Abstract
A unique, new palaeotaxodont (Protobranchia) genus and species, Yuexiconcha duplicata Zhang, Niu and Johnston, is proposed and described. It is characterized by: a medium-sized, transversely elongated, sub-elliptical shell; heterotaxodont dentition; and posterior tooth row consisting of crowded gradidentate dentition, partially and dorsally overlapped by an additional tooth row that emanates from the beak to form a bitaxodont dentition (new term). Most significantly, a prominent resilifer separates the anterior and posterior tooth rows and shows slight to moderate excavation into the hinge-plate. While a resilifer indicates phylogenetic proximity with Nuculoidea, Yuexiconcha nov. gen. is readily distinguished by its bitaxodont posterior dentition and a more elongate posterior shell lobe and so is provisionally placed in the Family Nuculidae, Order Nuculida. The hinge of Yuexiconcha nov. gen. indicates that a resilifer in palaeotaxodonts first developed in the Ordovician, rather than in the Silurian (Wenlock) as thought previously. Specimens described herein were collected from a fine-grained siliciclastic rock unit in the upper part of the Dongchong Formation in western Guangdong, South China. Other components of the biota occurring with the bivalves are uncommon and include trilobites and brachiopods that indicate a late Middle–early Late Ordovician (late Darriwilian–early Sandbian) age.
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