Many of the organisms that had existed on the earth left no fossils because most of them can not meet the conditions that must be fulfilled before fossils can form. For fossils formation to occur, the organism soft tissue must decay leaving behind the hard part [majorly bone] in the sediment. Overtime, layers of sediment have to build up and pressed down on the buried remains. As time goes on, water and mineral seeped into the remains and form crystals.