Neumania paucicostata, Evaegerquist, 2003

Evaegerquist, 2003, New brachiopods from the Lower-Middle Ordovician (Billingen-Volkhov stages) of the East Baltic, Acta Palaeontologica Polonica 48 (1), pp. 31-38 : 33-35

Neumania paucicostata sp. nov.

Figs. 3 , 4 .

Holotype: PMU In 108 ( Figs. 3B and 4A ), ventral valve from the standard section in Putilovo Quarry, Zheltjaki Member (local name of bed: Krasnota), lower P. originalis conodont Zone.

Paratypes: 17 ventral valves, 11 dorsal valves and 1 articulated specimen ( PMU In 107–109, 112–119, 213–230) .

Diagnosis.—Ventribiconvex, subtrapezoidal atelelasmatine withproclineventralinterarea;ornamentcoarselycostatewith imbricate growth lamellae; spondylium simplex U−shaped.

Description.—Shell up to 4.8 mm long and 7.2 mm wide with maximum width at hinge line; cardinal angles acute to slightly alate; lateral margins straight to convex, anterior margin rounded. Anterior commissure slightly sulcate. Radial ornament coarsely costate with rounded costae which only rarely branch; costae of equal size; one to two costae per mm at the anterolateral margin. Concentric ornament with fine fila (not visible on all specimens) and irregularly arranged, imbricate growth lamellae, which vary in number from three to eight on examined specimens; average number 1.5 per mm valve length.

Ventral valve high, lateral profile with highest point at apex. Interarea plane, procline, sometimes with faint growth linesparalleltohingeline,butalsowithfaintstriaeparallelto the delthyrial opening; length of interarea on average 35% of width. Delthyrium about as high as wide, open, on some specimens bounded by narrow deltidial plates. Interior surface of valve smooth. Hinge teeth small, simple. Elevated, U−shaped spondylium simplex, supported anteriorly by short rounded septum. Diductor scars rounded anteriorly, not readily distinguished on all specimens; adductor scars weakly impressed.

Dorsal valve moderately convex, with shallow sulcus beginning at umbo, broadening anteriorly. Cardinal extremities depressed. Interarea short, concave, orthocline. Notothyrium closed by chilidium; cardinal process developed as low ridge; dental sockets small. Notothyrial platform almost flush with interarea, well developed with prominent anterior border, merging into narrow median ridge which extends as long as the muscle field. Adductor field impressed, anteriorly lobed, extending half−length of the shell. Anterior adductor scars not readily distinguished from the smaller posterior scars( Fig.3A ).Interiorsurfaceofvalvesmooth,musclefield flanked by two faint, diverging mantle canals.

Discussion.—This clitambonitid species is referred to Neumania based on the presence of imbricate growth lamellae and a U−shaped spondylium simplex with impressed muscle scars, a feature which is taken as diagnostic for the genus by Rubel and Popov (1994). The muscle scars are not as clearly distinguished as in the other two species from the East Baltic, N. erecta ( Pander, 1830) and N. costata ( Pander, 1830) . There is no obvious adductor scar, but a broad and, in some specimens, quite rough surface between the diductor scars probably served as attachment for the adductor muscles. The other two species also have finer radial ornaments than the new species (see Rubel and Popov 1994).

Compared with Neumania atlanticus ( Neuman, 1976) , the new species has larger, rounded costae, wider spaced growth lamellae and less prominent internal structures (see Harper in Bruton and Harper 1981).

The weakly impressed muscle scars somewhat resemble those of Apomatella Schuchert and Cooper, 1931 , another commonly occurring genus in the area. However, this genus has a more V−shaped spondylium simplex without visible muscle scars.

Distribution.—Upper part of the B. triangularis conodont Zone (lower Dikari Member) to the upper P. originalis Zone (Zheltjaki Member), found in Putilovo Quarry and at Lava River.

Figures

Fig. 3. Neumania paucicostata. A. Dorsal valve interior showing the anteriorlylobedmusclescars,drawnfromspecimenPMUIn107.B.Lateralview of ventral valve drawn from specimen PMU In 108 (holotype).

Fig. 4. Neumania paucicostata sp. nov. Specimens from Putilovo Quarry. A. Holotype PMU In 108; ventral valve interior (A1), enlargement of the spondylium showing the muscle attachment area (A2). B. PMU In 113, oblique internal view of ventral valve showing the U−shaped spondylium. Note that this specimen does not possess rounded muscle scars as in the holotype. C. PMU In 112, enlargement of spondylium on broken ventral valve, slightly oblique view showing theroughmuscleattachmentarea.D.PMUIn109,ventralviewofconjoinedspecimen.E.ExteriorofspecimenPMUIn228indifferentviews.F.PMUIn107, dorsal interior and exterior. G. PMU In 224; oblique posterior view of cardinalia (G1), dorsal valve interior (G2). Scale bars 1 mm.