Fossils are the visible evidence of life in past geologic periods, and those who study fossils are known as paleontologists. For some reason, these words in the past have brought to mind the notion of a dry, lifeless, and very complex science. True, paleontology is complex, but it is far from lifeless. It is a part of the earth's history which is far less abstract than the physics of this planet's earliest origins or the chemistry of what scientists believe to be the prelude to life.

  Paleontology can be considered a biological science married to and inseparable from a geologic counterpart. In order to discuss fossils, or even to understand them in the context of the geologic ages in which they lived, it becomes necessary to familiarize oneself with the geologic column and the geologic time scale.

The geologic column is to the geologist what a sequence of pages are to an author; just as pages of a story progress logically from the earliest events to the climax, so do sedimentary rocks deposited on the earth's surface. Arizona's geologic column includes rocks from the early Precambrian period to the Pleistocene Ice Ages, with several pages or chapters removed by erosion, or simply never printed (deposited) in this region. Geologists rely on previously determined geologic columns and the index fossils (fossils specific to one and only one age) to determine where in the overall rock unit they are exploring.

 The Geological Column can tell you a lot if you know what you're looking at

The geologic time scale
is composed of a sequence of artificial time units which coincide with the rock units of the geologic column, and represent the various ages of the earth's history. The time scale, worked out through various physical dating methods, is an invaluable tool for paleontologists who must communicate their research, but because of its artificial nature and because the science of geologic dating is constantly being refined, there exists no time scale which is truly correct.

The largest unit of geologic time is called an era. Each era is divided into periods, and each period into epochs. All of these units represent a dramatic change in lifeforms from those of the preceding unit, and have been given names which describe the characteristic stage of biological development. For example, Paleozoic means ancient life, a name derived from the simple or ancient stage of development of lifeforms in that era.

Those geologists from the 1800s
were smarter than we thought

Periods are smaller divisions of time which constitute the eras: each period derives its name from the geographical area in which it was first studied. Some Paleozoic examples are:

• Permian-from the Perm Province of Russia

• Pennsylvanian- from the American state of Pennsylvania

Still smaller are the units called formations. Formations are rock units which generally share common environments of deposition, and are relatively contemporaneous in age and are similar in mineral content. Some examples of Arizona formations are the Shinarump Formation, the Chinle Formation (named from the Navajo community of Chinle), and the Redwall Limestone (its name derived from its characteristic red cliff-forming sandstone).

 Boundaries exist between divisions of the geologic time scale not only because of paleontological differences, but also because of radical changes in geology. There are limitations to the usefulness of this scale, which become obvious when one realizes that geologic events did not always take place simultaneously with events in other geographical areas, and that animals may have evolved at different rates in different places.

Lifeforms existing on the boundaries between time periods often have many characteristics of animals in both units, this occasionally causes some confusion. However, for the sake of convenience, we have retained this system of time and rock classification and must work around the inherent problems of its artificial