Fossils forming

What fossils tell us

This chapter looks at fossils and their formation. Fossils are traces of ancient plants or animals preserved in rocks. The number of fossils and their placement in rocks is called the fossil record. The study of fossils is called palaeontology. See image 1.

Because fossils are buried in rock, most of the time they are found only when the rock is dug up, worn out or cut away. Fossils may include parts of animals, their footprints, eggs or nests. Plants and their spores, or seeds, also make up part of the fossil record. Most ancient animals and plants did not leave any fossils so we will never know of their existence.

Types of fossils

There are several types of fossils:

• mould fossils - a fossilised impression in the rock or a negative image of the organism
  
  
• cast fossils - formed when a mould is filled in
  
  
• trace fossils, or ichnofossils - fossilised nests, burrows and footprints
  
  
• true form fossils - fossils of the actual animal or animal part

Fossil formation

Any living organism can be fossilised if its body is quickly buried by sand or mud before it rots away. A quick burial is most likely to happen near the water which makes the fossils of sea creatures the most common. Fossils of land animals are quite rare. Usually all that remain to tell us what the creature looked like are just the hard parts like the shell, bones and teeth. That is why sometimes it is impossible to find out if that creature had fur, what colour it was, or what size its body parts were.
Here are the main steps in fossil formation:

• Animals are quickly buried after their death by sinking in mud or sand.
  
  
• Over time, more and more rocks and various sediments cover the remains.
  
  
• Parts of the animal or the entire animal that do not rot are covered by new sediments.
  
  
• Some parts of the dead animal (that do rot or get eaten by other animals) turn into fossils over time.
  
  
• After long periods of time, the chemicals in the buried animal's body change. As the animal's bone slowly rots, water seeps into the bone and replaces the bone (organic) chemicals with other rock-like (non-organic) minerals.
  
  
• A 'rock copy' of the original animal is formed as a result. That 'rock copy' is called a fossil. The fossil has the same shape as the original animal.

See animation 1.

Finding fossils

Quickly-eroding, soft sedimentary rocks are good places for finding fossils. For example, fossilised remnants of Plesiosaur were discovered recently by an amateur palaeontologist who stumbled on some bones sticking out of the cliff face in Yorkshire, England. But finds like that are very rare. Usually fossils do not just stick out of the ground. Palaeontologists must do enormous amounts of work to find, excavate and prepare fossils.

Dating fossils

There are several methods that can be used to date fossils:

1. The oldest method is studying how deeply a fossil is buried. The deepest rocks contain the oldest fossils.
   
   
2. The radiometric fossil dating method is the most accurate. Some chemical elements can turn into different elements over time. They are called radioactive chemical elements and their change into different elements is called radioactive decay. Radioactive elements are natural parts of rocks and they decay at a known rate. The original element, called 'parent', gradually decays into 'daughter' elements. This process is irreversible. By measuring the parent and daughter elements in a given rock (knowing the decay rate) it is possible to calculate the time passed from the fossil's formation.The fossil's age equals the age of rocks in which it is found.
   
   
3. Looking for index fossils is another method. This method is probably the most suitable for school students. Index fossils are fossils that have already been dated. These fossils are used as guides when working out the age of other fossils.

Coal - fossilised plant material

Coal is actually fossilised plant material. It was formed by ancient forests. When the trees died they were buried under other plants and trees. Thick layers of these plants e accumulated for a really long time. After millions of years of compression, the chemical element of these plants, called carbon, turned into coal. All natural organic fuels are called fossil fuels.

Oil

Like coal, oil is a fossil fuel. It forms from the remains of tiny sea creatures buried in the ocean floor under the layer of sediment. Their remnants do not rot because there is no oxygen on the ocean floor. Different bacteria and magma heat turned these organisms into carbon-containing chemical compounds that form industrial gas and oil. See image 2.