Onniella sp.

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 374-375

Onniella sp.

Figs. 8M, N, 9A, B; Table 5.

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

Description.—Shell small, subcircular, ventribiconvex, widest at about one−third length of shell. Cardinal extremities rounded. Anterior commissure rectimarginate. Ventral valve strongly and evenly convex, deepest around umbo; interarea apsacline, nearly planar, about one−eighth of shell length; beak slightly curved; delthyrium open, wide. Dorsal valve gently convex with shallow sulcus originating from umbo; interarea low, anacline, less than one−tenth of shell length; notothyrium open. Multicostellae fine, about 5–6 costellae per mm at 1 mm growth stage, with number of costellae increasing anteriorly by bifurcation (common) or intercalation (rare); first−order costae originating from apex, second−order at about one−third length, and third−order at three−quarters length of shell; concentric fila dense, evenly spaced over entire shell surface.

Teeth small, thin, triangular; dental plates thin, high, relatively long, inclined medially but subparallel at their bases. Ventral muscle field suboval, clearly impressed, about one−third length and one quarter width of shell; triangular adductor scar in middle of muscle field, slightly elevated, elongated; two lobes of diductor scars longitudinally semielliptical. Cardinalia delicate, about one−eighth to one−sixth length, and one−fifth to one−third width of shell; cardinal process with thin shaft and swollen myophore; brachiophores rod−like, projecting strongly in ventral direction; brachiophore bases thick, divergent from each other at acute angle; brachiophore supports thin, very short, tilting basomedially; notothyrial cavity deep, slightly elevated anteriorly. Sockets small, shallow, open antero−laterally. Dorsal muscle field about one−half length and two−fifths width of shell; outer, posterior pair of adductor scars subcircular, located anterior to notothyrial cavity; inner, anterior pair much more elongated and narrow. Minute tubercles reflecting shell endopunctae dense, irregularly spaced.

Variability.—Available specimens indicate that the angle between the pair of brachiophore bases becomes smaller with larger shell size ( Table 5). The brachiophore supports are commonly present and basomedially inclined, but they may be poorly developed in some shells ( Fig. 9B). The dental plates are well developed and extend anteriorly to form lateral bounding ridges of the muscle field ( Fig. 8M 1 ), but, in a few specimens, they are short and limited to the posterior part of the muscle field.

Discussion.— Williams and Wright (1963) pointed out that Dalmanella and Onniella are somewhat similar externally but Dalmanella can be distinguished from Onniella by having fulcral plates, smaller muscle scars and a more elongate ventral muscle field. On the basis of Onniella from the type Caradoc area of Shropshire, Hurst (1979) suggested that the differentiation of species within Onniella lies in the ribbing pattern, ventral muscle field, and other internal features. Most known species of Onniella , such as the type species, O. broeggeri Bancroft, 1928 , from the Caradocian of Shropshire, England and those from North America ( Cooper 1956) and Baltica ( Hints 1975), have an unevenly convex ventral valve and a median ridge in the dorsal interior. The Weixin species has an evenly convex ventral valve but lacks a clearly defined dorsal median ridge. It probably represents a new species, but additional and better preserved specimens are needed to study all the diagnostic characters and their range of variation.

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.