A fossil is any evidence of past life. Fossils are as diverse as the living things that left them behind, and can range from microscopic plant pollen to giant dinosaurs. There are 2 general categories of fossils:
- Body fossils are the actual remains of dead organisms, such as bones, teeth, shells and leaves.
- Trace fossils are preserved records of the activities of organisms, and include things like footprints, burrows, or dung
How Fossils Form
Most organisms die and decay, leaving no remains behind, but in rare circumstances a dead organism can become fossilized. Fossils are primarily found in sedimentary rock, which is formed on the Earth's surface. Layers of sedimentary rock are formed by accumulating particles like sand or silt. Sometimes, plants and animals become buried in this sediment, which protects them from scavengers, organic decay, or weathering. Once buried, water carrying dissolved minerals may flow through the remains, leading them to be preserved at the cellular level.
The chances against the remains of an organism becoming a fossil are so great, it is a wonder that any fossils are found at all. Some environments, however, are more likely to produce fossils than others. For example, 115 million years ago Dinosaur Park was the site of an oxbow, meaning a sharp bend in a river. Like a blocked drain, the oxbow created a natural trap that would snag debris flowing downsteam, including dead plants and animals. These remains were rapidly buried in the silt that continuously flowed. down the river, ensuring the abundance of preserved plants and animals found at the Park today.
How We Know the Age of Fossils
There are 2 different methods to determine the age of fossils. Scientists can learn the relative age of fossils using simple but important principles of geology. For example, if one layer of rock is found on top of another, then the lower layer must have formed first. That means that fossils found in the lower layer are older than fossils found in the higher layer.
Scientists can also determine the absolute age of fossils using an accurate and reliable method called radiometric dating. Certain types of rocks contain naturally-occurring radioactive isotopes, which decay, or change, at known rates. By comparing the abundance of isotopes with their decay products within a rock sample, scientists can determine how much time has passed since that rock was formed. While only certain types of rock contain radioactive isotopes, radiometric dating is used in tandem with relative dating methods to determine the numerical age of virtually any rock formation, and by extension, any included fossils.