The Early Cretaceous Jehol biota of China, preserved in lacustrine deposits, yields abundant well-preserved invertebrate and vertebrate fossils. However, the exact mode and nature of its extraordinary preservation remains unclear. Here we investigated the preservation of fossil insects from lacustrine successions at 12 Lower Cretaceous localities of China, and found that many insects in Jehol biota were preserved by pyrite (later pseudomorphed by iron oxide). These pyritized fossils were observed solely within lacustrine deposits with volcanogenic sediments. The pyritization of Jehol insects has been facilitated by the moderate sedimentation rate, anoxic bottom environment, abundant dissolved iron and sulfur compounds from frequent volcanic eruptions, and sediments low in organic carbon. In addition, the pyritization process has been evidently aided by microbial films, which may have been widespread in the Jehol paleolakes. Our result provides the first record of widespread pyritization of insects from the freshwater Jehol biota, and shows that pyritization is an important preservational pathway in lacustrine successions.

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