Santonian age (Pejovi} & Radoi~i}, 1987).The lowermost member of the Povlja section (eastern part of Bra~ Island) consists of foraminiferal limestones: grainstone with Accordiella conica Farincci, Scandonea mediterranea De Castro, numerous nubecularids etc., and subordinate layers with Pithonella and Stomiosphaera.Cyclothyris ?globata is found in these limestones.They continue upward following the first occurrence of the Keramosphaerina tergestina Stache with the same foraminiferal assemblage and numerous rudists (Bournonia, Gorjanovicia, Hippurites, Radiolites, Rajka, Vaccinites).This part of the Povlja section is Santonian ‡Early Campanian in age (Pol{ak & Mamuzi}, 1969;Pejovi} & Rado-i~i}, 1987).There is no information about the sedimentary rocks underlying the bed with Cyclothyris ?globata in the Povlja area.Pejovi} & Radoi~i} (personal communication, 1999) interpret this Senonian interval on the basis of the paleontological content of the superposed limestones as a part of Dol Formation in the sense of Gu{i} & Jelaska (1990).The Dol Formation is assigned mainly to the Campanian by these authors.It is interesting to note the following data, which were mentioned by Swinburne (in Morris & Skelton, 1995, p. 278) that "dated the Bra~ Marbles (= Pu~i{}a Formation) as Early ‡Middle Campanian, based on Sr isotope correlations, in agreement with Pejo-vi} & Radoi~i}".According to these data the age of the beds with C. ?globata is Early Campanian.
External morphology.Shell of medium size, up to 25 mm long, generally with subtriangular outline, rarely transversely ‡oval, always with twisted asymmetrical anterior commissure (11 specimens with right side up, 7 specimens with left side up).Dorsal valve usually much more convex than ventral valve.Greatest width in the anterior third, maximum thickness at midvalve.Lateral commissures straight.Beak massive, suberect to nearly straight with sharp and short beak ridges.Apical angle ranges between 72° and 102°.Foramen rounded, circular, relatively large, hypothyrid to submesothyrid.Interarea small, concave.Each valve is ornamented with 24 to 32 simple ribs, which anteriorly become stronger and sharper.Median septum occupying from 0.37 to 0.48 of the dorsal valve length.
Internal morphology (Figs. 3 ‡6).The description that follows is based on four sectioned specimens from the three localities: Gu~a, Bra~ and Nanos.A pedicle collar is developed.Dental plates divergent disappearing before the teeth arise, bounding large, subtrapezoidal delthyrial chamber.Lateral umbonal cavities small, semicircular.Hinge teeth massive, short and rectangular, crenulated, with distinct denticulae.Septalium not present.Outer hinge plates subhorizontal, very gently ventrally concave.Septum represented by a low ridge.Crural bases concave, oblique inclined at 35 ‡50° towards the hinge axis.Crura radulifer in type, triangular or with thickened distal ends, ("diabolo" like sections), slightly to strongly curved ventrally, and divergent anteriorly.
Shell microstructure.The preliminary data on the shell microstructure of Cyclothyris ?globata were published by Motchurova ‡Dekova (1992a, 1992b, 1995).At that time one of us (N.M. ‡D.) believed that the shell of genus Cyclothyris was composed of three calcitic layers: primary microcrystalline, secondary fibrous and tertiary prismatic.However, more recent extensive SEM observations on numerous samples of some representatives of the genus Cyclothyris have shown unambiguously that tertiary prismatic layer is not developed in the genus Cyclothyris and in all other Cretaceous rhynchonellide genera studied (Motchurova ‡Dekova, 2000, 2001).This called for the revision of the described previously shell microstructure of the Bulgarian material.The recent SEM observations showed that the diagenetically formed secondary calcite prisms perpendicular to the inner shell surface were previously misidentified for original tertiary calcitic layer (Pl.III, Figs. 4,5).Previously the pseudo ‡tertiary prisms were observed mainly in thin sections where they were obviously misidentified with the diagenetic calcite prisms.Thus now we maintain that the shell of C. ?globata is composed of two layers only: primary microscrystalline and secondary fibrous (Pl.III, Fig. 2).
The following data represent a revision of the published data about Cyclothyris ?globata (Motchurova ‡Dekova, 1992a, 1992b, 1995).Two close transverse sections at the middle of the shell length of one specimen from the Late Santonian, Shumen Formation, northeastern Bulgaria were examined under SEM.One of the sections was more posterior and crossed the muscle field, sectioning part of the myotest (Pl.III, Fig. 4).The shell is 500 to 800 µm thick.The primary layer is microcrystalline granular, possibly diagenetically recrystallized, 50 to 70 µm thick (Pl.III, Fig. 2).The dimensions of the fibres were measured in cross ‡section at the maximum width of the shell in the central part of the plane of symmetry.The fibres are anvil ‡like in cross ‡section (Pl.III,     It is interesting to note that the SiO 2 nodules were developed as a layer in the outermost and innermost part of the shell following the direction of the penetrating pore solutions enriched in amorphous SiO 2 during the early diagenesis.Thus a kind of "islets" remained non ‡altered in the center of the ribs (Pl.III, Fig. 6).
Remarks and discussion.Representatives of this asymmetric species found in several Santonian ‡Campanian outcrops of the Pannonides, Carpatho ‡Balkanides and the Dinarides of southeastern Europe have been assigned previously to the Cenomanian species Cyclothyris difformis Valenciennes in Lamarck or C. contorta d'Orbigny.We refer here these Santonian ‡Campanian asymmetric rhynchonellides to the species Rhynchonella globata, described by Arnaud (1877) from the Campanian of France (Charente, Dordogne, Gironde).The lectotype, as already mentioned (Motchurova ‡Dekova, 1995, p. 58), should be one of the two syntypes, figured by Arnaud (1877, Pl. 28,Figs. 36 ‡38), deposited in the Arnaud's collection, Université Paris 6, Jussieu.The revision of the original type material from France is still in progress (Gaspard, 1997;personal communication, 2000) and the range of its internal and external variability is still unknown.Considering the great external similarity with the species of Arnaud we assign the material from the Pannonides, Carpatho ‡Balkanides and the Dinarides of southeastern Europe to this French species.Having examined the internal morphology of 10 specimens from Bulgaria (Motchurova ‡Dekova, 1995) and four from former Yugoslavia (this paper) a wide range of internal variability was found.We think the totality of the external and internal elements allow us to assign this species tentatively to the genus Cyclothyris M'Coy.We prefer to use open nomenclature for Cyclothyris ?globata because of some differences with the Aptian-Cenomanian representatives of this genus.The species C. ?globata described in this paper is characterized by smaller foramen, less curved almost straight lateral commissures, poorly developed median septum, somewhat different angle of insertion of the teeth, better expressed inner socket ridges.However, the development of the hinge plates and the crural bases in "forked" condition as discribed in the revision of the genus Cyclothyris (Owen, 1962) are typical for Cyclothyris.Some of the Bulgarian representatives show deviation from the typical internal features of the genus (Motchurova-Dekova, 1995, Figs. 15, 16).In two sectioned specimens from Gu~a (Serbia) and one specimen from Nanos (Slovenia) the crura are with thickened distal ends i. e. "diabolo" shape.In the second specimen from Nanos the crura have triangular sections.Thus, the distal ends of crura of this species may vary in sections.In five figured specimens from Bulgaria (Motchurova ‡Dekova, 1995) the crura have subtrian-gular to arch ‡like sections but not "diabolo" shape.This could be due to the slightly different inclination of the serial sections.
While commenting on the French Late Cretaceous asymmetric rhynchonellides Gaspard (1997) stated that: "Observations of representative specimens of different series allow to recognize different species from difformis and globata in the Conacian ‡Santonian horizons.This is in contrast with data of Motchurova ‡Dekova (1995) who includes the Santonian specimens from Bulgaria in the species C. globata (Arnaud)".More recently D. Gaspard (personal communication, 2000) examined some of the Bulgarian material and agreed that it is quite probably that it could be referred to the species C. ?globata.
The abundant material of C. ?globata from northeastern Bulgaria has shown wide range of variability both in the internal and external morphology likewise as in the Cenomanian species C. difformis (Motchurova ‡Dekova, 1995).This confirms the observations made by Fürsich & Palmer (1984) that the overall morphological variability is usually higher in species with asymmetrically developed commissures than in symmetrical brachiopods.
C. ?globata differs from C. difformis and C. contorta in having fewer costae, more triangular outline, smaller foramen and permanently asymmetric shell (displaying the so called "obligate asymmetry"), whereas C. difformis and C. contorta can develop both asymmetric and symmetric shells ("facultative asymmetry" sensu Fürsich & Palmer, 1984).According to several authors the manifestation of the "non ‡symmetry" in asymmetric rhynchonellides is revealed after the juvenile stage.In the Bulgarian material of C. ?globata all the small ‡sized shells, supposed to be juvenile or young, have already well manifested asymmetry.
