Saucrorthis Xu, Rong, and Liu, 1974

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 372-374

Genus Saucrorthis Xu, Rong, and Liu, 1974 Saucrorthis minor Xu, Rong, and Liu, 1974

Figs. 7H–J, 8A–K; Table 3.

Saucrorthis minor sp. nov.; Xu et al. 1974: 151, pl. 66: 1–4.

Material.—24 ventral internal and 24 external, 31 dorsal internal and 35 external, and two internal moulds of conjoined valves respectively.

Description.—Shell small, planoconvex, subsemicircular to subcircular (e.g., Fig. 7I). Cardinal extremities rectangular to acute. Anterior commissure denticulate, rectimarginate. Ventral valve moderately to strongly convex, deepest in umbonal area; interarea moderately high, flat, apsacline; delthyrium open; beak erect. Dorsal valve plane, occasionally with gentle sulcus in antero−medial part of shell; interarea very low, anacline. Costae coarse, simple, rounded, 20–23 per valve, with rare bifurcations originating 0.5–1.0 mm from apex; minute tubercles sparse, regularly spaced on crests of costae ( Fig. 8I 2 ). Table 4. Shell measurements (mm) of Nothorthis perplexa Xu and Liu, 1984 .

Concentric growth lines fine, evenly spaced, interrupted by irregular concentric lamellae towards anterior margin.

Teeth strong; dental plates thick, very short, subparallel; delthyrial platform strongly elevated; muscle field slightly elongate, bilobed anteriorly, about one−third length and one−quarter width of shell, without anterior bounding ridge; adductor scar deeply impressed, elliptical, surrounded by pair of elongate−oval diductor scars; mantle canal system saccate. Cardinalia stout, about one−fifth length and one−quarter width of shell; cardinal process swollen, occupying whole notothyrial cavity, myophore strongly projecting posteriorly and ventrally; notothyrial platform highly elevated; brachiophores blade−like, projecting antero−ventrally; brachiophore bases high, long, ridge−like, divergent at 74–101°; dorsal adductor scars clearly impressed, about one−third length or width of shell, bisected longitudinally by low and wide median ridge in most specimens; anterior pair of adductor scars subcircular, slightly larger than posterior pair. Crenulations strong around peripheral rim of each valve, becoming abruptly weaker inside rim (e.g., Fig. 8E 2, G 2 ).

Discussion.—The Weixin specimens of S. minor may vary from elongate with rectangular cardinal extremities in some ( Fig. 7H) to transverse with prominent ears in most others. The fulcral plates may be well developed ( Fig. 8D, G 1 –G 3) or entirely absent ( Fig. 8E 1 , E 2 ) in half of the species population.

Among the three known species of Saucrorthis , S. minor is easily distinguished from S. obscura ( Xu and Liu 1984; see Zhan and Jin in press) of south China and S. irravadica ( Reed 1936) of Burma ( Cocks and Zhan 1998) by having a smaller shell size and fewer, usually non−bifurcating costae. Specimens of O. obscura from the Dashaba Formation also have minute tubercles on the outer surface of the shell.

Figures

Fig. 2. Middle Ordovician sedimentary facies on the Yangtze Platform, South China palaeoplate (modified from Zhang et al. 2002), with three fossil localities discussed in the text: 1, Shihtzupu Formation at Shizigou Valley near the Weixin County town, Yunnan Province; 2, Type area of Shihtzupu Formation at Donggongsi, Zunyi, Guizhou Province; 3, Dashaba Formation at Shuanghe, Changning, Sichuan Province.

Fig. 1. Location map of the study area. A. Map of China, with the South China palaeoplate outlined by thick line; inset box enlarged as B. B. The border region between Yunnan, Guizhou and Sichuan provinces; inset enlarged as C. C. Detailed map of the Shizigou Valley section near the Weixin County town, with the fossil locality marked by solid triangle.