Index Fossils

Index Fossils

Index Fossils

By studying fossils found in successive rock layers - from older to younger - Earth Scientists have discovered the evolutionary sequence of life.

 

Each time interval has an unique array of fossils, which can be used to date the rocks in which they occur. Some are particularly useful because they occured over a short time span before they quickly became extinct and had a wide geographical range. They are called Index Fossils.

 

Earth Scientists look for such index fossils in sedimentary rocks to figure out the relative ages of rocks within the worldwide geological time scale. When we find the fossils in rocks from different places in the world, we know that the rocks are of the same age.

Drepanura sp.
Drepanura sp.
01
Drepanura sp.

Trilobite, Upper Cambrian

Shandong, Chian
Sample size: 10x7cm
Many forms of Trilobites are index fossils that are associated with the Paleozoic Era and went extinct during the Permian Period ca. 248 million years ago.

Early Palezonic - Triobites
Cyrtograptus
Cyrtograptus
02
Cyrtograptus

Silurian, Shaanxi, China
Size: 15x15cm
The diverse forms of Graptolites serve as important index fossils for the Ordovician and Silurian period of the Early Paleozonic Era.

Early Palezonic - Graptolites
Glossopteris
Glossopteris
03
Glossopteris

Pteridospermatophyta (Seed Ferm), Permian, Australia
Size: 28x15cm
Glossopteris is an important Permian index fossil found in many continents today providing strong evidence that these continents were once joined together in an ancient Supercontinent Gondwana.

Cenozoic - Breakup of Gondwana
Ophiceras
Ophiceras
04
Ophiceras

Mollusca, Cephalopoda,
Ammonidea, Ceratitida
Lower Triassic, Guangxi, China
Size: 4.5x4cm
Ammonites were very common and diverse in the Mesozoic Era (251Ma-65Ma). They were not found in rocks formed after this time as they went extinct 65Ma ago together with the Dinosaurs.

Mesozoic - Aquatic Life

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Relative Age

Absolute Age