The shape of an ancient organism found in a rock.
A fossil is a trace of a living organism that lived in prehistoric times. Although we often find fossils when we dig down through layers of rock, fossil formation is actually quite rare. The fossilization process can only occur in certain conditions, such as when volcanic ash traps an organism or when rock formation captures hard body parts like bones or shells.
Effects
Fossils form when natural processes like rock formation and sedimentation preserve organisms or their shapes. But the actual bones and remnants of the bodies of ancient living organisms make up just one kind of fossil. Sometimes natural processes replace the bodies of living organisms with inorganic materials, leaving just their shape.
Types
Petrification fossils form in one of two ways, according to Bellarmine University. The first way, perimineralization, happens when ground water passes through the organic material of the body, leaving behind minerals in cellular spaces. The second way, replacement, occurs when ground water dissolves the organic material of the body and entirely replaces it with minerals. According to Georgia Perimeter College, scientists also consider footprints, tracks, dwellings and trails to be a type of fossil. They call these trace fossils.
Function
Geologists use fossils in two ways, according to Indiana University. The first is biostratigraphic correlation. This method uses what we know about the evolutionary development of plants and animals to date the rock layers in different regions. For example, if a geologist wants to know the relative age of two rock layers that are distant from each other, the geologist can compare the fossils found in one rock layer to those found in the other. If the geologist finds that the fossils in both are at a similar stage in evolutionary development, then it is clear that the two rock layers formed during the same era.
Features
The second way geologists use fossils, according to Indiana University, is to perform environmental interpretation. Since certain ancient organisms either could only live in fresh water or could only live in ocean water, their presence in a rock layer reveals how the rock layer formed. For example, if a geologist wants to know whether a layer of sandstone formed on an ocean bed or ended up there long after it formed in fresh water, the geologist can look at the type of fossils in the sandstone for an answer.
Expert Insight
Over the years, geologists have formed a set of rules which help them use the fossils they find to make determinations about the geological origins of the nearby region. As Indiana University notes, these rules are not always 100 percent accurate, but they are helpful. One rule says that finding many coral fossils means that the rock formed in shallow water. Another rule says a rock with both plant and animal fossils (other than fish) likely formed in fresh water.
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