Cast of a fossil of Coelophysis.
Image: Frank Coffa
Source: Museum Victoria
Question: How are fossils formed?
Answer: In simple terms, fossils are traces of dead animals preserved as a geological feature. Fossils can form in a number of ways. One of the better-known processes of fossil formation is when a dead animal or plant is covered by sediments and the hard tissues (such as bones) are slowly replaced by minerals. These sorts of fossils are usually formed in areas that once had water near them such as rivers or creeks; these carry sediment like sand and mud which cover the remains of the dead organism, and the speed of the flow of the water determines how quickly the remains become buried. This burial prevents the remains from being exposed to weathering or other factors that limit the process of fossilisation.
Most fossils that are found are the fossilised bones or exoskeletons of animals and the more solid parts of plants. This is because the soft parts of the animal’s body, or the plant, decompose quickly and there is not enough time for the minerals that replace the bone to replace the tissues.
It is important to note that fossils do not have to the remains of an animal; they can also be the evidence of a creature’s activity. These are known “trace fossils”. Some common trace fossils include animal tracks and burrows and even coprolites (fossilised ‘poo’). These types of fossils can give us clues on other aspects of how extinct organisms lived, telling us how fast they may have moved or what they ate.