Acambona sp.

Sun, Yuanlin & Baliński, Andrzej, 2011, Silicified Mississippian brachiopods from Muhua, southern China: Rhynchonellides, athyridides, spiriferides, spiriferinides, and terebratulides, Acta Palaeontologica Polonica 56 (4), pp. 793-842 : 818

Acambona sp.

Fig. 19B, F–I .

Material.—One slightly damaged juvenile dorsal valve, 14 very small fragments of umbonal region of dorsal valve, and one incomplete ventral valve from samples MH2, M2−8, M2/2, M2/3, and M2/4.

Remarks.—Although the specimens at our disposal are very fragmentary there is no doubt that they belong to the retziid athyridides. The characteristic internal features of the dorsal valves are: massive cardinal plate without perforation, high postero−ventrally projecting cardinal flanges, and low median septum. The cardinal plate is attached to the valve wall dorsally to the inner socket ridges ( Fig. 19G ). Of special interest is the presence of a distinct median ridge running on the underside of the cardinal plate, continuing anteriorly from the umbonal cavity also as a median ridge on the valve floor ( Fig. 19G ). This ridge can be interpreted as a low median septum. A strong median ridge is also observed between high and sharp crural bases on the upper (ventral) surface of the cardinal plate ( Fig. 19B 1 , F). Inside the ventral valve a short pedicle collar is developed. The presence of a median ridge on the floor of the dorsal valve and median ridges on both the upper and lower surfaces of the cardinal plate distinguish the specimens from Muhua from all known species of Eumetria Hall, 1863 . The absence of distinct radial costae on our specimens (which usually represent, however, only the umbonal region of the shell) reminds the condition observed in Acambona (e.g., Weller 1914: 449; Grunt 1986: 159). Also the presence of distinct pedicle collar in ventral valves from Muhua indicates that they should be included rather in the genus Acambona . The specimens clearly represent, at least, a new species, but more material is needed in order to allow a full description.

Figures

Fig. 19. Retzioid and nucleospiroid brachiopods from the Tornaisian Muhua Formation, southern China. A. Retzioid Hustedia sp.; nearly complete shell, PKUM02−0468, in dorsal (A1), lateral (A2), and enlarged dorsal (A3) views. B, F–I. Retzioid Acambona sp. B. Umbonal fragment of dorsal valve, PKUM02−0469, in interior (B1) and posterior (B2) views showing complexity of cardinalia. F. Slightly incomplete dorsal valve, PKUM02−0470, in interior (F1) and enlarged interior (F2) views. G, H. Umbonal region of two dorsal valves, PKUM02−0471 (G) and PKUM02−0472 (H), in anterior view with labelled morphology. I. Incomplete ventral valve, PKUM02−0473, in exterior (I1) and interior (I2) views. C–E. Nucleospiroid Nucleospira sp. C. Interior of incomplete dorsal valve, PKUM02−0474. D, E. Two posterior fragments of shell, PKUM02−0475 (D) and PKUM02−0476 (E), in anterior view showing cardinalia; ventral valve at the top. A, D, E, G, and H, SEM micrographs. Samples MH1 (A, C–E) and MH2 (B, F–I).

Fig. 1. Stratigraphic columns (A) and map (B) of the sampling sites around Muhua and Gedongguan villages in Guizhou Province, southern China (after Sun and Baliński 2008).