Rugosochonetes cf. celticus Muir−Wood, 1962

Sun, Yuanlin & Baliński, Andrzej, 2008, Silicified Mississippian brachiopods from Muhua, southern China: Lingulids, craniids, strophomenids, productids, orthotetids, and orthids, Acta Palaeontologica Polonica 53 (3), pp. 485-524 : 502-504

Rugosochonetes cf. celticus Muir−Wood, 1962

Figs. 12, 13E.

cf. 1962 Rugosochonetes celticus sp. nov.; Muir−Wood 1962: 68–70

(partim); pl. 7: 3–5, 7, 8; text−fig. 24; [pl. 6: 8, 9; pl. 7: 6, 10–14 =

R. speciosus (Cope, 1938) ]. cf. 1968 Rugosochonetes celticus Muir−Wood ; Brunton 1968: pl. 7:

28–30; pl. 8: 1–9. cf. 1970 Rugosochonetes celticus Muir−Wood ; Brand 1970: 108–111;

pl. 10: 12–20; pl. 13: 5, 6, 9.

Material.—Thirty−seven complete and damaged shells, 32 broken dorsal and 67 broken ventral valves.

Description.—Shell medium to large in size for the genus; the estimated width and length of the largest broken specimen is about 25 mm and 16 mm, respectively; subelliptical to subquadrate in outline with the greatest width near the hinge line in subquadrate forms and near the mid−length of the shell in subelliptical forms, and length:width ratio 0.67–0.76; cardinal extremities rounded to subangular, with poorly demarcated, small ears; concavo−convex in lateral profile, about one−fourth as deep as long; three to five pairs of orthomorph oblique, symmetrically arranged spines, inclined at 32–50 ° to the hinge line. Shell surface smooth over the first 1 to 2 mm of the beaks and anteriorly with round radial capillae that increase anteriorly mainly by bifurcation on ventral and by intercalation on dorsal valve; interspaces narrower than capillae, 3.5 to 5 capillae per mm at 3 mm distance from ventral beak and 3–5 (on one specimen exceptionally 6.5) near mid−anterior margin.

Ventral valve moderately to strongly convex with the greatest convexity occurring at about one−third of valve length from the beak, with depth about one−fifth to one−third of valve width; ventral interarea flat and orthocline, with small pseudodeltidium. Dorsal valve generally slightly concave to strongly concave; dorsal interarea hypercline with small chilidium; elongate protegular node prominent on the dorsal beak.

Ventral valve interior with short and plate−like teeth, subparallel to hinge line ( Fig. 12M); median septum high, posteriorly confined but commonly extended anteriorly as low ridge for about one−fifth of valve length ( Figs. 12M, 13E); adductor scars large and subovate; endospines confined to antero−lateral marginal area, radially arranged and correspond to intercapillary spaces.

Dorsal interior with bilobate, posteriorly directed, and externally narrowly confined quadrifid cardinal process ( Fig. 12I 3). Some specimens show a large and deep cardinal process pit; inner socket ridges straight or slightly curved ( Figs. 12H, I, 13E), anteriorly divergent at 140–160 °; anderidia long, narrow, anteriorly divergent at 57–65 ° ( Fig. 13E); median septum ridge like, variable, becoming strong late in ontogeny; accessory septa absent; endospines radially and evenly arranged, prominent at central region of the visceral disc ( Figs. 12D, 13E).

Remarks.—The specimens from the Muhua Formation seem to be closest to Rugosochonetes celticus described from the Carboniferous (Late Viséan to Early Namurian) of England, Scotland, and Ireland ( Muir−Wood 1962: 69; Brunton 1968: pl. 8: 1–9; Brand 1970). As Muir−Wood (1962: 69) and Brunton (1968: 54–55) remarked, the species is quite variable as far as radial ribbing is concerned. The studied specimens are intermediate between fine− and coarse capillate morphotypes described by these authors. The only important difference between specimens described by Muir−Wood (1962) and those from China is the inclination of ventral spines which in the former attain 20–30 ° whereas in the latter the angle reaches 32–50 ° to the hinge line. On the other hand Brand (1970: 109) noted that on larger specimens of R. celticus examined by him the spine inclination varies from about 80 ° near the umbo to about 45 ° near the cardinal extremities (see also Brunton 1968: 56–57). It seems that specimens from China possesses more elongated teeth in comparison to triangular in R. celticus examined by Brand (1970: 108), but this difference is poorly discernible on the illustrated Scottish specimens.

The Chinese specimens can be distinguished readily from Tournaisian R. distinctus Afanasjeva, 1976 , R. sargensis ( Nalivkin, 1979) , and Viséan R. annae Afanasjeva, 1976 of the East European Platform and the Urals by having coarser capillate shell ornament (see Sokolskaja 1950; Afanasjeva 1976; Nalivkin 1979). The specimens described here are similar externally to Rugosochonetes chesterensis ( Weller, 1914) described from the Paint Creek Formation (Mississippian) of Illinois, USA ( Weller 1914), but differ from the latter mainly in having less developed ears and coarser radial ornament.

The young specimens of this species are very similar to Subglobosochonetes cf. malevkensis described above in shell outline and ornamentation. However, the latter does not show any trace of the dorsal median septum and has relatively higher length/width ratio. Furthermore, the endospines in the dorsal valve seem to be more evenly and regularly disposed in Rugosochonetes cf. celticus than in S. cf. malevkensis .

Stratigraphic and geographic range.— R. celticus was described from Late Viséan–Early Namurian of North Wales, England, Scotland, and Ireland ( Muir−Wood 1962). The form under consideration was found in the Muhua Formation in Samples GB, GT, M2/1, and Mu−42.