Protoskenidioides weixinensis, Zhan & Jin, 2005

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 375-378

Genus Protoskenidioides Williams, 1974 Protoskenidioides weixinensis sp. nov.

Fig. 9C–P; Table 6.

Derivation of name: After Weixin county , the type area of the new species .

Holotype: NIGP 139097 , dorsal internal mould.

Type locality: Collection AFI 1680, Shizigou Valley near the Weixin County town , Yunnan Province .

Type horizon: Upper part of Shihtzupu Formation. Material.—28 ventral internal and five external, 51 dorsal internal and 5 external moulds.

Diagnosis.—Shell very small, weakly transverse, ventribiconvex. Shell costellae increasing anteriorly through intercalation and bifurcation. Sessile spondylium undercut anteriorly. Cardinal process ridge−like, separated from median ridge by brachiophore supports; median ridge well−developed, extending to anterior margin of shell. Posterior outer pair of dorsal adductor scars larger than anterior inner pair.

Description.—Shell very small, ventribiconvex, transversely subsemicircular to subcircular, with maximum width at or slightly anterior to hinge line. Cardinal extremities rectangular to rounded. Anterior commissure rectimarginate to broadly sulcate. Ventral valve strongly convex, deepest in umbonal area; interarea high, apsacline, with planar surface; delthyrium narrow, open; beak erect. Dorsal valve gently convex, deepest around umbo; narrow sulcus starting immediately anterior of umbo; interarea low, planar, anacline; notothyrium open, relatively wide. Costellae rounded, different in size, 15–18 per valve; intercalation and bifurcation beginning about mid−length of shell. Growth lines fine, evenly spaced.

Teeth strong, wedge−shaped; dental plates thin, basolaterally divergent, becoming subparallel to medially convergent at their bases to form primitive, sessile, anteriorly undercut spondylium. Muscle field about one−quarter length and one−third width of shell. Cardinalia stout, about one−quarter length and one−third width of shell; cardinal process ridge−like, with swollen myophore, extending for whole length of notothyrial cavity in adult specimens, but limited to the posterior end in juveniles ( Fig. 9I). Sockets deep, relatively large, with variously developed fulcral plates; brachiophores rod−like, projecting antero−ventrally; brachiophore bases high, thin, divergent at about 74°; brachiophore supports variously developed, meeting anteromedially to median septum at 94°, forming cruralium−like structure; notothyrial cavity deep, subrombic to subcircular in outline, bounded posteriorly by brachiophore bases and antero−aterally by brachiophore supports. Median septum very high, extending to shell anterior margin, reaching maximum height at about two−thirds length of shell. Muscle field clearly impressed, about two−fifths length or width of shell, without bounding ridges; outer, posterior pair of Type species: Halirhachis leonina sp. nov. adductor scars elliptical to longitudinally semicircular; inner, Diagnosis.—Shell small, concavoconvex, semicircular, parvianterior pair smaller, more elongate. costellate. Dental plates thin, short, low; ventral muscle field Variability.—The new species shows some degrees of varia− small, bilobed; a small platform immediately anterior to mustion in the development and configuration of brachiophore cle field, undercut anteriorly, supported by high, thick median supports, which may be strong, thick, and merging onto the ridge limited to visceral area; pair of short, antero−laterally dimedian septum in some shells ( Fig. 9N 1 , N 2 ), or weak and vergent ridges present in posterior part of shell. Cardinal prodisjunct from the median septum in others ( Fig. 9L 1 , L 2 ). The cess simple ridge, discrete from thick, widely divergent socket angle between the brachiophore support and the median sep− ridges; platform well−developed, tilting antero−ventrally tum varies from acute ( Fig. 9M) to obtuse ( Fig. 9N 1 , N 2 ). The above valve floor in medial part; median ridge high, thick. angle of divergence between the pair of brachiophore bases Species included.—In addition to the genotype, the followhas similar variations. In shells of similar size, the dorsal me− ing species is probably assignable to the new genus: Diamdian septum varies from thin and low to thick and high. bonia miaopoensis Chang, 1983 , Miaopo Formation, lower Ontogenetically, small shells tend to have a weak cardinal Caradocian, Yichang, Hubei Province. process, a short dorsal median septum that is disjunct from Discussion.—The new genus differs from all other members the cardinal process and does not extend to the shell anterior of the superfamily Plectambonitoidea in its ventral interior. margin ( Fig. 9I, O), and a pair of subparallel brachiophore Its strong dorsal median ridge and well−developed platform supports. are similar to those of Bockelia Neuman (in Neuman and Discussion.— Protoskenidioides weixinensis is the youngest Bruton 1989) and some representatives of Leptellina Ulrich known species of the genus. It is characterized by a weakly and Cooper, 1936. Compared to the new genus, Bockelia diftransverse shell with fine costellae that increase anteriorly fers in having a generally smaller cardinalia and Leptellina via bifurcation and intercalation. Despite its small shell size, having a well−defined dorsal muscle field.

