What it a fossil?
The word fossil is derived from the Latin fossilis meaning an object that has been dug up from the ground. Fossils are the evidence for the existence of once-living animals and plants and may be either the preserved remains of an organism or evidence of its activity.
Types of fossils
Trace fossils
Trace fossils are the preserved impressions of biological activity. They provide indirect evidence for the existence of past life. They are the only direct indicators of fossil behaviour. As trace fossils are usually preserved in situ they are very good indicators of the past sedimentary environment. Trace fossils made by trilobites have provided an insight into trilobite life habits, in particular walking, feeding, burrowing, and mating behaviour.
Chemical fossils
When some organisms decompose they leave a characteristic chemical signature. Such chemical traces provide indirect evidence for the existence of past life. For example, when plants decompose their chlorophyll breaks down into distinctive, stable organic molecules. Such molecules are known from rocks more than 2 billion years old and indicate the presence of very early plants.
Coprolites
Coprolites are fossilized animal feces. They may be considered as a form of trace fossil recording the activity of an organism. In some coprolites recognizable parts of plants and animals are preserved, providing information about feeding habits and the presence of coexisting organisms.
Body fossils
Body fossils are the remains of living organisms and are direct evidence of past life. Usually only hard tissues are preserved, for example shells, bones, or carapaces. In particular environmental conditions the soft tissues may fossilize but this is generally a rare occurrence. Most body fossils are the remains of animals that have died, but death is not a prerequisite, since some body fossils represent parts of an animal that are shed during its lifetime. For example, trilobites shed their exoskeleton as they grow and these molts may be preserved in the fossil record.