A new Carboniferous species of Isogramma (Dyctionellida, Brachiopoda) from NW Serbia

Representatives of a new Early Moscovian, Carboniferous, species of the dictionelid brachiopod Isogramma MEEK & WORTHEN, 1870 from northwestern Serbia are described. Small dimensions, weakly developed dorsal lateral ridges and the absence of dorsal anterior sulcus characterize this new taxon. A comparison with other representatives of genus Isogramma and some remarks of this genus are given.

One of the most fossiliferous localitites is the Hamlet of Obradovi}i, situated 4.4 km N-NW of Krupanj at the edge of an unsealed road towards the village of Cerova, 150 m NW of the trigonometric point 572.All Isogramma specimens in this locality, as in the majority of other localities, are preserved as moulds and impressions.Among this material, a well preserved dorsal valve (the natural counterpart) was found, and this is proposed as a holotype of a new species.The name of this new species, Isogramma serbica, was earlier introduced by the author (1966)(1967) but without formal description.This is now provided.
As far as the author is aware, this is the first find of complete impressions of the interior and exterior of both valves of this species.Although the descriptions of new representatives of this genus appear sporadically in the literature, many questions remain unanswered: the exact function of the internal structure of the ventral valve, the question of communication within the valves and with the marine environment in which they have been lived.The reason for this is the relatively poor preservation of the fossil materials, although every new discovery places a new piece in the jigsaw puzzle of this unusual group.
The author and her colleagues I. FILIPOVI] and V. PAJI] revisited the locality several times and found new material which are presented here as paratypes.The number of Isogramma specimens is low in comparison with the majority of other localities reported elsewhere.
One poorly preserved specimen of Isogramma was also discovered by the author in the locality Eljdi{te in the region of Sana's Paleozoic of Bosnia (STOJANOVI]-KU- ZENKO, 1966ZENKO, -1967)), which is not included in this paper.Superfaminiliar classification follows WILLIAMS et al. (1996).
Derivatio nominis.Name given from the land of Serbia.
Paratypes.Specimens collected at a later date: two dorsal valves external moulds (OB101, OB103); ventral valve internal moulds (OB104) and natural counterparts of the exterior and interior of two ventral valves (OB102, OB105).
Description.Shell small, the largest specimen is a holotype with a length of 11.0 mm and a width of 22.0 mm, transversely sub-eliptical in outline, ventral valve convex, dorasal valve shallowly concave.
Ventral valve.Posterior outline arched, the greatest convexity in the umbonal region.The beak does not extend beyond the hinge line.The notch (depressed triangular area) with well-defined lateral ridges, is positioned in the central part of the triangular depression and is divided medially by a narrow furrow extending anteriorly to its forked end.The length of the notch is two thirds of the valve (Pl. 1, Fig. 7) or slightly shorter (Pl. 1, Fig. 13).The platform extends from near the beak (Pl.1,Figs. 7,8).A notch divides the platform and forms two triangular symmetrical, posteriorly slightly depressed, anteriorly elevated areas with well developed lateral margins extending anteriorly and slightly longer than the notch.A very clear outline of the two triangular parts of the platform can be recognised (Pl.1,Figs. 8,13).
The platform is anteriorly curved with channels between radial grooves, which represent muscle scars and reach the lateral margins.The hinge line of the ventral interior valve is covered by a row of open punctae.The additional interior ridge with shallow furrows which is connected with the hinge line and it would appear that both parts of the platform are hanging from the hinge line (Pl. 1, Fig. 13).
The exterior surface of the ventral valve is covered with elevated lines with punctae in the interspaces and is interrupted with the posterior part of the notch (Pl. 1, Fig. 9).In some specimens, the notch is of different length or the whole valve is covered with ornaments.
Dorsal valve.Lateral margins rounded, extend from the cardinal process, greatest width just anterior to the hingeline.Exterior ornaments of fine concentric ridges, 2-4 per mm in the anterior half of the shell.The interspaces between the ridges contain a great number of small open punctae.The ventral ridges are sometimes irregular, with disruptions and with variation of ridge heights.In case of injury to the valves, the ridges intervene in the reparation (Pl. 1, Fig. 11).The interior of the dorsal valves is covered by a smaller number of randomly distributed rounded punctae.Weak lateral ridges extend from the cardinal process laterally, slightly shorter than the hinge line (Pl. 1, Fig. 4) The exterior of the dorsal valve shows a trace of the outline of muscle scars positioned on the inside of the valve.Muscle scar distinct, located on both sides of the prominent median septum which extends over two thirds of the valve length and is enclosed by the buttress plates.
The myophore is monolobate and distinct.The shaft, which is located anteriorly of the cardinal process, together with buttress plates, supports the myophore.

Comparison with other species of Isogramma
One of the closest species to I. serbica sp.nov. is I. manchoukuensis HATAI & OMURA (1940) from the Moscovian of Northern China.This latter lacks the dorsal sulcus but possesses coarser external ornaments.The shape of the shell is similar and so is the size.The ornamentation, however, consists of much broader and rounded commarginal ridges, from which the anterior margin posteriorly, are fewer and widely separated, almost disappearing at the lateral margins of the shell and the punctation is hardly visible.I. salteri BRAND (1970) from the Early Carboniferous of Britain, is a less elongated species of greater cancavo-convexity.I. carinthiaca (AIGNER, 1931) from the Early Carboniferous of Germany is close to a new species in terms of the external ornamentation but possesses a distinct, narrow dorsal anterior sulcus.I. millepunctata (MEEK & WORTHEN, 1873), described by VOL-GIN (1957) from southern Fergana, is also similar to I. serbica, but is a larger species.I. pachti (DITTMAR, 1972) from the Early Carboniferous of the Moscow basin, is of similar size and ornamentatation to I. serbica but is strongly concavo-convex, and possesses a sulcus.The stratigraphicaly younger forms I. paotechowensis (GRA-BAU & CHAO), 1927, and I. millepunctata (MEEK & WOR-THEN), 1873, found in the Carnic Alps, are geographically the closest to NW Serbia.

Remarks on the genus Isogamma
Isogramma is a sporadic genus in terms of its geographical distribution and is known from the Early Carboniferous and Permian.Comprehensive reviews of the genus include those of AIGNER & HERITSCH (1931), CO-OPER (1952), BRAND (1970) andWARDLOW et al. (1987).Early species of the genus from the Early Carboniferous and Moscovian tend to be small in size, whereas Late Carboniferous and Permian forms often attain a larger size (LIKHAREV, 1936;GAURI, 1965;NAKAMU-RA, 1970).Isogramma serbica sp.nov.belongs to the earlier group of small species with fine concentric ornamentation and is proposed as a new species on the basis of the absence of a narrow dorsal sulcus to the anterior of the prominent dorsal median ridge.
The genus Isogramma does not have teeth or sockets, so a solution is needed for the opening and closing of valves.MUIR-WOOD & COOPER (1960) and BRAND (1970), stated the probability of the role of lateral ridges in the dorsal valve in articulation.WARDLAW et al. (1987), believe that articulation was accomplished by the additional ridge in the ventral valve.
Specimens of I. serbica sp.nov.contain puntae in the hinge line of the interior of the ventral valve, as well as the additional ridge.There is no foramen for the pedicle in the ventral valve and the only possible communication with the interior of the shell is through the bordering punctae along the margin of the umbonal plate, as WARDLAW et al. (1987) suggested.