Calyptolepta huanghuaensis ( Chang, 1983 )

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 384-385

Calyptolepta huanghuaensis ( Chang, 1983)

Fig. 12E–M; Table 12.

Bilobia huanghuaensis sp. nov.; Chang 1983: 477, pl. 1: 22.

Material.—Two ventral internal and three external, 10 dorsal internal and 10 external moulds.

Description.—Shell small, concavoconvex, subsemicircular; maximum width along hinge line. Cardinal extremities acute, with long, narrow ears. Anterior commissure rectimarginate. Ventral valve evenly, strongly convex, deepest at mid−length of shell; interarea relatively high, about one−fifth length of shell, apsacline, with planar or slightly curved surface ( Fig. 12E, F); delthyrium narrow; pseudodeltidium arched. Dorsal valve about half as long as wide; unevenly concave, deepest centrally ( Fig. 12G, L, M); interarea about one−tenth length of shell, catacline to hypercline; notothyrium small; chilidium convex. Unequal parvicostellae; 9–11 accentuated costae, bifurcating twice at about 1.0 mm and 1.5 mm growth stages respectively.

Teeth small, wedge−shaped; dental plates thin, short. Muscle field clearly impressed, bilobed, about one−fifth length and width of shell; adductor scars small, subtriangular, slightly elevated, located in postero−medial part of muscle field; diductor scars elongate, widely divergent from each other. Vascula media saccate, originating from anterior ends of diductor scars.

Cardinalia stout, about one−seventh length and one−quarter width of shell; cardinal process undercut, simple knob or trifid, projecting ventrally ( Fig. 12H 1 –H 3); sockets small, narrow, open antero−laterally; socket ridges thick, discrete from cardinal process, terminating as blade−like processes. Bema well−developed, elevated, undercut anteriorly, about one−half length and one−third width of shell; central side septa beginning in front of notothyrial cavity, highest at anterior margin of bema; one or two pairs of longitudinal transmuscle ridges weak, short, developed mainly in anterior half of adductor scars; inner pair of adductor scars slightly smaller but higher than outer pair; connecting plate between central side septa variously developed, confined to bema, undercut ( Fig. 12H 2 , H 3). Platform marked by closely arranged but discrete longitudinal septules, particularly on both sides ( Fig. 12L, M). Shell marginal area strongly curved dorsally, covered by dense crenulations and pseudopunctae.

Variability.—The species shows a number of ontogenetic variations. With increasing shell size, the bema becomes stronger and more elevated, the ratio of dorsal interarea height to shell length increases, and the angle between the pair of socket ridges decreases. The connecting plate between central side septa is absent in juveniles (e.g., Fig. 12J) but well−developed in most adults ( Fig. 12H 3). There are also some intrapopulation variations in the present collection. The central side septa, for example, may change from poorly developed ( Fig. 12K) to prominent ( Fig. 12H, L, M) in shells of similar sizes. The length of central side septa is similarly variable. The number and strength of transmuscle ridges on bema are different from one individual to another or even from one side to another in a single valve (e.g., Fig. 12K, L).

Discussion.—The holotype of this species from the Miaopo Formation (lower Caradocian) of Yichang, western Hubei Province, is about 9 mm long and 12 mm wide, nearly three or four times as large as the Weixin material, but all the external and internal characters of the specimens from both localities are identical. The dorsal internal structures, particularly the undercut cardinal process and the strong central side septa, are different from those of Bilobia Cooper, 1956 . Some specimens of Calyptolepta rarum ( Neuman 1976) are similar to the Weixin shells of C. huanghuaensis in having an undercut cardinal process, a pair of strong side septa, and a highly elevated and undercut connecting plate.

The type species, C. diaphragma Neuman, 1976 from the Middle Ordovician (Darriwilian) tuffs of Newfoundland, can be distinguished from C. huanghuaensis by having a more prominent dorsal platform and more numerous costellae. Calyptolepta chengkouensis ( Xu, Rong, and Liu 1974) reported from various Middle Ordovician rocks of South China ( Xu and Liu 1984; Zhan and Jin in press) is different from C. huanghuaensis in its more numerous costellae, smaller dorsal platform, and stronger ventral platform.

Figures

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.

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.