The oldest era was the Paleozoic era. It ranged from 600 to 210 million years ago. Paleozoic era saw the origin and adaptive radiation of sponges, starfishes, snails, insects, crabs, and terrestialised amphibians and reptiles.

The Paleozoic era was followed by middle period named as Mesozoic era. This era ranged from 65 to 210 million years ago. Its duration was 145 million years. During this era, among animals the reptiles came to prominence. Hence this era is known as the golden age of reptiles. Further this era saw the origin and development of birds and reptiles.

Cenozoic era is characterised by rapid evolutionary changes in mammals. This era is known as the Age of mammals.

Tamil Nadu Biology Text, Class: XI, Unit VII: Origin of life, Page no: 226.