Geology is a four-dimensional science — geologists have to think as much about time as about up and down, and north, south, east and west. The study of meteorites and the solar system has allowed the age of the Earth to be measured at about 4 600 000 000 years old. Years of studying the Earth’s rocks and collecting fossils have proven to scientists the animals and plants that lived on the Earth, and even the types of sedimentary rocks deposited, have changed immensely over time.
In order to guide their research, geologists have constructed a geological time scale (see simplified time scale and links on this page) that divides the history of the Earth into a number of time slices. These time slices are united by common elements relating to the earth environments or organisms at that time. Originally, divisions of the time scale were based entirely on the assemblages of fossils seen in rocks of that age.
The largest subdivisions in the geological time scale are called eons. There are four eons — the Hadean (4600 million to 4000 million years ago), Archean (4000 million to 2500 million years ago) Proterozoic (2500 million to 541 million years ago), and Phanerozoic (541 million years ago to today). Within the eons are a number of finer subdivisions — eon > era > period > epoch > age. The word ‘Phanerozoic’ means ‘visible life’ and originally, it was thought all life originated at the start of that eon. We now know that life and fossils extend far back into the Archean, although the forms of life prior to about 600 million years ago were mostly dominated by bacteria and algae.
As geologists have studied the fossils of each geological period for such a long time, paleontologists can use the fossils found in a new area to understand the age of the rocks in that area. This is called biostratigraphy. Biostratigraphy is regularly used to constrain the age of sedimentary rocks, which are difficult to date using radiometric methods carried out on igneous and metamorphic rocks. However, unlike radiometric dating, which gives an absolute age for a rock in numbers of years, biostratigraphy is a relative dating method. Each fossil or group of fossils represent organisms that were around for a specific slice of time and finding that fossil tells us the rock was formed somewhere in that slice.
Western Australian fossils in perspective
Western Australia is endowed with both very recent and very old rocks, including some of the oldest geological materials known on Earth (from Jack Hills), the oldest convincing evidence of life (Pilbara stromatolites, see page below), and rocks that chart the oxygenation of the Earth’s early atmosphere (in the Pilbara and Hamersley Ranges). As a result, the State has a number of interesting and important fossil sites extending more than 3000 million years of geological history.
The time scale above shows the distribution of some important Western Australian fossils according to their age. You can find more information on these localities using the links below.
1. Stromatolites and other evidence of early life — Western Australia has a rich collection of the oldest evidence of life on the planet, including fossil mats and mounds formed by bacteria. The State also has many living examples of these enigmatic structures.
2. Trace fossils of the Tumblagooda Sandstone — Arthropods were the first animal group to leave the seas and tackle life on land. Find out about traces of this landward transition seen near Kalbarri.
3. Dinosaurs and other giant reptiles — Although dinosaur fossils are rare in Western Australia, they weren’t the only large animals to live during the Mesozoic era — pterosaurs and various groups of giant marine reptiles of this age have also been recorded from Western Australia.
4. ‘Strings of beads’ — indications of early multicellular life? — These enigmatic and iconic fossils are found in the southern Pilbara and Mid West regions within rocks between 1171 and 1070 million years old. Although there has been some debate about what these fossils represent, they are generally considered some of the oldest evidence for multicellular life within the State.
External links covering a range of topics relating to Australian fossils, paleontology and geological time can be found on External websites.
More information on fossils and paleontology data within GSWA
For further information contact: