Enteropleura guembeli ( Mojsisovics, 1874 )

Chen, Jin-Hua & Stiller, Frank, 2007, The halobiid bivalve genus Enteropleura and a new species from the Middle Anisian of Guangxi, southern China, Acta Palaeontologica Polonica 52 (1), pp. 53-61 : 59-60

Enteropleura guembeli ( Mojsisovics, 1874)

Fig. 6 .

1874 Daonella Gümbeli sp. nov.; Mojsisovics 1874: 8, pl. 3: 12, 13.

1912 Enteropleura Gümbeli (Mojsisovics); Kittl 1912: 162, pl. 1: 16, 17.

1974 Enteropleura guembeli (Mojsisovics) ; Chen et al. 1974: 334, pl. 175: 22 [same specimen described by Gu et al. (1976: 225, pl. 38: 18) and Gan and Yin (1978: 351, pl. 118: 6)].

1980 Enteropleura guembeli Mojsisovics ; Chen 1980: 1192.

Material.—Only the figured right valve, NIGP 15959, from the lower part of the Xinyuan Formation (late Middle Anisian, early Middle Triassic); Bomo area, Xinyuan, Ziyun District, southwestern Guizhou, southwestern China. Although the specimen was registered with a NIGP repository catalogue number, it apparently has not been handed over to the collections of NIGP and was not available for this study; most probably it is lost. The figure provided herein is a reproduction of that in Chen et al. (1974).

Original description.— Chen et al. (1974: 334, translated from Chinese): “Shell small, subcircular. Hinge line straight and short, about 2/3 of the shell length. Umbo situated slightly anterior to mid−length. Posterior slope of umbo with a shallow groove. Shell surface with 5–6 commarginal lines and fine, weak, closely spaced radial lines.” Re−description.—Shell small, 9 mm long, rounded−subquadrate with the dorsal part being slightly longer than the ventral part; longer than high, height/length ratio about 0.8. Beak orthogyrous, slightly projecting above the dorsal margin, positioned at 0.45 of the shell length from anterior. Dorsal margins slightly descending from the beak, ventral and posterior margins broadly curved; anterior margin curved, but anterior part probably slightly incomplete. Hinge line straight and short, equalling about 0.6 of the shell length; ligament area not preserved; auricles absent. Prodissoconch small, about 1 mm in length. External surface of dissoconch with commarginal sculpture of 5–6 irregular rugae and several very fine lines; rugae restricted to early and middle ontogenetic stages; radial striae very fine and weak, threadlike, limited to the central and posterior sectors.

Discussion.— Enteropleura guembeli is a small species with nearly subcircular (slightly longer than high) shell. Its surface exhibits several irregular commarginal rugae and weakly developed radial ribs, and lacks regular and fine commarginal lirae ( Mojsisovics 1874: 8, pl. 3: 12, 13; Kittl 1912: 162, pl. 1: 16, 17). These sculptural features distinctly differ from those of the taxa of the Enteropleura bittneri group (i.e., E. bittneri , E. jenksi , E. lamellosa , E. walleri ).

In the original description of the Guizhou specimen, Chen et al. (1974: 334) mentioned a short “shallow groove” on the umbonal part ( Fig. 6 ). However, position and direction of this “groove” seem to differ from those of the grooves generally found in the species of Enteropleura , including E. guembeli , which indicate the internal ridges. This “groove” may be an artefact. However, the other features of this specimen such as shell shape, shell size, beak, hinge line, and surface sculpture are close to those of the type specimens of E. guembeli ( Mojsisovics 1874; Kittl 1912). Therefore, the assignment by Chen et al. (1974) is acceptable (e.g., Gu et al. 1976: 225; Gan and Yin 1978: 351; Chen 1980: 1192; Yin 1985: 588; Chen 2004: 659). According to Gan (1983), the Guizhou specimen of E. guembeli comes from a dark−green sandy shale of the lower part of the Xinyuan Formation, which yields the ammonoids “ Paraceratites ” cf. binodosus, Judicarites sp. , and Balatonites sp. , indicating a late Middle Anisian age.

Figures

Fig. 6. Halobiid bivalve Enteropleura guembeli (Mojsisovics, 1874) from the lower part of the Xinyuan Formation (late Middle Anisian, Middle Triassic), Mobo, Xinyuan, Ziyun District, southwestern Guizhou, southwestern China. Right valve, NIGP 15959; reproduction of Chen et al. (1974: pl. 175: 22).