What is a Fossil?

The image below is a photo of this exhibit poster.

Approximate dimensions: H:23 in W:18 in
Black Hills Institute of Geological Research poster 1991
Text by Peter L. Larson; Illustrations by Dorothy Sigler Norton, Science Graphics

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POSTER TEXT

WHAT IS A FOSSIL?

Classification
Once humans developed curiosity about their world, they began to name and classify the things they saw. Classification schemes use hierarchies which denote relationships of organisms to each other. Traditional classification recognizes five kingdoms for living organisms: Plantae, Animalia, Fungi, Protista, and Monera. Each kingdom is divided into phyla which are subdivided into classes, then orders, families, genera, and finally, species. Organisms are classified according to similarities and differences of form, to achieve a sense of order. Organisms are often grouped together on the basis of shared characters assumed to be derived from common origins. Classification systems are only attempts to explain the complexities and the possible relationships of living things.

Image captions: Kingdom Plantae, Kingdom Protista, Phylum Brachiopoda, Phylum Arthropoda, Class Crinoidea, Order Ammonoidea, Family Tyrannosauridae, Stomatolite, Hallucigenia sparsa

Time
The passage of time is recorded in the layers of sedimentary rocks. This stratigraphic record reveals the chronological sequence of events on Earth. When humans began studying the stratigraphic occurrence of fossils, they discovered that the record of life formed a world-wide pattern. This pattern has been used to construct a Geologic Time Scale that is divided on the basis of the first and last occurrence of organisms and points of mass extinction. Certain radioactive isotopes of elements are found in fossil-bearing sedimentary rocks and in igneous rocks found in contact with sedimentary rocks. Utilizing the analysis of these radioactive isotopes, which decay at known rates, scientists assign dates to positions in the stratigraphic record. Levels in sedimentary rocks are dated radiometrically in years, while fossils provide relative ages that may be radiometrically calibrated. Fossils are indispensable instruments for dating and correlating rock units.

Ancient Life
A fossil is any evidence of ancient life. This evidence may be the actual remains, such as bone, teeth, shells or plant tissues; chemically altered remains; or traces of the activities of extinct organisms, such as burrows and trackways. Paleontology is the study of fossils. Scientists who study fossils are paleontologists. Paleontologists use fossils to reconstruct ancient life. Paleontology is often confused with archaeology which is the study of ancient humans and their cultures. The focus of paleontology is the history of life on earth, from its beginnings approximately 3.6 billion years ago, up to the advent of human culture about 10,000 years ago. The study of fossils draws upon the sciences of geology and biology to place organisms in time, space and in relation to other forms of life. Except for rare occurrences, fossils are found only in sedimentary rocks.

Image caption: Archaeopteryx lithographica

Evolution
The fossil record reveals the appearance and disappearance of species that seem to have arisen suddenly, then remained unchanged for millions of years only to abruptly disappear. During the years since Darwin, there have been many changes in the way we view this record. The Cambrian fossil record documents the swift appearance, geologically speaking, of virtually every modern animal phylum as well as many now extinct. Apparently, there was a dramatic and early diversification of life. The fossil record shows the extinction of at least 25% of the phyla that existed in the Cambrian. We see, thereafter, a rise in diversity within animal phyla but no new phyla developing. Geologic time is punctuated by the disappearances or mass extinctions of vast numbers of organisms, possibly provoked by environmental catastrophes, followed by new diversification within existing phyla. The fossil record shows extinction as a major contributor to evolution. Perhaps, only when populations are under stress and when ecological niches open, do new organisms appear, filling gaps left by the demise of former species. It is possible that surviving major environmental change allows some organisms to diversify because others have perished.

Image caption: Bird Trackway; Pelecypod Borings; Crocodile Coprolite

Trace Fossils
Trace fossils are evidence of the activities of ancient organisms. Trackways of dinosaurs and trilobites, or the burrows of worms, clams and other organisms are often the only fossil evidence preserved. Trace fossils yield information about how organisms lived. Rarely is an organism preserved at the end of its trail. In the case of most trace fossils, we are able to identify the perpetrator only in general terms. Coprolites (fossil excrement) can reveal what animals ate. Groups of nests and multiple, parallel trackways indicate herding behavior for some dinsoaur species. Trace fossils can help us determine feeding habits, body size, locomotion, social behavior and other aspects of day to day existence. They are glimpses of the activities of extinct organisms.

Fossilization
Upon death, most plant and animal remains are soon digested by bacteria and other living things. Thus some of the organic elements of life are recycled to build new organisms. Rare and special circumstances are necessary for any parts or traces of an organism to be preserved as a fossil. Preservation requires that an organisms or its remains to be buried rapidly, be insulated from oxygen and decay-producing organisms, and remain buried and undisturbed. Under even the best of conditions, the preservation of soft parts is exceptionally rare. Usually only hard skeletal elements are preserved as fossils.

Image caption: Refrigeration (Mammoth); Dessication (Sloth Dung); Interment (Fish); Recrystallization (Pelecypod); Permineralization (Dinosaur Bone); Replacement (Wood); Distillation (Fern); Dissolution (Wood); Pseudomorphosis (Wood)

Fossilization Processes
Various processes may produce fossilization. The longer a fossil lies buried in the earth, the more changes it may undergo. Therefore, many fossils are preserved by a combination of two or more of the following processes. Refrigeration: Animals and plants may be preserved frozen in permafrost. Desiccation: Animals that die in dry caves may dehydrate and be preserved for thousands of years. Interment: Most fossils are preserved as a result of being covered by sand, mud or volcanic ash. Recrystallization: Some mollusk and echinoderm exoskeletons undergo a crystal structure modification. Permineralization: Wood and bone are often preserved with the orginal cellular spaces filled with minerals. Replacement: After cellular spaces have been filled, cell walls often dissolve and are replaced with minerals leaving visible structure but little or no organic material. Distillation: Fragile organisms may be preserved as carbon films when volatile chemicals dissipate leaving residual, organic material. Dissolution: When buried remains are completely dissolved, a void may be left in the rock which is a mold of the original organism. Pseudomorphosis: If a mold fills with minerals, the result is a cast of the organism with no internal structure or organic material preserved.

Value
Fossils have been collected, admired and pondered since humans assumed the power to reason. Today, fossils are used by geologists to correlate and date sedimentary rocks. Petroleum geologists use fossils in the search to locate oil reservoirs beneath the earth's surface. To the biologist, fossils provide insight into the classification of living species and the evolution of anatomical features. Fossils are the primary interest of the paleontologist. Fossils are utilized in studying exteinctions, evolution and plate tectoncis. Individuals, schools and museums use fossils to teach geology, biology, ecology, astronomy and evolution. People collect and enjoy fossils as objects of curiosity, beauty and inspiration. Fossils are windows to the past. Through them we see the continual metamorphosis of our planet. Fossils are the ancient ones and they have much to teach us if we are willing to learn.