Melvin Colbert. age 13, of Annapolis, Md. for his question:
HOW IS A FOSSIL PRESERVED?
A fossil is a record or remains of a plant or animal that lived in the past. Fossils may be preserved in several ways. The four chief kinds of fossils are petrified fossils, molds and casts, prints and whole animals and plants.
Petrified fossils are the remains of plants and animals that have been turned to stone. The remains may be petrified in three main ways.
(1) Many fossils are formed by replacement. Water dissolves away the original substance of the plant or animal. As the substance dissolves, minerals replace it.
(2) In permineralization, minerals fill in the small air spaces in bones or shells with changing the original shape of the object. The actual bone or shell remains, strengthened by the minerals.
(3) In carbonization, leaves or the soft parts of animals turn to carbon. Other chemicals escape, leaving a record of the shape of the plant or animal as a thin film of carbon.
Living things sometimes become buried in mud, clay or other material that hardens around them. Later, the bodies dissolve away, leaving openings within the hard material that are natural molds of the original. Scientists fill these molds with wax, plaster or plastic.
Prints may be molds of thin objects, such as leaves or feathers, or they may be tracks or footprints left by extinct animals. Prints are preserved when the soft mud in which they are made hardens into stone.
Whole animals and plants are rarely preserved. Most fossils consist only of shells, teeth, bones and other hard parts. Flesh and other soft parts almost always decay too quickly to be preserved.
The earlist fossil egg known came from rocks in Texas that are probably 270 million years old. It is of an early mammal like reptile.
The famous dinosaur eggs found by the explorer Roy Chapman Andrews in 1923 came from 135 million year old rocks in Mongolia.
The oldest and earliest forms of life were simple, one celled creatures. The oldest fossils known to science are algae and bacteria, or one celled plants, that were alive over 3.1 billion years ago.
The next oldest fossils are plants and animals that had developed hard skeletons and shells that have left good records of their remains in rocks. These fossils have been determined to be about 600 million years old.
Scientists have found deposits of carbon more than 2.7 billion years old. They believe plants formed this carbon, but they cannot identify the plants.
Plants and animals that live in dry areas leave fewer fossils than those that live in damp places or in water. Dissolved minerals in the water help form fossils.