Skeleton of the bird-like specimen (Aurornis xui) found in Yizhou Fossil & Geology Park, China.
Credit: Thierry Hubin/IRSNB
When animals, plants and other organisms die, they typically decay completely. But sometimes, when the conditions are just right, they're preserved as fossils.
Several different physical and chemical processes create fossils, according to the New York State Geological Survey.
Freezing, drying and encasement, such as in tar or resin, can create whole-body fossils that preserve bodily tissues. These fossils represent the organisms as they were when living, but they're very rare.
Most organisms become fossils when they're changed through various other means.
The heat and pressure from being buried in sediment can sometimes cause the tissues of organisms — including plant leaves and the soft body parts of fish, reptiles and marine invertebrates — to release hydrogen and oxygen, leaving behind a residue of carbon.
This process — which is called carbonization, or distillation — yields a detailed carbon impression of the dead organism in sedimentary rock.
The most common method of fossilization is called permineralization, or petrification. After an organism's soft tissues decay in sediment, the hard parts — particularly the bones — are left behind.
Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock.
In another fossilization process, called replacement, the minerals in groundwater replace the minerals that make up the bodily remains after the water completely dissolves the original hard parts of the organism.
Fossils also form from molds and casts. If an organism completely dissolves in sedimentary rock, it can leave an impression of its exterior in the rock, called an external mold. If that mold gets filled with other minerals, it becomes a cast.
An internal mold forms when sediments or minerals fill the internal cavity of an organism, such as a shell or skull, and the remains dissolve.