What is a fossil?
Fossils are the preserved remains of plants and animals whose bodies were buried in sediments, such as sand and mud, under ancient seas, lakes and rivers. Fossils also include any preserved trace of life that is typically more than 10 000 years old. Soft body parts decay soon after death, but the hard parts, such as bones, shells and teeth can be replaced by minerals that harden to rock. In very exceptional cases, soft body parts like feathers, plant ferns, or other evidence of life, such as footprints or dung may also be preserved.
The word fossil is derived from the Latin fossilis meaning ‘unearthed’.
Preserved evidence of the body parts ancient animals, plants, and other life forms are called body fossils. Trace fossils are the evidence left by organisms in sediment, such as footprints, burrows and plant roots.
Why do we study fossils?
Fossils give us a useful insight into the history of life on Earth. They can teach us where life and humans came from.
Fossils provide important evidence for evolution, and the adaptation of plants and animals to their environments. Fossil evidence provides a record of how creatures evolved and how this process can be represented by a tree of life showing that all species are related to each other.
Fossils can show us how the Earth and our environment have changed through geological time, how continents, now widely separated, were once connected.
Fossils can be used to date rocks. Through the process of evolution, different kinds of fossils occur in rocks of different ages, enabling geologists to use fossils to understand geological history. For geologists, fossils are one of the most important tools for age correlation. Ammonites, for example, make excellent guide fossils for stratigraphy i.e. guide fossils can be used to determine the relative age of two or more layers of rock, or strata, that are in different places within the same country or somewhere else in the world.
Fossils can be used to recreate different worlds. For example, worlds populated by dinosaurs or dragonflies with a two-metre wing span.
How do fossils form?
Fossils are typically found in sedimentary rocks and occasionally some fine-grained, low-grade metamorphic rocks. Sometimes the fossils have been removed, leaving moulds in the surrounding rock or the moulds may have later been filled by other materials, forming casts of the original fossils.
Rapid burial by sediments, previously suspended in water, is required for fossilisation to occur. The burial process separates the remains from the biological and physical processes that would otherwise breakup or dissolve the body material. Fossils are more likely to be preserved in marine environments for example, where rapid burial by sediments is possible. Less favourable environments include rocky mountaintops or similar environments where carcasses decay quickly and/or few sediments are being deposited to bury them.
There are four main ways of describing fossil preservation:
The most common method of fossilisation is petrification through a process called permineralisation. After a shell, bone or tooth is buried in sediment, it may be exposed to mineral-rich fluids that move through the porous material such as a shell or bone and becomes filled with preserving material such as calcium carbonate or silica. Eventually the minerals entirely replace the organic material and they are literally turned into stone or petrified.
Some fossils form when their remains are compressed at depth. A dark imprint of the fossil is produced as a result of high-pressure forces exerted by the weight of overlying sediments and perhaps sea water.
Moulds and casts
In cases where the original shell or bone is dissolved away it may leave behind a space in the shape of the original material called a mould. At some point in the future, sediments may fill the space to form a matching cast. Soft-bodied sea creatures such as snails are commonly found as moulds and casts because their shells dissolve easily. A cast is a positive impression of the original material formed by contact with the mould. More about gastropods.
The rarest form of fossilisation is the preservation of original skeletons and soft body parts. For example, insects that have been trapped and preserved perfectly in amber (tree resin).