Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. The study of fossils is called paleontology. There are several different types of fossils and fossilization processes. In order for an organism to be fossilized, the remains are typically covered by sediment very quickly. However, an organism may also become a fossil by becoming petrified or remaining in an oxygen-deprived environment, such as at the bottom of a lake. The oldest known fossils are approximately 3.5 billion years old.
