Leptellina spatiosa, 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 : 378-380

Genus Leptellina Ulrich and Cooper, 1936 Leptellina spatiosa sp. nov.

Figs. 9Q, 10J–O, 11A–C; Table 8.

Derivation of name: From the Latin, spatiosus, large, spacious, referring to the large shell size of the new species.

Holotype: NIGP 139113 , ventral internal and external moulds.

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

Type horizon: Upper part of Shihtzupu Formation.

Material.—Six ventral internal and three external, nine dorsal internal and seven external moulds. Collection AFI1680.

Diagnosis.—Shell medium to large, transverse, concavoconvex, with short dorsal geniculation; ventral muscle field comprising two widely separated lobes; cardinal process simple; dorsal muscle bounding ridges prominent; median ridge terminating at junction with platform margin.

Description.—Shell medium to large, concavoconvex, subsemicircular, with maximum width along hinge line. Cardinal extremities acute, with short ears in some shells ( Figs. 10O, 11C). Anterior commissure rectimarginate. Ventral valve strongly convex, about three−fifths as long as wide; interarea high, about one−seventh of shell length, apsacline, with planar to weakly concave surface; delthyrium narrow, covered by arched pseudodeltidium. Dorsal valve about half as long as wide, unevenly concave, with short, sharp geniculation; interarea about one−eighth of shell length, planar, hypercline; notothyrium small, covered by strongly convex chilidium. Unequal parvicostellae; 7–9 accentuated costae, with accentuated costellae inserting at one−third and two−thirds shell length respectively; 3–4 finer costellae between two adjacent accentuated costae or costellae. Concentric fila dense, evenly spaced; weak concentric rugae present in some specimens, especially in postero−lateral parts of shell.

Teeth small; dental plates absent; ventral muscle field bilobed, about one−third length and one−quarter width of shell; adductor scars subtriangular, deeply impressed, located in postero−medial part of muscle field, flanked by larger, much longer, antero−laterally divergent diductor scars; mantle canal system saccate; vascula media originating from anterior ends of diductor scars, extending antero−laterally for a short distance, then bifurcating into antero−medial and lateral branches and further into numerous branches; vascula genitalia located on both sides of muscle field, antero−laterally bounded by anteriorly pointed coarse tubercles; ventral platform variously developed, with its margin marked by low ridge or discrete septules.

Cardinalia stout, about one−eighth length and one−fifth width of shell; cardinal process simple, thick, ridge− or rod−like, projecting ventrally; sockets small, deep, open antero−laterally; inner socket ridges high, discrete from cardinal process, divergent from each other at 94–95°, terminating into rod−like processes. Muscle field elevated, about one−third length and one−quarter width of shell, with high, thick, postero−lateral bounding ridges and weak antero−lateral ridges; anterior pair of adductor scars slightly larger and more elongate than posterior pair. Median ridge thick, highest at anterior end of muscle field, terminating at junction with anterior margin of platform. Platform semicircular in outline, with its marginal ridge ranging from low to relatively high and tilted towards shell margin. Vascula media beginning from anterior ends of adductor scars, extending along median ridge to platform margin, then turning postero−laterally to form many branches; vascula myaria originating from junction of two pairs of adductor scars, extending and branching antero−laterally.

Variability.—The margin of the ventral platform is usually marked by a row of anteriorly projecting septules but, in some specimens, it is a low, continuous ridge ( Fig. 10N). The ventral muscle bounding ridges are typically weak but may become high and thick, particularly in specimens with a well−developed ventral platform margin. In some ventral interior a median septum is present in the central part of the valve but in some others it is absent. The margin of the dorsal platform is normally a simple ridge tilted towards shell margin, but rarely the platform is poorly defined ( Fig. 9Q). The irregular concentric rugae are visible only in some specimens.

Discussion.—Most of the known species of Leptellina are Darriwilian in age ( Cocks and Rong 1989), but none of these have the large, bilobed ventral muscle field and the prominent bounding ridges of dorsal muscle field (resembling a bema) of the new species. Various species of Leptellina from North America ( Cooper 1956) and Scotland ( Williams 1962), as well as those from Bohemia (lower Caradocian, Havlíček 1967), for example, have a comparatively smaller ventral muscle field. Leptellina qianjiangensis (Liang in Liu et al. 1983) from the Changwu Formation (middle Ashgillian) of western Zhejiang Province ( Cocks and Rong, 1989) is similar to L. spatiosa , especially in its large and bilobed ventral muscle field, but differs in having a more elongate and convex ventral valve, more numerous accentuated costae and costellae, and a much stronger dorsal platform. Lepidomena pulchra Laurie (1991) from the base of the Lower Limestone Member of the Benjamin Limestone (Caradocian) of Tasmania has a similar ventral muscle field and mantle canal system, but it has a less transverse shell, more numerous accentuated costae, and a high and blade−like cardinal process, and lacks dorsal muscle bounding ridges.