Paleontology is the branch of science devoted to the understanding of past life as revealed by the fossil record. Normally, when an organism dies, its physical remains are scattered and destroyed by the elements in a short span of time. Such elements include not only wind, weather, and decay, but also water and the activities of carnivores and scavengers of many kinds. Occasionally, however, bony remains (and, very exceptionally, some soft tissues) lying on the surface or in superficial cavities may be covered by accumulating sediments (most often river or lake muds in the case of terrestrial organisms, and sea-bottom particulates in that of marine forms) before they are totally destroyed.
Among the remains that escape destruction, complete articulated skeletons are extremely rare; more commonly preserved are individual bones and teeth, often broken. Unless the enclosing sediments are chemically hostile, or become melted by heat from the Earth’s interior or pressure from above, bones thus incorporated into the accumulating sediment pile can survive more or less indefinitely, though their distortion due to local Earth movements is not uncommon.
As water carries minerals in solution through both the sediments and the contained animal or plant remains, the organic constituents of those remnants become replaced by minerals, in a process most often known as mineralization. If erosion of the enclosing sediment pile subsequently sets in, the now fossilized remains may become exposed once more at the Earth’s surface, where they are yet again subject to the forces of natural destruction.
However, for a brief period they are also available to be collected by human beings, who have been picking up unusual re-exposed objects for the last few hundred thousand years, at least. Fossils are found all over the world, and are a vast storehouse of information about past life. Those who professionally find fossils and extract such information from them are known as paleontologists.
From the very beginning, paleontology has been integral to the study of Earth history, which began with attempts to order the sedimentary rocks, laid down by water and wind, which contain fossils. The other grand categories of rocks exposed on the Earth’s surface include igneous rocks (extruded from Earth’s molten core by volcanoes and by the physical rising through the solid crust of lighter rocks such as granite), and metamorphic rocks, which are sedimentary rocks that have been re crystallized by pressure and heat.
Volcanic rocks are particularly helpful in dating the ages of various events in Earth history because reliable techniques exist by which to measure the time that has elapsed since they last cooled. These techniques depend on the phenomenon of radioactivity, by which unstable forms of certain elements “decay” to stable states at known rates. Volcanic rocks do not contain fossils, but they represent single points in time and are often interleaved among sedimentary fossil-bearing layers, which can be dated by reference to them.