具有坚硬部分，如壳、骨、牙或木质组织的硬体生物，在死亡后没有被压碎或腐烂、风化，被某种能够阻碍分解的物质迅速埋藏起来，经过长期埋藏并发生石化作用，最终形成化石。东兰国家地质公园有珍贵的化石遗迹-兰木化石群，菊石作为最典型的代表，生存于泥盆纪至白垩纪，是已绝灭的海生无脊椎动物化石。

After the death of those creatures which have hard part, such as shell, bone or woody tissue, instead of being crushed, decaying or weathering, they were quickly buried by some materials that prevent decomposition . After a long period of burial and lithification, there eventually became the fossils. There are precious fossils in Donglan National Geopark, such as Lanmu fossil group, among which ammonite is the most representative one, which lived from Devonian to Cretaceous and is a kind of extinct Marine invertebrate fossil.