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What is a fossil?

How are fossils formed?

Where are fossils found?

Determining the age of fossils

Geological time scale

Eras and periods

Pseudofossils

Further information

The geological time scale.

Source: Museum Victoria.

The Geological Time Scale was developed from the study of the sequence of rock layers, or strata in the Earth's crust. These studies enabled geologists to determine the order in which geological events had occurred.

The Geological Time Scale is usually presented as a table. The dates on this Time Scale, which were determined by radiometric means, may vary slightly depending upon the particular methods used. The accumulation of new data, however, is gradually reducing the size of this variation. Palaeontologists have built up a lot of information about the sorts of plants and animals that lived in the various periods of time represented in the time scale. (For more background on geological time see ‘How old is the Earth?’ ‘Measuring the age of the Earth’ ‘The immensity of Earth time’ in Dinosaurs & Fossils.)

Many of the names on the time scale relate to the area in which the rock formations were first studied. Other names describe the general nature of the rocks deposited at that time. For more information, scroll down to the table below.

Name (click for more detail)	Time (Millons of Years Ago)	Derivation
Cainozoic Era	65 mya to present	Greek—'new animal life'
Quaternary Sub-era	1.78 mya to present
Tertiary Sub-era	65 to 1.78
Mesozoic Era	251 to 65 mya	Greek—'middle animal life'
Cretaceous Period	141 to 65	From Latin creta, meaning chalk, as the rocks were first studied in the 'White Cliffs of Dover', England.
Jurassic Period	205 to 141	From the Jura mountains on the Swiss/French border
Triassic Period	251 to 205	Greek—'threefold'
Palaeozoic Era	545 to 251 mya	Greek—'ancient animal life'
Permian Period	298 to 251	From the province of Perm, in eastern Russia
Carboniferous Period	354 to 298	From the widespread, thick, coal-bearing deposits of this age in the northern hemisphere
Devonian Period	410 to 354	From Devon in England
Silurian Period	434 to 410	From Silures, an ancient tribe in north Wales
Ordovician Period	490 to 434	From Ordovices, an ancient tribe in north Wales
Cambrian Period	545 to 490	From Cambria, the Roman name for Wales
Precambrian	4600 to 545 mya	'before Cambrian', see above