Shell microstructures of the Early Cambrian Anabarella and Watsonella as new evidence on the origin of the Rostroconchia

Internal moulds of the laterally compressed monoplacophoran Anabarella plana Vostokova, 1962, and likely earliest rostroconch Watsonella Grabau, 1900, from the Early Cambrian of the Siberian Platform, show similar microstructures. The moulds are covered with a thin phosphatic crust replicating the inner morphology and microstructure of the shells. The shells were completely removed during etching of the samples in 10% acetic acid, except for some moulds of Watsonella sp., which retained an incompletely preserved and recrystallized wall. Three types of microtexture were found in moulds of Anabarella and Watsonella : polygonal, spiny and step-wise. The polygonal texture is well exposed in the apical area and dorsal margin and is interpreted to represent a prismatic outer layer. The polygons can be partially overlapped by spiny and stepwise textures that may represent a crossed-lamellar inner layer. Prisms and lamellae were first-order structural units, probably consisting of fibers. The similar shell microstructures of Anabarella and Watsonella , especially at the dorsal margins, support the hypothesis that a laterally compressed monoplacophoran such as Anabarella plana was the first evolutionary step from monoplacophorans towards Early Cambrian bivalves via the earliest rostroconch-like Watsonella.