The best-preserved fossils form when the organism is quickly buried. If the organism is not buried quickly, external factors such as scavengers, decay and the weather can deteriorate the quality of the fossil to be. The right conditions for fossilization are exceptional. In most cases an organism will not fossilize. If the organism is buried in an anoxic environment, even the soft parts can be preserved. Mostly only the harder parts, such as shells or skeletons, are fossilized.

The fast burial of organisms occurs mainly in sediments of watery environments (i.e. clay, sand in rivers and oceans). Fossils of land animals are more rare. On land you need for example a volcanic eruption or flashflood to bury the organism fast enough with sediment.

After burial it depends on the chemical composition of the sediment whether the organism is well preserved or not. In the sediment chemical and mineralogical composition of the fossil can be altered. The internal structure remains preserved.
Otherwise the fossil can be dissolved in water. In this instance the cavity can later be filled up by another mineral. In this case the internal structure is lost.

After fossilization the fossil is buried deeply in the earth. Geological processes such as volcanism and mountain building the fossil can come to the surface by erosion of the overlying layers. Then we can find the fossil at the surface.