Glyptorthis sarcina, 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 : 372

Glyptorthis sarcina sp. nov.

Fig. 7C–G; Table 2.

Derivation of name: From the Latin sarcina , bundle, referring to the fascicostellae of the new species.

Holotype: NIGP 139065 , dorsal internal and external moulds of conjoined valves.

Type locality: Shizigou Valley near the Weixin County town , northeastern Yunnan Province .

Type horizon: Upper part of Shihtzupu Formation.

Material.—Five ventral internal and two external, three dorsal internal and two external moulds.

Diagnosis.—Biconvex shell; relatively sparse, coarse costae with uncommon bifurcation in posterior half, and more intense fascicostellate branching anteriorly; ventral muscle field considerably smaller than in typical Glyptorthis ; dorsal adductor scars marked by strong, radial ridges.

Description.—Shell small to medium, equally biconvex or dorsibiconvex, subquadrate to subsemicircular, with maximum width at or near hinge line. Cardinal extremities rounded or rectangular. Anterior commissure rectimarginate or weakly denticulate. Ventral valve moderately convex; beak erect; interarea flat or weakly concave, about one−fifth to one−seventh of shell length, apsacline; delthyrium narrow, open. Dorsal valve convex to strongly convex; sulcus narrow, starting from umbo, becoming wider and shallower anteriorly; dorsal interarea flat, anacline, less than one−tenth of shell length; notothyrium open. Costae coarse, sparse, 9–11 per valve, with rare bifurcation in posterior part of shell and intense fascicostellate branching near anterior margin of some specimens. Concentric fila dense, evenly spaced over entire shell surface; concentric lamellae irregular, best developed near anterior margin of shell. Minute tubercles dense, irregularly distributed on shell surface ( Fig. 7G 2 ).

Teeth wedge−shaped; dental plates receding, subparallel; muscle field small, deeply impressed, occupying one−quarter length and one−fifth width of shell; adductor scar subtriangular, located in antero−medial part of muscle field, flanked by diductor scars; vascula media originating from anterior ends of two diductor scars, extending anteriorly to two−thirds of shell length, then diverging antero−laterally. Cardinalia about one−fifth length and one−third width of shell; cardinal process simple, ridge−like, projecting postero−ventrally, with crenulated crest; brachiophores blade−like; brachiophore bases stout, diverging from each other at 78°; notothyrial platform high, continuous anteriorly with thick median ridge; sockets large, deep; fulcral plates weak; median ridge thick, high, limited within muscle field; muscle field about half shell length or width; posterior pair of adductor scars small, located immediately anterior of notothyrial platform; anterior pair much larger, marked by strong radial ridges; mantle canal system saccate, poorly impressed.

Discussion.—The new species is assigned to Glyptorthis on the basis of its prominent ventral interarea, fascicostellate branching of costae, strong concentric lamellae, and the configuration of cardinalia. This Llanvirnian species is among the oldest forms of the genus and differs from all other congeneric species in its considerably weaker concentric lamellae, smaller ventral muscle field, strong radial ridges in the dorsal muscle field, and minute tubercles on the outer shell surface. The relatively sparse primary costae of the new species are somewhat similar to those of Glyptorthis glypta Cooper, 1956 but the latter has strong lamellose frills. The sparse costae of G. sarcina are reminiscent of Ptychopleurella but the new species lacks the prominent, catacline to strongly apsacline, pyramidal, ventral interarea that is diagnostic of Ptychopleurella .

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.