Animals that have hard body parts, such as bones or shells, are most likely to be preserved during fossilization. These rigid materials are more durable to the forces that cause fossilization; they are more likely to survive intact or to leave imprints that later become fossilized.
The conditions under which fossilization occurs are very specialized; only a fraction of a percent of the animals that have lived on Earth are fossilized. Typically, a fossil forms when animal remains are rapidly buried in materials that do not degrade the organic materials but instead maintain them in some form. Very dry regions preserve bone and other hard structures fairly well, and acidic peat bogs are ideal at pickling plant materials for millions of years.
Soft tissues, such as skin, cartilage, muscle or soft plant materials disappear rapidly, both through natural decomposition and as a result of scavenging. Consequently, the fossil record primarily records boned animals or shellfish. However, some conditions allow preservation of softer-tissued animals. Insects that become stuck in amber may be preserved, as the amber hardens and creates a protective environment. Deep freezing can also preserve soft tissue; the lack of oxygen and cold temperatures prevent the tissue from decomposing, leading to highly intact animal specimens even after millions of years. Mammoths and at least one ancient human have been found frozen in glacial ice because of this process.