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.