Early Cambrian Anabarella plana from Three Gorges area, South China

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Anabarella, a conspicuous taxon of early mollusc, is widely distributed in the early Cambrian strata and is considered an important link in the evolutionary lineage that reflects a transitional form from helcionelloids to bivalves. In South China,Anabarellahas mainly been documented from Yunnan, Shaanxi, Sichuan, and Hubei provinces. However, the taxonomy ofAnabarellais questionable, which has implications for the interpretation of the genus’ temporal and spatial distribution. New and abundant well-preserved specimens of the helcionelloid molluscAnabarellawere recovered from the Member 5 of the Yanjiahe Formation in the Three Gorges area. Through morphological study, these specimens can be definitely identified asA. plana. On the basis of this new material, the species ofAnabarellapreviously reported in the literature from South China were taxonomically revised, and, with the exception ofA. planafrom the Yanjiahe Formation, other species should be assigned toIgorella. Therefore, at present,A. planais the only valid species of the genusAnabarellain South China and is limited to Cambrian Stage 2. Study of the available specimens ofA. planareveal three types of microstructures: convex polygonal impressions, concave polygons, and lamello-fibrillar microstructure. In addition, the thicker shell of the sub-apical area and the three different structures of the sub-apical area provide more evidence thatA. planamight have adapted a semi-infanual mode of life and indicate thatAnabarellais a likely ancestor ofWatsonella.