Fossils are found in sedimentary rocks. Usually a fossil is a preserved specimen of some living organism. Therefore the assumption is that the organism was buried in some sedimentary environment. The development of the sedimentary formation then affects the fossil development. If the sediment has the proper chemical conditions the original life form is replaced, atom by atom, with a new element. For example the carbon atoms are replaced by silica and the resulting life form has all the original shape and structure, but the chemical composition is altered. I have seen tree branches that extended through severall layers of coal. The structure was maintained, but the composition varied, one section being carbon and the two adjacent sections were sandstone. The branch structure remains clearly identifiable continuously throughout the structure. If the development of the sediment is harsh, we find fossil fragments. Not all sedimentary rock formations contain fossils, but those that do suggest information about the history of the development, and occassionally indicate information about the environmental conditions during the deposition of the sediment.