GEOLOGY | FOSSILS
What are fossils?
Fossils are the remains of plants and animals that were buried in sediments of ancient seas, lakes and rivers and later preserved in the rock record. Soft body parts decay soon after death, but the hard parts, such as bones, shells and teeth can be replaced by minerals that harden into rock. Remains can include microscopically small fossils, such as single-celled foraminifera or pollen grains, as well as more familiar, larger animals like trilobites and ammonites. Preserved evidence of the body parts of ancient animals, plants and other life forms are called body fossils. In very exceptional cases, soft parts like skin, feathers and plant leaves may also be preserved in exquisite detail.
Life can also be detected by the chemicals and physical tracks and trails left behind by an organism. Trace fossils are the evidence left by organisms in sediment, such as footprints and burrows.
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 that were previously suspended in water is required for fossilisation to occur. The burial process isolates the remains from the biological and physical processes that would otherwise break up or dissolve the body material.
Fossils are more likely to be preserved in the marine environment for example, where rapid burial by sediments is possible. Less favourable environments are for example, rocky mountaintops, where carcasses decay quickly, because 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 moving through the porous rock material and becomes filled with preserving minerals such as calcium carbonate or silica. Eventually, the minerals entirely replace the organic material and the remains are literally turned into stone or ‘petrified’. (Petra was the Latin word for rock.)
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.
Plant leaves and ferns are good examples of fossils produced by compression.
MOULDS & 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.
Image: This image is a mould of a marine organism, similar to modern-day whelks (gastropod - a class pf mollusc). Fossils can form when a mould of the interior of the shell is made by water-borne minerals percolating through it. The shell material eventually dissolves away, leaving the mould.
The rarest form of fossilisation is the preservation of original skeletons and soft body parts. Insects that have been trapped and preserved perfectly in amber (fossilised tree resin) are examples of preserved remains.
Image: Insect preserved in Baltic amber.