It is usually maintained that the external asymmetry of such rhynchonellides does not affect the internal morphology (Fürsich & Palmer, 1984, Gaspard, 1991).However, according to Fage (1934) in R. difformis (= Cyclothyris difformis) the internal apparatus participate in the deformation of the shell as well.Our observations show both symmetric and asymmetric development of the internal morphology, especially in the crura and the position of the septum.
All specimens are from Shumen Formation, uppermost Late Santonian, northeastern Bulgaria.Figures are in natural size.

PLATE III TABLA
SEM micrographs of two transverse sections through a specimen of Cyclothyris ?globata (A r n a u d ) , Shumen, NE Bulgaria, Shumen Formation, Late Santonian.
Arrangement of the fibres in a rib.
Section through the whole thickness of the rib.
Secondary fibres.Internal part of the shell on the left, overgrown by diagenetic calcite prisms (arrow).
Part of the shell in its entire thickness showing three ribs.Silica nodules developed in the innermost and outermost part of the shell (SiO 2 ).Small islets of non ‡altered secondary layer (s.l.) are seen in the center of the ribs.

Fig. 2 .
Fig. 2. Plots of length versus width (black symbols) and of length versus thickness (open symbols) for 18 specimens of Cyclothyris ?globata (A r na ud ) from former Yugoslavia.

Figs. 1
Figs. 1, 3, 4).They rarely have rhombic outline in the lateral internal part of the shell.The fibres are 20 ‡35 µm wide and 7 ‡13 µm thick in cross ‡section.For comparison, the summarized data about other representatives of the genusCyclothyris (C.difformis, C  vespertillio, C. antidichotoma, C. zahalkai)  show that the fibres are anvil ‡like anysometric, 15 ‡30 µm wide and 2 ‡10 µm thick(Motchurova ‡Dekova, 2001).Thus C. ?globata from the Upper Santonian of NE Bulgaria shows somewhat larger fibres than the above mentioned species.Similarly to other rhynchonellides, thinner and smaller fibres are developed close to the external surface of the shell.Towards the interior of the shell the fibres increase in size.Part of the shell of the examined specimen was strongly silicified.It is interesting to note that the SiO 2 nodules were developed as a layer in the outermost and innermost part of the shell following the direction of the penetrating pore solutions enriched in amorphous SiO 2 during the early diagenesis.Thus a kind of "islets" remained non ‡altered in the center of the ribs (Pl.III, Fig.6).Remarks and discussion.Representatives of this asymmetric species found in several Santonian ‡Campanian outcrops of the Pannonides, Carpatho ‡Balkanides and the Dinarides of southeastern Europe have been assigned previously to the Cenomanian species Cyclothyris difformis Valenciennes in Lamarck or C. contorta d'Orbigny.We refer here these Santonian ‡Campanian asymmetric rhynchonellides to the species Rhynchonella globata, described by Arnaud (1877) from the Campanian of France (Charente, Dordogne, Gironde).The lectotype, as already mentioned(Motchurova ‡Dekova, 1995, p.  58), should be one of the two syntypes, figured by Arnaud (1877,Pl.28, Figs.36 ‡38), deposited in the Arnaud's collection, Université Paris 6, Jussieu.The revision of the original type material from France is still in progress(Gaspard, 1997; personal communication, 2000)  and the range of its internal and external variability is still unknown.Considering the great external similarity with the species of Arnaud we assign the material from the Pannonides, Carpatho ‡Balkanides and the Dinarides of southeastern Europe to this French species.Having examined the internal morphology of 10 specimens from Bulgaria(Motchurova ‡Dekova, 1995)  and four from former Yugoslavia (this paper) a wide range of internal variability was found.We think the totality of the external and internal elements allow us to assign this species tentatively to the genus Cyclothyris Each specimen was coated with ammomium ‡chloride before photographing.Figures are in natural sized.a = dorsal view; b = lateral view; c = anterior view.Svaki primerak je zapra{en sa amonijum ‡hloridom pre fotografisawa.Slike su u prirodnoj veli~ini.a = dorzalna strana; b = bo~na strana; c = predwa strana.

Fig . (
Fig. (Sl.) 4. Detail from the lower part of Fig. 3. Internal part of the shell below.The dashed white line indicates the boundary between the myotest and the diagenetically grown calcite prisms perpendicular to the internal surface (d.c.).