Fossils

Fossils are the direct evidence of an organism that lived more than 10 000 years ago. Fossils are rare events, and are often incomplete or distorted.
Many fossils were formed when an animal or a plant fell into a swamp or a lake and was immediately covered by a fine layer of silt. This blocked off the oxygen, needed by the decaying bacteria to decompose the organism quickly. Instead, the organism decayed very slowly over thousands of years, leaving the remains intact.

Types of Fossils

Unaltered Remains - e.g. dinosaur bones, frozen mammoths, animals preserved in peat bogs, insects preserved in amber which is hardened tree resin, mummified remains of desert animals
Petrified Wood - The original wood has slowly been replaced by minerals, and is now a rock that resembles the original wood structure
Carbon Residues - As a fern leaf died and slowly decayed, there was a residue of carbon left behind in the shape of the original fern
Imprints - e.g. footprints, tracks
Moulds - the depression or imprint formed by the hard parts of an animal, but no original remains exist
Casts - If a mould is filled by another type of rock over time, it becomes a cast, e.g. casts of burrows
Coprolites - fossilised faeces that indicates the diets of ancient animals

Structure of the Organism - After examination of today's animal and plant structures, palaeontologists piece the fossilised bones and leaf structures together to form an image of the fossil organism
Type of Movement - The structure indicates whether the organism walked, swam or flew
Diet - Presence of seeds or small bones in fossilised stomach contents or faeces indicates what was eaten
Habitat - Fossilised seeds indicate forests of trees, grassland or swamps
Gender - Larger animals were probably male. Also some male birds such as emus have distinct skull protrusions in the male of the species
Social Group - If fossils of different-sized animals of the same type were found together, this may indicate they roamed in family or social groups
Age of Trees- By counting the growth rings of fossilised trees, palaeontologists can estimate the age when they died
Age of Fossil - Various dating methods can determine the approximate time that a fossilised was living thousands or millions of years ago

Law of Uniform Change - Scientists assume that the earth's geological changes (e.g. erosion, deposition, earthquakes, volcanoes) probably occurred at a similar rate throughout the history of the earth.
Law of Superposition - Scientists assume that rock layers deeper under the earth's surface are older than rock layers close to the earth's surface.
Correlation - Scientists assume that if the same distinctive fossils (index fossils) are in the same kind of sedimentary rock layers at different places (e.g. on either side of the Grand Canyon), they are probably of the same age.
Radioactive Dating - Radioactive elements, such as uranium, potassium and carbon, within the earth's rocks and fossils are like 'clocks'. Many elements have radioactive isotopes containing different numbers of neutrons which are unstable and can break down to form other elements. The time taken for half the original element to break down is a definite time called a half-life.
For example, when each half of the original mass of Uranium-238 in rocks breaks down to form Thorium-230, the process takes about 80 000 years. Another example is when half an original mass of Carbon-14 in fossil remains breaks down, the process takes about 5 730 years.