How Are Fossils Formed?
One way is “permineralizaation”
To start with, an animal or plant must die in water or near enough to fall in shortly after death. The water insulates the remains from many of the elements that contribute to decomposition.
As time passes, land and mudslides, river delta sediments bury the exoskeleton.
As the sediments continue to pile on, the lower layers become compacted by the weight of the layers on top. Over time, this pressure turns the sediments into rock. In time, the entire shell is replaced leaving rock in the exact shape of the original life form.
As the continental plates move around the earth, crashing into each other, mountains are formed. Former sea floors are lifted up and become dry land. Now a fully formed fossil is buried under hundreds or even thousands of feet of rock! Thanks to the movement of the plates, the fossil will come closer to the surface and nearer to discovery.
Rain, wind, earthquakes, freeze and thaw all work toward erosion. The mountains that were built up are worn away over time. The fossil sees the light of day!
There are other ways that organisms can turn into fossils, including:
unaltered preservation (like insects or plant parts trapped in amber, a hardened form of tree sap)
replacement (An organism’s hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron)
carbonization=coalification (in which only the carbon remains in the specimen – other elements, like hydrogen, oxygen,
and nitrogen are removed)
recrystalization (hard parts either revert to more stable minerals or small crystals turn into larger crystals)
authigenic preservation (molds and casts of organisms that have been destroyed or dissolved).