Winston Hedge, age 14, of Harrisburg, Pa., for his question:
HOW DOES A FOSSIL FORM?
A fossil is the remains or the outline of an animal or plant that lived in the past. These remains, or their impressions, are often preserved in the earth's cruet. Although fossils can be formed in several ways, most of them are petrified, or turned into stone, by the process of mineralization.
Hard parts of an animal or plant, such as the bones or wood, are replaced by minerals when water, with minerals dissolved in it, infiltrates the remains. The water fills the bones, pores and all open spaces. As the remains decompose, the water evaporates and the minerals harden.
At times there is only a general outline or t:ace of the organism that has been fossilized. Shells, leaves and insects that fell into moist mud or clay often left imprints when the mud or clay hardened into rock. Footprints of animals have also been found preserved in this way.
Sometimes bones and other hard parts fossilize when the original material substance that composed them are not destroyed. Shells, skeletal parts of echinoderms, corals and bones of mammals have been found unchanged from their original forms. Fossils of this type may be formed when plants or animals get stuck in mud or other sticky material.
The famous La Brea Tar Pits in Los Angeles have provided scientists with another type of fossils. Long ago many animals in California, including bison, walked into ponds of tar and were trapped. Saber toothed tigers and vultures came to eat them but they were also trapped.
Slowly the animals sank into the tar and were completely covered. The bones of these creatures were preserved and turned into fossils.
Many fossils were also formed when layers of mud on the ocean bottom collected on top of shells and the skeletons of various ma:ins animals. As time passed, layer upon layer accumulated and these layers formed sedimentary rock, which enclosed the fossilized remains.
Sometimes, but only rarely, animals have been preserved completely.
Animals that have been preserved in their entirety were enclosed in some material, such as ice or frozen soil, which prevented decay. We have found some good examples of this type of fossil in the mammoths uncovered in Alaska and Siberia.
Many of the complete animal fossils have skin, hair, flesh and even stomach contents preserved.
Insects and flowering plants have also been nearly perfectly preserved when they were covered over by sap from trees. The sap hardened to become amber, trapping the undamaged flower oz insect for millions of years.
Paleontology is the name of the science that studies fossils of ancient life. The paleontologist is able to tell us a great deal about how conditions were on Earth when the fossils were formed.