After the dinosaurs died, if they were not eaten by other animals and were quickly buried in soft sandy soil, the abundant inorganic salts in groundwater — such as calcium carbonate, silica, pyrite, etc. — would chemically replace their bones. Over time, the uncorroded bones in the remains would be completely replaced by these inorganic salts, forming fossils.
However, the chances of meeting these conditions for fossil preservation are very rare. To form fossils, animal carcasses must be quickly buried without being damaged. Terrestrial animals like dinosaurs, after dying, are easily eaten by scavengers. Even if they are not eaten, their bodies are easily decomposed when exposed to the air, and their bones will be weathered and destroyed. On land, only when sandstorms, volcanic eruptions, etc. occur by chance, or when dinosaurs happen to die in environments with better burial conditions such as lakes, rivers, and swamps, and are quickly buried by the sedimentation of rivers and lakes, can fossils be formed. Therefore, only a very small number of dinosaur remains can be preserved as fossils.