Macropotamorhynchus Sartenaer, 1970 Hemiplethorhynchus von Peetz, 1898

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 : 805-808

Genus Macropotamorhynchus Sartenaer, 1970

Type species: Camarotoechia mitcheldeanensis Vaughan, 1905 ; Carboniferous Limestone (Tournaisian); Gloucestershire, England. Macropotamorhynchus cf. insolitus Carter, 1987

Fig. 8 .

Material.—Two almost complete shells and 49 fragments of conjunct and single valves from samples D2 and M2−8; the material is poorly preserved, consists of highly fragmented specimens which are additionally obscured by silica and frequently attached bioclasts.

Description.—Shell small in size (dimensions of a largest complete shell in mm: length 6.2, width 6.4, thickness 2.5; Fig. 8D ), subequally biconvex, subovate to subrounded; immature shells elongate, adults usually as wide as long to slightly transverse; cardinal margin obtuse angled, passing gently into the arched lateral and anterior margins; anterior commissure weakly uniplicate and serrate. Ventral valve weakly convex; beak elongate, suberect; palintrope high, delthyrium narrow and high, deltidial plates not observed; sulcus very weak, marked in the anterior half of the valve. Dorsal valve regularly convex in lateral profile, posteriorly with shallow median depression, anteriorly with very low median fold; anterolateral margins in large specimens slightly deflected toward the ventral valve marking the reduction of the shell growth in length.

Shell ornament of strong, simple, rounded costae extending almost from beaks; sulcus with 2–3 and fold with 3–4 costae, flanks with up to 5 costae, those on ventral valve sometimes with their anterior ends reflected in ventral direction.

Interior of ventral valve with quite long, slightly divergent dental plates. Dorsal valve interior with long median septum reaching half of the valve length; septum supports V−shaped septalium; outer hinge plates triangular, slightly concave ( Fig. 8E, G ); crura wide, anteriorly divergent, slightly convex laterally, tips wide, curving ventrally Fig. 8C ).

Remarks.—The material is very fragmentary and obscured by siliceous deposits attached to specimens but reveals external morphology and internal structures which seem to be characteristic for the genus Macropotamorhynchus . Morphological characteristics of the species from Muhua, especially the small shell dimensions, elongate outline of the immature and more transverse in adult specimens, elongate ventral beak, and shell costation pattern suggest close similarity to Macropotamorhynchus simplex ( Roberts, 1971) from the Tournaisian Burt Range Formation, Bonaparte Gulf Basin, north−western Australia ( Roberts 1971) and to M. insolitus from the lower Carboniferous of western Alberta, Canada ( Carter 1987) and south−eastern Idaho, USA ( Christensen 1999). From the former species our specimens differ by having less uniplicate commissure, weaker shell costation which commences slightly more anteriorly to the beaks, and usually slightly greater number of the median costae. The material from Muhua seems to be very close to the highly variable externally M. insolitus . Both forms have similar shell shape and character of the surface costation. The former differs in having less pronounced sulcus and fewer median and flank costae, although the latter feature falls within limits

http://dx.doi.org/10.4202/app.2010.0106

of variability of the Canadian species. Besides these differences the species from Muhua and western Australia seem to be quite close morphologically.

The specimens from the Muhua Formation are similar externally to Camarotoechia tuta (Miller, 1881) described by Weller (1914: 179–180) from the Mississippian of the Mississippi Valley Basin, USA. The former differ in having less inflated shell, less developed sulcus, fold, tongue, and less numerous costae in the sulcus and on the flanks. From the type species of the genus, i.e., Camarotoechia mitcheldeanensis Vaughan, 1905 described from Carboniferous Limestone (Tournaisian) of England and Macropotamorhynchus sp. aff. M. insolitus Carter, 1987 described from the Strunian–Tournaisian of southern Morocco and the Tournaisian of Algeria ( Brice et al. 2005), our specimens differ in having smaller shell with more elongate outline and less developed sulcation.

Figures

Fig. 8. Rhynchotrematoid brachiopod Macropotamorhynchus cf. insolitus Carter, 1987 from the Tornaisian Muhua Formation, southern China. A. Incomplete juvenile shell, ZPAL V.XXVI/60, in dorsal (A1), ventral (A2), and lateral (A3) views. B. Shell, ZPAL V.XXVI/61, in dorsal (B1), ventral (B2), lateral (B3), posterior (B4), and anterior (B5) views. C. Incomplete juvenile shell, ZPAL V.XXVI/62, in oblique lateral view showing distal parts of crura. D. Almost complete shell, PKUM02−0413, in dorsal (D1), ventral (D2), lateral (D3), posterior (D4), and anterior (D5) views. E. Incomplete and slightly deformed dorsal valve, ZPAL V.XXVI/63, in interior view (E1), and enlargement showing cardinalia (E2). F. Incomplete shell, ZPAL V.XXVI/64, in dorsal view. G. Interior of dorsal valve, ZPAL V.XXVI/65. Sample D2 (A–C, F, G) and M2−8 (D). C, E–G, SEM micrographs. Scale bars 1 mm.