the internal structures of the new species are more prominently developed than other congeneric forms, including the Halirhachis leonina sp. nov. genotype. Protoskenidioides revelata Williams, 1974 , the Fig. 10A–I; Table 7. type species, from the upper Arenigian of Shropshire, Eng− Sowerbyites ? sp.; Wang and Yan 1978: 220, pl. 54: 5–7, 9. land, has a larger, strongly transverse shell with less frequent Derivation of name: From the Latin, leoninus, of lion, referring to the intercalation of costellae. Protoskenidioides minor Xu and type locality, Shizigou, meaning “lion's valley” in Chinese. Liu, 1984 from the lower Meitan Formation (lower Are− Holotype: NIGP 139102, ventral internal mould. nigian) of Guizhou Province (SW China) can be distin− Type locality: Shizigou Valley near the Weixin County town, northeastguished by its more strongly transverse shell with notably ern Yunnan, SW China. coarser and fewer costae. Its dorsal median septum is gener− Type horizon: Upper part of Shihtzupu Formation. ally absent or poorly developed. Protoskenidioides huang− Material.—Five ventral internal, and four dorsal internal and huaensis Zeng (in Zeng et al. 1983) from the upper Dawan one external moulds. Collection AFI1680. Formation (upper Arenigian) of Hubei Province is similar to Description.—Shell small, concavoconvex, semicircular,

P. weixinensis in ribbing style but differs in having a larger with maximum width along hinge line. Cardinal extremities

and more strongly transverse shell. Its cardinal process and acute, with small ears developed in some strongly transverse

median septum are absent or poorly developed. shells. Anterior commissure rectimarginate. Ventral valve

Order Strophomenida Öpik, 1934 strongly convex, deepest in central part of valve; interarea high, about one sixth of shell length, apsacline, with planar

Superfamily Plectambonitoidea Jones, 1928 surface; beak erect; delthyrium narrow, covered by small,

Family Taffiidae Schuchert and Cooper, 1931 arched pseudodeltidium. Dorsal valve gently concave to flat

Genus Halirhachis nov. in visceral area, with short, sharp geniculation; interarea low, Derivation of name: From the Greek, halis, many, and rhachis, ridge. hypercline, about one−sixteenth of shell length; notothyrium Halirhachis is a feminine noun, referring to the longitudinal ridges in small, covered by narrow, arched chilidium. Parvicostellae the ventral interior. mainly of two sizes; accentuated costae 7–9 in number; 4–5

finer costellae between adjacent costae at anterior margin. Concentric fila evenly spaced but usually poorly preserved.

Teeth small, wedge−shaped; dental plates short, thin; pair of blade−like plates present on both sides of teeth along hinge line, resembling accessory denticular ridges but lacking denticles. Muscle field bilobed, occupying one−fifth length and width of shell, bounded postero−laterally by dental plates; adductor scars small, slightly elevated, flanked by much larger diductor scars; small, short platform about 0.8–1 mm long, located immediately anterior to muscle field, with its surface covered by longitudinal tubercles, elevated anteriorly, supported by thick median ridge extending slightly beyond mid−length of shell. Pair of prominent ridge present in postero−lateral parts of valve, extending antero−laterally for about 1 mm, bearing anteriorly tilted, elongate tubercles; peripheral rim marked by discrete, anteriorly tilted tubercles.

Cardinalia about one−fifth length and one−third width of shell; cardinal process a simple ridge, mainly in posterior part of umbonal cavity; brachiophore processes short, rod−like; sockets small, narrow, deep, open antero−laterally; socket ridges thick, divergent from each other at about 120°; muscle field poorly impressed, about three−fifths length and one−half width of shell; outer, posterior pair of adductor scars large, longitudinally semicircular; inner, anterior pair subelliptical, less than half width of posterior pair; median ridge developed in anterior part of valve, terminating at its anterior junction with platform; platform highly raised and unsupported antero−medially ( Fig 10H, I).

Discussion.— Sowerbyites ? sp. illustrated by Wang and Yan (1978, no description) from the Miaopo Formation (lower Caradocian) of Yichang, Hubei Province, is virtually identical to the new species except for its weaker postero−lateral ridges in the ventral interior. Diambonia miaopoensis Chang, 1983 resembles Halirhachis leonina in most aspects of the ventral interior but differs in having a larger, more elongate shell and lacking accessory denticular ridges along the ventral hinge line.

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.