Chris Ohman, age 13, of Tulsa, Okla., for his question:
HOW WERE FOSSILS FORMED?
Oldest fossils known to scientists are some one celled plants that were alive more than 3.1 billion years ago. Next oldest fossils have been traced back to about 600 million years ago. These were a number of animals with hard skeletons that were found with fossils of plants and shells that had been imprinted in layers of rock.
Plants and animals that lived in dry areas left fewer fossils than those that lived in damp places or in water. Dissolved minerals in the water helped to form fossils..
A fossil is the remains or a record of an animal or plant that lived in the past. A fossil could be a whole animal that was preserved in ice, a footprint, a bone, a tooth or even an imprint of a leaf.
Fossils can tell scientists a great deal about life on our planet millions of years before man existed.
Chief kinds of fossils are petrified fossils, molds and casts, prints and whole animals and plants.
A petrified fossil is one of an animal or plant that has turned to stone. The remains could have been petrified in three ways: replacement, permineralization or carbonization. In replacement, water dissolves away the original substance of the plant or animal. As the substance dissolves, minerals replace it. In permineralization, minerals fill in the small air spaces in bones or shells without changing the original shape of the object. The actual bone or shell remains, strengthened by the minerals. In carbonization, leaves or soft parts of animals turn to carbon. Other chemicals escape, leaving a record of the shape of the creature as a thin film of carbon.
Molds and casts are formed when living things become buried in mud or clay. As the bodies dissolve away, an opening within the hard material leaves a mold of the original.
Prints are preserved when the soft mud in which they are made hardens into stone. Prints preserve details of the skin of some dinosaurs and the veins and pores of leaves.
Whole animals and plants are rarely preserved. Most fossils consist only of shells, teeth, bones and other hard parts. In a few cases, whole animals are preserved in tar or ice.
Almost all fossils are found in sedimentary rocks, or rocks that have been built up in layers from small particles. These rocks lie beneath about three fourths of the land surface of the earth. Fossils from different times in the earth's history are found in different places.
Age of a fossil can be determined by measuring the amount of certain chemicals in the fossils.