What are the typical conditions necessary for fossii formation?
The body needs to be cut off from oxygen and covered in fine sediment quickly after death, with fairly calm, unchanging thermal and mechanical conditions.
Fossil formation rarely takes place on land because most of the geological processes involve erosion more than deposition, which means things are taken away rather than piled on.
Fossils are mostly made in the ocean where deposition takes place on the ocean floor.
The body also can't decompose, as the corpse has to make a strong imprint on the rock, and decomposition would weaken its structure. Oxygen is the main catalyst for decomposition, and water is mostly oxygen, which means the organism, once dead, is covered by a layer of fine mud through which water can't easily seep. Water can travel through sand very quickly and so it is useless for fossilisation.
After being covered by mud, layers of sediment are deposited and build up over millions of years, weighing down on the organism, with the heat and pressure forming sedimentary rock and the imprint of the animal remaining as a fossil.
Palaeontologists think that the fossil record is biased, because certain species have no opportunity for lack of oxygen or have no hard skeletons to make an imprint, like jellyfish, and so they have almost no representation in the record.
Fossilisation on land can take place in volcanic ash, which fossilises well because it is so fine and can record minor details, while coarse gravels are not so fluid.
Constant temperature also helps in fossilisation so there is no thermal disruption.