Fossils are the remains and traces (such as footprints or other marks) of ancient plant and animal life that are more than 10,000 years old. They range in size from microscopic structures to dinosaur skeletons and complete bodies of enormous animals. Skeletons of extinct species of humans are also considered fossils. An environment favorable to the growth and later preservation of organisms is required for the occurrence of fossils. Two conditions are almost always present: (1) The possession of hard parts, either internal or external, such as bones, teeth, skulls, shells, and wood; these parts remain after the rest of the organism has decayed. Organisms that lack hard parts, such as worms and jellyfish, have left few geologic records. (2) Quick burial of the dead organism, so that protection is afforded against weathering, bacterial action, etc. Nature provides many situations in which the remains of animals and plants are protected against destruction. Of these, marine sediment is by far the most important environment for the preservation of fossils, owing to the incredible richness of marine life. The beds of former lakes are also productive sources of fossils. The rapidly accumulating sediments in the channels, floodplains, and deltas of streams bury fresh-water organisms, along with land plants and animals that fall into the water. The beautifully preserved fossil fish from the Green River oil shale of Wyoming in the western United States lived in a vast shallow lake. The arctic ground in the far north acts as a remarkable preservative for animal fossils. The woolly mammoth, a long-haired mammal, and other mammals have been periodically exposed in the area of Siberia, the hair and red flesh still frozen in cold storage. Volcanoes often provide environments favorable to fossil preservation. Extensive falls of volcanic ash and coarser particles overwhelm and bury all forms of life, from flying insects to great trees. Caves have preserved the bones of many animals that died in them and were subsequently buried under a blanket of clay or a cover of dripstone. Predatory animals and early humans alike sought shelter in caves and brought food to them to be eaten, leaving bones that have been discovered.
单选题
Which of the following is true of the environments in which fossils are found?