Middle Jurassic unionids (non-marine Bivalvia) from the Shiwandashan Basin, southern China, with special emphasis on Cuneopsis Simpson

Abstract

Articulated non-marine unionid bivalves from red beds of the Middle Jurassic lower member of the Nadang Formation of Banyou, Shiwandashan Basin, Guangxi province, southern China, comprise five morphotypes of Cuneopsis johannisboehmi (Frech). They have been recognised on the basis of a population study by their: (1) transversely elliptical shape, with subparallel and substraight dorsal and ventral margins, (2) elongated cuneiform shape, with a shallow concavity near the posteroventral margin, (3) elongated cuneiform shape, with a sharp posterior end, but without a clear posteroventral concavity, (4) sub-triangular shape, with a sharply pointed posterior and a relatively rounded anterior margin and gently convex ventral margin and (5) suboval shape, with a convex ventral margin. The analysis demonstrates that these unionids are morphologically variable and leads to revision of at least 15 species of unionids, which are merged into Cuneopsis johannisboehmi. In the Shiwandashan Basin, the unionids are preserved in two types of shell assemblage (2D and 3D). Morphological features such as relatively thick shells, medium to large inflation, a large H/L ratio (more than 0.5) and anteriorly placed umbones all indicate a habitat of large rivers for this unionid.