Tritoechia Ulrich and Cooper, 1936

Zhan, Renbin & Jin, Jisuo, 2005, Brachiopods from the Middle Ordovician Shihtzupu Formation of Yunnan Province, China, Acta Palaeontologica Polonica 50 (2), pp. 365-393 : 387-391

Genus Tritoechia Ulrich and Cooper, 1936 Tritoechia sp.

Fig. 15A–C ; Table 16.

Material.—Four ventral internal and three dorsal internal moulds.

Description.—Shell small, ventribiconvex, subsemicircular (about three−fifths as long as wide), with maximum width along hinge line; cardinal extremities acute to rectangular; anterior commissure rectimarginate. Ventral valve strongly convex, deepest in umbonal area; interarea relatively high, about one−sixth length of valve, apsacline; beak erect; notothyrium small, covered by a thick, strongly arched deltidium. Dorsal valve gently convex; interarea low, about one−tenth length of valve, anacline; notothyrium narrow, completely covered by thin, strongly convex chilidium. Ornament of roughly equal−sized costellae, about 7 per mm along anterior margin of shell.

Teeth relatively strong; dental plates high, thick, subparallel, extending to anterior margin of muscle field; delthyrial chamber wide, deep. Muscle field clearly impressed, confined to delthyrial chamber; adductor scar narrow, elevated; diductor scars wider, longitudinally rectangular. Mantle canal system saccate, weakly impressed; vascula media originating from anterior ends of diductor scars, divergent, extending and branching anteriorly and laterally.

Cardinalia about one−fifth length and one−half width of shell; cardinal process small, thin, ridge−like, projecting posteriorly; notothyrial platform strongly elevated, continuous laterally with prominent socket ridges to form arched, free−hanging, transverse plate; sockets deep, narrow, open laterally; socket ridges diverging strongly. Muscle field poorly impressed, with weak myophragm.

Discussion.—Various species of Tritoechia of Tremadocian to Arenigian age commonly have a weakly transverse to equidimentional shell, a weak cardinalia and a variously developed median ridge. The Weixin material of Darriwilian age is one of the youngest representatives of the genus, and differs from other congeneric species in having a more transverse shell, stronger cardinalia and lacking a median ridge. The shells of Tritoechia dawanensis Zeng (1977 : pl. 13: 8, length/width ratio 0.84) and T. gaoluoensis Zeng (1977 : pl. 13: 9, ratio at 0.74) from the lower Dawan Formation (lower Arenigian) of western Hubei Province, for example, are less transverse than those from Weixin.

clearly impressed, subtriangular, about two−fifths length and two−sevenths width of shell; adductor scars minute, elongate−oval, located at posterior end of muscle field; diductor scars much larger, subtriangular; myophragm low.

Discussion.—The outline and internal characters of the ventral valve are similar to those of Peritritoechia imbricatia illustrated by Xu et al. (1974) from the Shihtzupu Formation (Mid Ordovician) of Meitan, northern Guizhou Province, but the latter differs somewhat from the present specimen by having a pair of subparallel dental plates and a long ventral median ridge extending anteriorly beyond the ventral muscle field. The uncertain species−level identification of the Weixin specimen is denoted here by a question mark.

Genus Peritritoechia Xu, Rong, and Liu, 1974 Order Pentamerida Schuchert and Cooper, 1931

Superfamily Porambonitoidea Davidson, 1853 Peritritoechia imbricatia? Xu, Rong, and Liu, 1974 Fig. 15D ; Table 17. Family Porambonitidae Davidson, 1853 Peritritoechia imbricatia sp. nov.; Xu et al. 1974: 152, pl. 66: 25–27. Genus Porambonites Pander, 1830

Porambonites transversus Xu, Rong, and Liu, 1974 Material.—One ventral internal mould.

