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Geology : Paleontology
If we begin at the present and examine older and older layers of rock, we will come to a level where no fossils of humans are present. If we continue backwards in time, we will successively come to levels where no fossils of flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates, no land plants, no fishes, no shells, and no animals. The three concepts are summarized in the general principle called the Law of Fossil Succession: The kinds of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.
How do scientists explain the changes in life forms, which are obvious in the record of fossils in rocks? Early explanations were built around the idea of successive natural disasters or catastrophes that periodically destroyed life. After each catastrophe, life began anew. In the mid-nineteenth century, both Charles Darwin and Alfred Wallace proposed that older species of life give rise to younger ones. According to Darwin, this change or evolution is caused by four processes: variation, over-reproduction, competition, and survival of those best adapted to the environment in which they live. Darwin's theory accounts for all of the diversity of life, both living and fossil. His explanation gave scientific meaning to the observed succession of once-living species seen as fossils in the record of Earth's history preserved in the rocks.
Scientific theories are continually being corrected and improved, because
theory must always account for known facts and observations. Therefore, as new
knowledge is gained, a theory may change. Application of theory allows us to
develop new plants that resist disease, to transplant kidneys, to find oil, and
to establish the age of our Earth. Darwin's theory of evolution has been refined
and modified continuously as new information has accumulated. All of the new
information has supported Darwin's basic concept--that living beings have
changed through time and older species are ancestors of younger ones.
The Law of Fossil Succession is very important to geologists who need to know the ages of the rocks they are studying. The fossils present in a rock exposure or in a core hole can be used to determine the ages of rocks very precisely. Detailed studies of many rocks from many places reveal that some fossils have a short, well-known time of existence. These useful fossils are called index fossils.
Today the animals and plants that live in the ocean are very different from those that live on land, and the animals and plants that live in one part of the ocean or on one part of the land are very different from those in other parts. Similarly, fossil animals and plants from different environments are different. It becomes a challenge to recognize rocks of the same age when one rock was deposited on land and another was deposited in the deep ocean. Scientists must study the fossils from a variety of environments to build a complete picture of the animals and plants that were living at a particular time in the past.
The study of fossils and the rocks that contain them occurs both out of doors
and in the laboratory. The field work can take place anywhere in the world. In
the laboratory, rock saws, dental drills, pneumatic chisels, inorganic and
organic acids, and other mechanical and chemical procedures may be used to
prepare samples for study. Preparation may take days, weeks, or months--large
dinosaurs may take years to prepare. Once the fossils are freed from the rock,
they can be studied and interpreted. In addition, the rock itself provides much
useful information about the environment in which it and the fossils were