Fossils are the remains, or traces, of animals and plants. The bodies of most living things decompose very quickly. Fossils are created when the remains of plants and animals are covered by sediments before they decompose. Most fossils are of hard body parts like teeth and bones or shells; soft body parts are very rarely fossilised.
The oldest fossils are of stromatolites - rock built from layers of sediment trapped by blue-green bacteria. Known as cyanobacteria, they were responsible for oxygenating the Earth's oceans and atmosphere. Stromatolites still exist today, they are sometimes known as 'living fossils'.
You can see living stromatolites in Shark Bay, West Australia.
Fossils can be formed by different processes.
Molds are formed after the buried remains are dissolved, leaving an organism-shaped hole in the rock. Casts are solid mineral deposits that filled a mould, leaving a copy of the living thing.
Carbonaceous film fossils are traces of plants or animals preserved as a thin film of carbon.
Petrification - plant or animal tissue replaced by minerals.
Cast and Mould Shell
Carbon Film Fossil Leaf
Petrified Shark Tooth
Fossils can also be the remains of tracks, burrows, eggs, nests and droppings (scientifically named coprolites - fossilised poo) and are known as trace fossils.
Below: dinosaur eggs in nest, dinosaur coprolite.
Trace fossils can also leave clues to animal behaviour (see videos).
Amber is fossilised tree resin, sometimes containing the remains of insects, plants, and just occasionally traces of larger animals. Organic matter is preserved in amber. The oldest pieces of amber with fossils discovered so far, dates to the Triassic era - about 320 million years old.
Tips of dinosaur feathers in amber
A spider trapped in amber
TASK 31: Make your own fossils. Make a shell cast and mould fossils using plaster and petroleum jelly (vaseline). Make trace fossils by pouring wet plaster onto a paper plate, and make your own tracks before the plaster sets. Have a story behind the tracks and be ready to explain what happened. You can find more information about dinosaur tracks here. Make an 'amber' fossil using a small object to represent an insect, which can be encased in blobs of hot glue (simulating tree resin) from a glue gun and allowed to dry.
Evidence from amber, and new studies of the shape and the arrangement of microscopic pigment structures in dinosaur feathers, have allowed scientists to determine for the first time, an accurate coloured image of a dinosaur fossil discovered in China. Image at left is the original fossil, at right an accurate reconstruction of what Anchiornis huxleyi looked like.
TASK 32: Identify each fossil type and the earliest era in which it could have been formed.
Next we'll return to rock forming processes and look at metamorphic rocks.