Yangtzedonta and the early evolution of shelled molluscs
2001
Yi, Qian
The holotype ofYangtzedonta Yu, 1985 and its type species,Y. primitiva Yu, 1985, is an incomplete specimen ofXianfengella prima He et Yang, 1982. Therefore, based on the principle of priority, the namesYangtzedonta and Y.primitiva Yu (1985) should be abandoned. The fact that ven-tral valve ofXianfengella is bilaterally symmetrical indicates that they are not the left valve of a bivalved mollusc, as originally proposed by Yu. At present, the oldest certain bi-valve isXianfengoconcha elliptica Zhang, which occurs in the Early Cambrian Qiongzhusi Stage in China. The oldest known rostroconch isHeraultipegma yunnanensis He et Yang, which first occurs in the third small shelly fossil assemblage zone of the Early Cambrian Meishucunian Stage. The oldest known monoplacophoran,Maikhanella pristinis (Jiang), first occurs in the first small shelly fossil assemblage zone (Me-ishucunian Stage). Evidence derived from analyses of shell morphology, mode of growth, histology, and stratigraphic occurrence suggests that bivalves evolved from a branch of rostroconchs, and that rostroconchs in turn evolved from a branch of monoplacophorans. The hypothesis that these three groups originated simultaneously and independently at the beginning of the Cambrian and subsequently evolved in parallel, probably is incorrect.
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