Fossils

What is a fossil?

A fossil is the preserved remains or traces of a living thing from the distant past, usually more than 10 000 years old. Scientists who study fossils are called palaeontologists. Fossils are our main evidence of what lived on Earth before humans, how living things have changed over time, and what past environments were like.

Kinds of fossil

• Body fossils are parts of the organism itself, usually hard parts such as bones, teeth, shells and wood. Often, groundwater carrying dissolved minerals soaks into the buried remains, filling the tiny spaces or slowly replacing the original material, making a stone copy. Wood turned to stone like this is petrified wood. At Lightning Ridge in NSW, some bones and shells have been replaced by opal.
• Moulds and casts. If a buried shell dissolves away, it leaves a hollow mould in the rock. If the mould later fills with minerals or sediment, it forms a cast with the shell's shape.
• Trace fossils are signs of an organism's activity rather than its body: footprints, burrows, bite marks, nests and fossilised droppings (coprolites). They show behaviour, such as how fast a dinosaur walked or whether animals moved in groups.
• Imprints and carbon films: soft leaves, feathers or bodies pressed into fine mud can leave a thin outline or a black film of carbon.
• Whole organisms are rare, but insects and spiders have been trapped in sticky tree resin that hardened into amber, and woolly mammoths have been found frozen in the permafrost of Siberia with skin, hair and stomach contents.

How fossils form

Most living things never become fossils. When an organism dies, scavengers eat it, bacteria and fungi decompose it, and weathering breaks down what's left. For a fossil to form, the remains need to escape all three. The best conditions are:

• Hard parts such as bones, teeth, shells or wood, which decay slowly.
• Rapid burial by fine sediment such as mud, silt, sand or volcanic ash, which hides the remains from scavengers and cuts off the oxygen that decomposers need.
• Water carrying dissolved minerals, which can fill and replace the remains.
• Undisturbed rock for millions of years: not melted, strongly metamorphosed or eroded away.

This is why most fossils are of animals that lived in the sea or in lakes and rivers, where sediment is always settling, and why fossils are found almost only in sedimentary rocks. Igneous rocks form from molten rock that would destroy any remains, and metamorphism usually destroys fossils too.

From bones to animals

Fossils are usually found when weathering and erosion expose them, or when quarries, mines or road cuttings break open rock. Palaeontologists record exactly where each piece is found and which rock layer it came from, then dig it out carefully, often wrapping it in a plaster jacket to protect it. In the lab, technicians remove the rock grain by grain with tiny drills, needles and even weak acids.

Complete skeletons are rare, so palaeontologists compare bones with those of related living and extinct animals to fill gaps. Scientists also use CT scanners to see inside fossils without damaging them, measure the chemistry of teeth to find out what animals ate, and compare growth rings in bones to estimate age. A fossil's surrounding rock tells a story too: ripple marks, mud cracks and other fossils reveal what the environment was like.

Australian fossil treasures

• Pilbara, WA: stromatolites about 3.5 billion years old, among the oldest evidence of life on Earth. Stromatolites are mounds built by layers of microbes, and living ones still grow at Shark Bay.
• Ediacara Hills, SA: imprints of soft-bodied animals about 550 million years old, among the earliest complex animals. In 2004 a new period of geological time, the Ediacaran, was named after this site: the first new period added in more than 120 years.
• Canowindra, NSW: thousands of fish from the Devonian period, about 360 million years ago, fossilised together when their pool dried up.
• Gogo, WA: beautifully preserved 380-million-year-old fish, including Materpiscis.
• Riversleigh, Qld (Waanyi Country) and Naracoorte Caves, SA: World Heritage sites full of fossil mammals, including ancient platypuses, marsupial lions and giant wombat relatives, the megafauna.
• Winton, Qld: dinosaurs such as Australovenator and the giant sauropod Australotitan, and the Lark Quarry trackways, with more than 3000 small dinosaur footprints.