Martellia ichangensis Wang, 1956

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 386-387

Martellia ichangensis Wang, 1956

Fig. 14A–J; Table 15.

Martellia ichangensis sp. nov.; Wang 1956: 11, pl. 2: 1–5.

Martellia ichangensis Wang, 1956 ; Xu et al. 1974: 148, pl. 65: 5–7.

Martellia ichangensis Wang, 1956 ; Zeng 1977: 47, pl. 15: 7, 8.

Martellia orbicularis sp. nov.; Zeng 1977: 47, pl. 15: 10–12.

Martellia ichangensis Wang, 1956 ; Xu et al. 1978: 296, pl. 116: 14.

Martellia ichangensis Wang, 1956 ; Xu and Liu 1984: 196, pl. 12: 27–29, 31; text−fig. 12.

Material.—Seven ventral internal and four external, 16 dorsal internal and seven external moulds.

Description.—Shell medium−sized, subcircular, ventribiconvex, with maximum width near one−third shell length; cardinal extremities rectangular to rounded; anterior commissure rectimarginate to weakly uniplicate. Ventral valve nearly as long as wide, strongly convex, deepest in umbonal area; interarea very high, slightly greater than one−third length of shell, apsacline, with planar surface bearing fine striae parallel to hinge line; beak erect; delthyrium small, covered by thick, arched deltidium; sulcus weak, narrow, developed in anterior third of shell. Dorsal valve evenly convex, deepest around umbo; interarea low, about one−tenth of shell length, anacline, with planar to gently concave surface; notothyrium narrow, covered by thick and strongly arched chilidium; fold low, narrow, present in anterior third of valve. Costellae equal−sized, dense, hollow, increasing anteriorly by bifurcation and intercalation, about 5 per mm at 5 mm growth stage; concentric fila dense, evenly spaced, interrupting costellae; 5–7 concentric lamellae unevenly spaced.

Teeth stout; dental plates high, thin, inclined basomedially, becoming subparallel at their bases, extending anteriorly slightly beyond muscle field; delthyrial chamber large, deep. Muscle field strongly elongate, about two−fifths length and one−quarter width of shell, slightly elevated; pair of elongate−oval adductor scars located in posterior part of muscle field, enclosed laterally and anteriorly by larger and more elongate−oval diductor scars. Median ridge low, extending through muscle field and beyond for two−thirds of shell length. Mantle canal system saccate, clearly impressed; vascula media originating from anterior ends of diductor scars, extending parallelly for short distance before dividing into many branches medially and laterally.

Cardinalia large, about one−quarter length and three−fifths width of shell; cardinal process thick, ridge−like, projecting posteriorly; notothyrial platform strongly elevated, transversely continuous with thick socket ridges to form free−hanging transverse plate supported by strong median ridge (Fig. 14F 2); sockets deep, narrow, open laterally; socket ridges divergent at about 115°. Muscle field clearly defined, about one−half length and width of shell; outer, posterior pair of adductor scars elongate, subtriangular, strongly divergent, separated from inner, anterior pair by two thin, long side septa; anterior, inner pair of adductor scars much larger, more elongate, subtriangular, bearing two weak, longitudinal transmuscle ridges. Median ridge (myophragm) high, thick. Mantle canal system apocopate; vascula media dividing into many branches shortly after originating from anterior end of muscle field.

Discussion.—This species has been reported from the upper Dawan and Meitan formations (upper Arenigian) at many localities on the Upper Yangtze Platform from central to southwestern China (e.g., Zeng 1977; Xu et al. 1978; Xu and Liu 1984). It is characterized by a subcircular shell outline and weak anterior fold and sulcus. Martellia fenxiangensis Zeng (1977 : pl. 15: 4–6) from the upper Dawan Formation (upper Arenigian) of Yichang, Hubei Province, has a weak fold and sulcus, but its shell is elongate. Martellia orbicularis Zeng (1977) , from the same type locality and horizon as M. ichangensis and M. fenxiangensis , was established on the basis of its nearly circular shell outline without fold and sulcus, but Zeng's (1977: pl. 15: 10–12) illustrated ventral valve clearly has a weak and narrow sulcus and its internal structures are identical to those of M. ichangensis . This implies that M. orbicularis is a junior synonym of M. ichangensis .

During ontogeny, the shells of M. ichangensis tend to become less transverse due to accelerated longitudinal allometric growth. The two relatively small ventral internal moulds (Fig. 14I, J), for example, are somewhat more transverse than other ventral valves in the same population (e.g., Fig. 14A, B).