Specimens are usually considered to be fossils if they are over 10,000 years old.The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.
The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater.
Minerals precipitate from the groundwater, occupying the empty spaces.
The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
There are many processes that lead to fossilization, including permineralization, casts and molds, authigenic mineralization, replacement and recrystalization, adpression, carbonization, and bioimmuration.