In contrast to the numerous excellently preserved arthropods, vertebrates and plants from the Mesozoic lacustrine fossil lagerstätten of northeastern China, which have calcium phosphate or organic skeletons, the preservation of taxa with a calcareous skeleton is fairly poor. Here we investigate, using a scanning electron microscope and energy dispersive X-ray spectrometer, the preservational modes of bivalves from the Jurassic Daohugou Fossil Beds of Inner Mongolia and the Lower Cretaceous Yixian Formation of eastern Liaoning. The Jurassic bivalve Ferganoconcha sibirica is preserved as strongly compressed composite moulds which contain remains of the organic periostracum. In the Yixian Formation, the bivalves Sphaerium anderssoni and Arguniella ventricosa occur as compacted internal, external or composite moulds or are preserved with a silicified shell, and rarely with a shell consisting of iron hydroxides, which had replaced pyrite during late diagenesis/weathering. Silicification produced partly fabric-replacive microcrystalline quartz and partly void-filling megaquartz crystals after the carbonate shell had been dissolved. Films of authigenic aluminosilicate minerals, partly secondarily silicified, cover the exterior and interior shell surfaces. Occasionally, early diagenetic pyrite crystals, now oxidized to iron hydroxides, filled shell cavities forming internal moulds and rarely replaced the bivalve shell. The poor preservation of the bivalves reflects the environment and water chemistry of these lakes, which were heavily influenced by volcanic processes. Frequent ash deposition and decomposition of volcanic glass particles created acidic and alkaline lake and interstitial waters, which led to early diagenetic formation of authigenic aluminosilicate minerals, ferruginous internal moulds, dissolution of shell carbonate and silicification of shells.

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