Say the word “fossil” and most people immediately think of dinosaur bones on display in a museum. While these are indeed fossils, the term is considerably more inclusive. A fossil is the preserved remains of an ancient organism from the geologic past. There are many different kinds of fossils and the science of studying fossils and how they form is called paleontology.
How a Fossil Forms
It is very unlikely for any given individual organism to become a fossil. Upon death, the bodies of most plants and animals are left exposed to the elements and scavengers which scatter and destroy most of the remains. As a result, the fossil record is biased to larger organisms with harder body parts. In order to become fossilized, an organism must be buried rapidly after death. Mineral-laden ground water form the surrounding sediment seeps into the hollow spaces and begins replacing organic tissues with nonliving crystal. The process is called permineralization. Sometimes to original parts decay and leave a hollow space known as an external mold. If minerals later fill this hole it becomes a cast. Fossils of this variety generally contain none of any of the original organic material. Remains that are younger than 10,00 years, however, often do contain organic material and are known as subfossils. Subfossils form under conditions where true fossilization is not possible. Often they are found in protected sites such as caves and can be preserved for many thousands of years.
Where to Find Fossils
Not all environments are conducive for the creation of fossils. For example, forests generally have soil that is too acidic for the preservation of bones so it is rare to find fossils there. Deserts often leave remains unburied to be eroded away by wind and sand . Many fossil are found in river or marine deposits, as the current helps to cover exposed remains with mud while carrying smaller pieces away. Anoxic waters are particularly good an preserving soft tissue remains due to the lack of scavenging microorganisms.
Types of Fossils
The most well-known type of fossil is the body fossils. Body fossils are the physical remains of ancient organisms. The fossilization process is biases towards the harder, more durable parts such as bones, teeth, and shells. Under exceptional circumstances, it possible to get soft tissue preserved like skin, hair, and even pigment cells. It is very rare to find a complete animal, as smaller bones get lost or eaten, and rarer still to find the bones articulated as they were in life.
Fossils used to identify specific geologic time periods are called index fossils. Certain species are only found in certain time periods, so even if a paleontologist is digging in different sediments and they find the same fossil it is safe to conclude that the sediments are the same specified age. Index fossils are usually common and easy to recognize. Trilobites and ammonites are two well-known varieties of index fossil.
Not all fossils are direct remains of organisms. Sometimes the fossil record yields evidence of their behavior. This can come in the form of footprints, burrows, and toothmarks. These are called trace fossils, or sometimes ichnofossils. Unlike with body fossils, it is usually impossible to determine which exact species made which trace fossil. To make up for this deficit, trace fossils are categorized and named by what they are. Apart from footprints, the best known trace fossils are coprolites. These are fossilized feces and tell us not only about an animal’s diet but the stricture of its environment as well.
Living organisms leave more evidence of their presence that just physical remains or alterations in their environment. They secrete unique substances that serve as direct evidence for their presence. These chemical traces are called biosignatures. The study of biosignatures is being used by astrobiologists in search for life on other planets.