|Three concepts are important in the study and use of fossils: (1) Fossils represent the remains of once-living organisms. (2) Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth. (3) The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.|
|Stratigraphic ranges and origins of some major groups of animals and plants.|
|Scientists look for ancestors and descendants through geologic time. The fossil Archaeopteryx lithographica was a Jurassic animal with the skeleton of a reptile, including fingers with claws on the wings (solid arrows), backbone extending into the tail (open arrow), and teeth, but it was covered with feathers. We can see fossils of many other reptiles in rock of the same age and even older, but Archaeopteryx lithographica is the oldest known fossil to have feathers. We conclude that this animal is a link between reptiles and birds and that birds are descended from reptiles. The specimen is about 45 centimeters long. Photograph courtesy of the National Museum of Natural History, Smithsonian Institution.|
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.
A species is the most basic unit of classification for living things. This group
of fossil clams shows likely ancestor-descendant relationships at the species
level. These fossils from the Mid-Atlantic States show the way species can change
through time. Notice how the shape of the posterior (rear) end of these clams
becomes more rounded in the younger species, and the area where the two shells
are held together (ligamental cavity) gets larger. Paleontologists pay particular
attention to the shape of the shells and the details of the anatomy preserved as
markings on the shells.
Numbers in the left-hand column refer to the following geologic time segments: 1, Pliocene; 2, Miocene; 3, Oligocene; 4, Eocene; 5, Paleocene; 6, Late Cretaceous.
Figure courtesy of G. Lynn Wingard.
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 formed.
|Fossils can be used to recognize rocks of the same or different ages. The fossils in this figure are the remains of microscopic algae. The pictures shown were made with a scanning electron microscope and have been magnified about 250 times. In South Carolina, three species are found in a core of rock. In Virginia, only two of the species are found. We know from the species that do occur that the rock record from the early part of the middle Eocene is missing in Virginia. We also use these species to recognize rocks of the same ages (early Eocene and latter part of the middle Eocene) in both South Carolina and Virginia. The study of layered rocks and the fossils they contain is called biostratigraphy; the prefix bio is Greek and means life.|