Enteropleura walleri, Chen & Stiller, 2007

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 : 58-59

Enteropleura walleri sp. nov.

Figs. 4, 5 .

2002 Enteropleura sp. (sp. nov.?); Chen and Komatsu 2002: 436. Derivation of the name: In honour to Thomas R. Waller, who has made important contributions to the research on Mesozoic bivalves.

Holotype: NIGP 140170 ( Fig. 4A); right valve of an adult individual.

Paratypes: NIGP 140171–140186 ( Figs. 4B–O, 5 ) .

Type locality: Jinya, Fengshan District, northwestern Guangxi, southwestern China.

Type horizon: Middle part of Unit D of the Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic).

Material.—More than 100 external and internal moulds of right and left valves as well as numerous fragments from the same locality and horizon as the type specimens.

Measurements.—The holotype is 25.5 mm long and 16.0 mm high.

Diagnosis.— Enteropleura of medium size with a large prodissoconch and fine, regular commarginal sculpture; no radial sculpture. Differs from all other congeneric species in the lack of radial shell sculpture.

Description.—Shell of medium size, usually 20–25 mm (up to 30 mm) in length, D−shaped, longer than high (height/length ratio 0.62–0.82), nearly flat. Beak orthogyrous or slightly prosogyrous, projecting slightly above the dorsal margin, positioned anterior to mid−length. Hinge line short, about onethird to two−fifth of the shell length. Anterodorsal and posterodorsal margins nearly straight, descending very slightly from the beak; ventral margin broadly curved; anterior margin stronger curved and passing tangentially into the hinge margin; posterior margin almost straight, sharply curved posterodorsally and passing tangentially into the posterodorsal margin. Relative longitudinal elongation of the shell, relative length of the hinge margin, and relative position of the beak fairly variable, resulting in variable shell outlines (cf. Figs. 4, 5 ); however, besides intraspecific variability, diagenetical deformation may also partly be responsible for these variable shapes. Anterior auricle absent; posterior auricle small and very narrow (5–6 mm long, 0.2–0.3 mm high), separated from the disk by a narrow sulcus ( Fig. 4L 2 ); posterior margin of auricle forming a very obtuse angle with its dorsal margin, angle subtended by the dorsal margin and the base of the auricle about 5–10 °. Prodissoconch large, up to 2.8 mm in length, suboval, with comparatively indistinct, weak commarginal growth sculpture or almost smooth ( Fig. 5 ). Posterior radial grooves (indicating the internal ridges anteriorly and posteriorly bordering the track of the ontogenetically migrating posterior adductor muscle scar) narrow and shallow ( Figs. 4C, D, J, 5A, D ); posterior groove generally present, 2.5–6.3 mm long; anterior groove occasionally present but usually indiscernible. Ligament area not preserved.

Surface (not including the surfaces of the prodissoconch and the posterior auricle) divisible into three contiguous radial sectors (triangular fields):

(1) Anterior sector with irregular commarginal growth lines, its ventral margin subtending an angle of about 25–35 ° with the dorsal margin;

(2) Central sector bounded by the anterior sector and the posterior groove, enclosing an angle of about 100–110 °, bearing regular, fine, sharply rounded commarginal lirae; lirae somewhat weakening anteroventrally, occasionally obscure on the whole sector; spacing of the lirae narrow during earlier ontogenetic stages and gradually widening with progressing ontogeny;

(3) Posterior sector ventrally bordered by a broad, shallow radial groove that diverges from the dorsal margin at an angle of 35–45 ° in early ontogenetic stages and 20–25 ° in late ontogenetic stages; exhibiting broad, closely spaced commarginal undulations; each undulation usually branching into two or three lirae ventrally.

Juvenile shells (10–15 mm long, 6–12 mm high; Figs. 4J, 5 ): shape variable, ranging from reversed−subtrigonal to subelliptical, generally suboval, occasionally subcircular ( Fig. 5D ). Posterior sector relatively wide, its ventral margin subtending an angle of about 35–45 ° with the dorsal margin. Differentiation of the sculptural features of the three sectors not distinct; posterior sector usually without ventrally branching commarginal undulations but exhibiting commarginal lirae that are slightly broader than those on the central sector.

Discussion.—In most specimens of Enteropleura walleri sp. nov., the shell sculpture of the central sector consists of fairly distinct, regular commarginal lirae ( Figs. 4A, B, D–F, J, M, 5A, B, D ). But in some specimens, the commarginal lirae are very weak or even nearly indiscernible ( Fig. 4G, H, L). These differing sculptural features might lead to an assignment to different species. However, these variably sculptured shells usually co−occur (e.g., Fig. 4H) and some intermediate forms ( Figs. 4C, I, 5C ) indicate that the differing sculptural features are due to intraspecific variation. Although these differently sculptured specimens may be found very close to each other on the same slab (e.g., Fig. 4H), the varying sculpture also may partly be due to preservational reasons since these very thin shells certainly were susceptible even to slight differences in taphonomical and diagenetical conditions.

With regard to shell outline, shell size, commarginal sculpture, and presence of a posterior auricle, Enteropleura walleri is closely related to the three other species of the Enteropleura bittneri group, namely E. bittneri Kittl, 1912 , E. jenksi Hopkin and McRoberts, 2005 , and E. lamellosa ( Kittl, 1912) . However, these latter species all have distinct radial shell sculpture which is lacking in E. walleri .

Occurrence.—Unit D, Lower Member of the Banna Formation (late Middle Anisian, early Middle Triassic); Fengshan District, Guangxi, southwestern China.

Figures

Fig. 5. Prodissoconch and juvenile shell morphology of the halobiid bivalve Enteropleura walleri sp. nov. from Unit D of the Lower Member of the Banna Formation (late Middle Anisian, Middle Triassic), Jinya, Fengshan District, northwestern Guangxi, southwestern China. A. External mould, open−articulated specimen (left valve below, right valve above), with clearly discernible indications of the internal posterior radial ridges, paratype, NIGP 140183; A1, dorsal valve parts, arrows point to the margins of the prodissoconchs. B. External mould, left valve, paratype, NIGP 140184. C. External mould, right valve, paratype, NIGP 140185. D. External mould, left valve, with clearly discernible indications of the internal posterior radial ridges, paratype, NIGP 140186; D2, dorsal valve part, arrow points to the margin of the prodissoconch.

Fig. 2. Stratigraphical subdivision and correlation of the Middle Triassic in northwestern Guangxi and southwestern Guizhou, southwestern China (Fm., Formation; Mb., Member) (not to scale with regard to unit thicknesses).