Sulevorthis Jaanusson and Bassett, 1993

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 370

Genus Sulevorthis Jaanusson and Bassett, 1993 Sulevorthis ? sp.

Fig. 7A, B; Table 1.

Material.—Two ventral internal, one dorsal internal and two external moulds.

Description.—Shell small, planoconvex, subcircular with maximum width at or near hinge line. Cardinal extremities round or rectangular. Anterior commissure rectimarginate. Ventral valve moderately to strongly convex, highest around umbonal area; interarea weakly curved, about one−sixth of shell length, apsacline; delthyrium narrow, open ( Fig. 7A 2 ). Dorsal valve nearly flat with broad, gentle, median sulcus; interarea linear, anacline; notothyrium open. Costae simple, coarse, evenly spaced, 12–13 per valve, crests rounded, interspace of equal width as costa; concentric fila dense, evenly spaced over entire shell surface.

Teeth massive, with shallow fossetts on inner sides; dental plates absent ( Fig. 7A 2 ); muscle field confined to delthyrial cavity, slightly elevated anteriorly, about one−third length and one−sixth width of shell; adductor and diductor scars not clearly separated; vascula media originating just anterior to muscle field, extending parallel to two−thirds of shell length, then turning towards shell antero−lateral margins. Cardinalia about one−tenth length and one−quarter width of shell; cardinal process small, ridge−like, confined to deep and narrow notothyrial cavity ( Fig. 7B 3); brachiophores rod−like, projecting mainly ventrally, without brachiophore supports; brachiophore bases thick, divergent at 98°; sockets shallow, open antero−laterally; muscle field poorly impressed. Crenulations extending to mid−length of shell.

Discussion.—Compared with the type species of Sulevorthis , the Weixin material differs in having a flatter dorsal valve, weaker concentric ornamentations, smaller cardinal process, and narrower delthyrium. Orthis sp. reported by Williams (1974) from the Stapeley Volcanic Group (lower Llanvirnian) of Shropshire is similar to the Weixin specimens in its dorsal interior but has more coarse costae. Several Cooper's (1956) species of Orthambonites , subsequently assigned to Sulevorthis by Jaanusson and Bassett (1993), can be readily distinguished from the Weixin species in having a gently convex dorsal valve, or more densely populated costae, or thicker cardinal process.

Sulevorthis has been reported from Baltoscandinavia, British Isles and North America ( Jaanusson and Bassett 1993). This is the first possible record of the genus in South China.

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.