REQUIREMENTS OF FOSSILIZATION
Although a large number of factors ultimately determine whether an organism will be fossilized, the three basic requirements are:
1. The organism should possess hard parts. These might be shell, bone, teeth, or the woody tissue of plants. However, under very favorable conditions of preservation it is possible for even such fragile material as an insect or a jellyfish to become fossilized.
2. The organic remains must escape immediate destruction after death. If the body parts of an organism are crushed, decayed, or badly weathered, this may result in the alteration or complete destruction of the [fossil] record of that particular organism.
3. Rapid burial in a material capable of retarding decomposition. The type of material burying the remains usually depends upon where the organism lived. The remains of marine animals are common as fossils because they fall to the sea floor after death, and here they are covered by soft muds which will be the shales and limestones of later geologic periods. The finer sediments are less likely to damage the remains, and certain fine-grained [Jurassic] limestones in Germany have faithfully preserved such delicate specimens as birds, insects, and jellyfishes.
Ash falling from nearby volcanoes has been known to cover entire forests, and some of these [fossil] forests have been found with the trees still standing and in an excellent state of preservation.
Quicksand and tar are also commonly responsible for the rapid burial of animals. The tar acts as a trap to capture the beasts and as an antiseptic to retard the decomposition of their hard parts. The Rancho La Brea tar pit at Los Angeles, California, is famous for the large number of [fossil] bones that have been recovered from it. These include such forms as the sabre-tooth cat, giant ground sloths, and other creatures that are now extinct. The remains of certain animals that lived during the Ice Ages have been incorporated into the ice or frozen ground, and some of these frozen remains are famous for their remarkable degree of preservation.