Most fossils are thick shells or skeletons.
Most of the Earth’s surface is covered by a type of rock called sedimentary, because it forms from layers of sediment that accumulate. One of the characteristics of sedimentary rocks is the large number of fossils they contain. All kinds of extinct plants and animals are preserved in the sand, silt or mud of ancient times.
When wood petrifies, the cellulose is replaced by minerals.
Over time, animal carcasses on sedimentary rock surfaces become covered in silt. Bones undergo mineralization, whereby minerals replace the original organic compounds to leave a cast of the body. Over time, the body is completely replaced by long-lasting minerals. In the petrifaction of wood, cellulosic and woody fibers are replaced by minerals such as silica.
Earth’s sedimentary rocks contain a relatively large number of fossils that formed from plants, animals, and marine life preserved long ago in sand, silt, or mud.
Even delicate features are sometimes preserved in fossils. The eyes of flies and the delicate wings of butterflies have been found preserved in fossils. However, most fossils are thick shells or skeletons. Animals with enduring fossils are also some of the most studied and understood.
It is surprising that something as soft and subtle as sand can become as rigid as a fossil. The reason lies in the pressurization that the sediments undergo as they are buried deeper and deeper. For every 100 feet (31 meters) of depth, the average temperature increases about 1°C. The average pressure increases about 1 ppsi (pound per square inch), or about 7 kilopascals, for every foot (31 centimeters) of depth. Increased pressure and temperature lead to a process called compaction, in which delicate sediment particles stick together and form rocks.
Sedimentary rocks often contain a large number of fossils due to the way their layers build up over time.
The mud is purified from the water during the compaction process, turning into rock. Sometimes the increased heat causes the material to undergo chemical transformations, called cementations, which cause minerals such as calcite, silica, and iron oxide to form films on the sediment surface. This is how fossils are made. The minerals also occupy air pockets between the sand molecules.
Fossils can also preserve things other than real bodies. Tracks, tracks, trails and burrows were found. Since the charcoal itself is compacted vegetation, many fossils can be found inside it.