EOPHILOBRYOIDELLA SINOANISICA NEW GENUS AND SPECIES, AN EARLY PHILOBRYID BIVALVE FROM THE UPPER ANISIAN (MIDDLE TRIASSIC) OF QINGYAN, SOUTHWESTERN CHINA

Journal of Paleontology, Mar 2004 by Stiller, Frank, Jinhua, Chen

Shells attaining lengths of up to almost 12 mm; however, specimens longer than 10 mm are rare; most specimens smaller with lengths of about 5-8 mm. Height of shells equaling 0.8 to 1.1 times the length; height mostly slightly smaller than length with height/length about 0.9 to 1.0. Width of doublc-valved shells (slightly) smaller than height, width/height mostly about 0.8 to 0.9. Articulated specimen (Fig. 3.1-3.5) with length of 7.7 mm, height of 6.7 mm, and width of 6.0 mm.

Etymology.-The species name refers to the occurrence of the new taxon in the Anisian of China.

Types.-Holotype, NIGP 134838 (Fig. 2); paratypes, NlGP 134839 (Fig. 3.1-3.5), NIGP 134840 (Fig. 3.6-3.10); all specimens from the lower upper Anisian (Leidapo Member) of Leidapo/Bangtoupo near Qingyan, Guizhou Province, southwestern China.

Other material examined.-Altogether about 580 specimens from the lower upper Anisian (Leidapo Member) of Leidapo/ Bangtoupo near Qingyan, Guizhou Province, southwestern China; repository catalog numbers: NIGP 134838-134852 (figured herein), and GPIMMS B6A-33.L15.

Occurrence.-Middle Triassic, lower upper Anisian (Leidapo Member); Qingyan, Guizhou Province, southwestern China.

Discussion.-The features of the new taxon Eophilobryoidella sinoanisica n. gen. and sp., particularly those of the hinge region, strongly suggest an assignment to the Philobryidae. The hinge with an internal ligament located in a narrow, elongate groove that runs obliquely posteriorly across the hinge plate, thereby dividing the hinge plate into two portions each showing numerous, fine but distinct, transverse crenulations, represents a type of hinge uniquely found in the Philobryidae and not found in any other bivalve group (compare e.g., Cox et al., 1969-1971). The other features of the new taxon such as the general shell shape also do not contradict an assignment to the Philobryidae. Tevesz (1977, p. 5, 8) also stressed the internal ligament and these hinge features as typical for the Philobryidae, "a ligament pit that lies between two rows of numerous, interlocking transverse denticles" is regarded to be characteristic for these bivalves. The features of the soft body anatomy discussed by Tevesz (1977), such as gills or foot, naturally are not available for study in the Triassic fossil specimens. The descriptions and figures in Cotton (1961) and Tevesz (1977) may serve for comparison of Eophilobryoidella sinoanisica n. gen. and sp. with modern philobryids; especially the figure of Micromytilus crenatuliferus in Cotton (1961, p. 71) shows hinge features that in some important aspects are rather similar with those of the new taxon.

Eophilobryoidella sinoanisica n. gen. and sp. differs from the modem philobryids by its anterior hinge plate portion being distinctly larger than the posterior portion. In the modern Philobryidae the anterior crenulated portion is smaller than the posterior one or attains about the same size at the most (compare the descriptions and figures in e.g., Cotton, 1961 and Tevesz, 1977). In relation to the respective shell size, the anterior hinge plate portion of the Triassic taxon generally is distinctly larger than the hinge plate portions in modern philobryids, whereas the posterior portions do not show such significant general differences in size. In the course of evolution, besides other modifications, the anterior crenulated hinge plate portion has been reduced.