Pleuropugnoides calcaris, Sun & Baliński, 2011

Sun, Yuanlin & Baliński, Andrzej, 2011, Silicified Mississippian brachiopods from Muhua, southern China: Rhynchonellides, athyridides, spiriferides, spiriferinides, and terebratulides, Acta Palaeontologica Polonica 56 (4), pp. 793-842 : 808

Pleuropugnoides calcaris sp. nov.

Fig. 9A–G, I, J.

Etymology: From Latin calcar, spur, refers to spurs formed by lateral ventral costae.

Holotype: Complete shell PKUM02−0416 illustrated in Fig. 9C.

Type locality: Muhua section, between villages of Muhua and Gedongguan (Guizhou province, South China).

Type horizon: Muhua Formation, correlated with the middle Tournaisian Lower Siphonodella crenulata Conodont Zone.

Diagnosis.— Pleuropugnoides with subelliptical shell outline and coarse, anteriorly angular radial costae; usually 4 costae in sulcus and 7–11 costae on each flank; lateral costae form distinct spurs on ventral valve; median septum attains nearly a half of the length of dorsal valve.

Material.—Four complete to almost complete shells and 20 nearly complete or fragmentary single valves, mostly with well preserved internal structures from samples MH1, M2−8, and D1.

Description.—Shell medium sized for genus, attaining about 18 mm in width, dorsibiconvex in profile, subelliptical in outline, wider than long, length attains 73–80% of width; commissure serrate, strongly uniplicate. Adult ventral valve weakly convex in posterior region but concave at flanks; umbo protruding, beak suberect; sulcus strong, anteriorly forms subtrapezoidal tongue. Dorsal valve regularly convex in posterior view, sometimes with flattened to weakly concave anterior in lateral profile and steep ventrally curving flanks resulting in slight geniculation at anterolateral margins; fold clearly separate at anterior half, becoming broader and much higher anteriorly. Shell ornament of strong, simple costae extending from beaks; costae rounded posteriorly, becoming angular and high anteriorly, those on ventral flanks reflected at their ends in ventral direction forming short but distinct spurs which protrude ventrally at valve margin ( Fig. 9C–E); sulcus with 4 and fold with 5 costae, flanks with commonly 7–8 costae, but may reach 11.

Interior of ventral valve with long divergent dental plates. In dorsal valve long median septum attains slightly less than half of the valve length and supports V−shaped septalium ( Fig. 9F 3, G 2); outer hinge plates triangular, flat, up to 2 mm long, flanked laterally by high inner socket ridges; crura poorly preserved, observed in one specimen, fairly long, divergent, curving ventrally; adductor scars well preserved in one valve ( Fig. 9F), impressed, anterior pair subelliptic, slightly divergent anteriorly, posterior pair smaller, strongly elongate, divergent anteriorly, slightly asymmetric.

Remarks.— Pleuropugnoides calcaris sp. nov. is morphologically close to, and was probably the ancestor of, P. pleurodon (Phillips, 1836) described from the early Viséan of England and Northern Ireland (see Ferguson 1966; Brunton 1984). The Chinese species is readily distinguished from the latter mainly by its smaller shell, stronger, more angular and more numerous costae in the sulcus and on the flanks. Internally P. calcaris differs in having a longer median septum in the dorsal valve. It is also similar to Pleuropugnoides greenleightonensis Ferguson, 1966 from the late Namurian of British Isles ( Ferguson 1966) in attaining comparable shell dimensions and in having angular costae and a long dorsal median septum. The characteristic marginal spurs on the ventral flanks which are formed by strongly reflected anterior ends of costae in ventral direction, appear also to be present in P. greenleightonensis (see Ferguson 1966: pl. 23: 11–13). P. calcaris differs in having more numerous costae in the sulcus (usually 4 in the former and 2–3 in P. greenleightonensis ) and on the flanks (usually 7–8 and 3–5, respectively) and a more trapezoidal, rather than rounded tongue.

It is noteworthy that a rhynchonellid similar to the species here described has been widely reported from China as Ptychomaletoechia kinlingensis (Grabau, in Chu 1933). The species is known from many localities and from sediments representing the late Famennian through the Tournaisian (e.g., Yang 1964; Tan 1987; Chen and Shi 1999). Unfortunately, Amadeus Grabau never published his description of this species, and Chu (1933) was the first to give a short description with figures of what was then called Camarotoechia kinlingensis from the Kinling Limestone (Tournaisian) of Jiansu. Evidently this quite important Chinese form urgently needs taxonomic and nomenclatorial revision, including its generic assignment, which seems problematic because externally it is quite different from the early Famennian Ptychomaletoechia omaliusi (Gosselet, 1877) , the type species of the genus.

Pleuropugnoides calcaris sp. nov. is generally more finely ribbed than Ptychomaletoechia kinlingensis having more ribs on the fold and sulcus (4 and 5 in the former, 3 and 4 in the latter) as well as on flanks (up to 11 and 8, respectively). In the former the flanks of the adult ventral valve are invariably distinctly concave whereas in the latter they are usually convex to flat and only rarely very weakly concave. Ptychomaletoechia kinlingensis does not form marginal spurs on its ventral flanks while in the species from Muhua they are characteristic. The ventral sulcus of the new species is less deep than in P. kinlingensis .

Stratigraphic and geographic range.—Known only from the type locality and horizon.