Fig. 15E . Description.—Ventral valve small, about two−thirds as long

Porambonites transversus sp. nov.; Xu et al. 1974: 153, pl. 66: 32, 33, 36. as wide, strongly convex, deepest in postero−medial part; cardinal extremities subrectangular; maximum width along Material.—One internal mould of conjoined shell, one venhinge line; interarea high, about one−fifth length of shell, tral internal and one external mould. apsacline, with planar surface; delthyrium small, covered by Description.—Shell very large (up to 27.9 mm long, 40.1 gently convex deltidium; pedicle foramen submesothyridid; mm wide), dorsibiconvex, transversely elliptical, with maxipedicle epithelium thick, well preserved, 0.47 mm in diame− mum width near mid−length of shell (hinge line about twoter ( Fig. 15D 2 ). thirds as wide as shell). Lateral parts of shell flattened to

Teeth small, wedge−shaped; dental plates thin, high, tilt− gently deflected in ventral direction. Cardinal extremities ing medially, extending anteriorly to serve as lateral muscle rounded. Anterior commissure broadly uniplicate. Ventral bounding ridges; delthyrial chamber deep. Muscle field valve gently convex posteriorly, planar antero−laterally;

+ Fig. 14. Martellia ichangensis Wang, 1956 . A. NIGP139155, ventral internal mould. B. NIGP139156, ventral internal mould (B1) and latex cast (B2).

C. NIGP139157, dorsal internal mould. D. NIGP139158, dorsal internal mould. E. NIGP139159, dorsal external mould (E 1) and latex cast (E 2). F. NIGP139160, dorsal internal mould (F 1) and latex cast (F 2). G. NIGP139161, dorsal internal mould. H. NIGP139162, dorsal external mould (H 1) and latex cast (H 2). I. NIGP139163, ventral internal mould. J. NIGP139164, ventral internal mould (J 1) and latex cast (J 2).

sulcus beginning immediately in front of umbo, widening and deepening rapidly to reach two−fifths width of shell at anterior margin; interarea low, apsacline; delthyrium open. Dorsal valve strongly convex, deepest at about one−third length of shell; fold originating in front of umbo; interarea anacline; notothyrium open. Costellae dense, evenly spaced, equal−sized, increasing anteriorly by intercalation, about 4 per mm at 5 mm growth stage. Growth lines evenly spaced, 6–7 per mm, showing “string of beads” along interspaces over entire shell surface.

Teeth stout, massive; dental plates long, about one−half length of shell, thin, high, tilting medially, becoming subparallel in their bases, fusing at their ends to form sessile spondylium. Muscle field well defined, not clearly differentiated into adductor and diductor scars. Cardinalia (7.2 mm long, 10.0 mm wide) about one−quarter length and width of shell; cardinal process absent; crura short, rod−like. Outer hinge plates thick, high, divergent from each other at 115°; inner hinge plates parallel, thick, high posteriorly.

Discussion.—The holotype (NIGP 22246) of Porambonites transversus from the Shihtzupu Formation of Fenggang, northeastern Guizhou Province is smaller and less transverse (L/W ratio 0.77) than the Weixin material, otherwise their external and internal characters are identical. One ventral internal mould of Porambonites spp. Cocks and Zhan (1998: 128, pl. 5: 8) from the Naungkangyi Group of Northern Shan States ( Burma) is similar to the Weixin specimen in outline and internal structures but is of smaller size. The Naungkangyi material is likely assignable to P. transversus , considering the close faunal relationship between the Sibumasu and South China palaeoplates during late Middle Ordovician time ( Fortey and Cocks 1998, 2003).

Figures

Fig. 15. A–C. Tritoechia sp. A. NIGP139165, dorsal internal mould (A1) and latex cast (A2) with enlarged view of cardinalia (A3). B. NIGP139166, dorsal internal mould. C. NIGP139167, ventral internal (C1) and latex cast (C2). D. Peritritoechia imbricatia? Xu, Rong, and Liu, 1974; NIGP139168, ventral internal mould (D1) with local enlargement (D2) showing muscle field and pedicle epithelium. E. Porambonites transversus Xu, Rong, and Liu, 1974; NIGP139169, ventral and dorsal views (E1, E2) of internal mould, ventral external mould (E3) and enlarged view of beaded costellae (E4).

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.