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Two recent databases show that the fossil record of insects is better than commonly supposed, comprising over 1000 families and 5000 genera. Insects (hexapods) first appeared in the Devonian, but by the Permian the number of orders present was similar to that of the present day. However, family data suggest an unsteady increase in diversity during the Phanerozoic, with four main peaks and troughs but no sudden change (supported by genera) at the Cretaceous/Tertiary transition. 相似文献
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