Index fossils dating
These eras are divided into periods, the system of which was established by the middle of the last century.The periods are in turn divided into epochs, and the epochs are divided into ages called ages.The generalized archosaurian reptiles of the Triassic gave way to the dinosaurs, a terrestrial megafauna the like of which the Earth has not seen before or since. Climatic conditions remained warm and tropical worldwide.
Late in the Proterozoic a runaway icehouse effect meant that the preceding warm conditions were replaced by a "Snowball Earth" with ice several kilometers deep covering the globe.
Warming conditions saw the short-lived Ediacarian biota and finally the appearance of first metazoa.
Early in the 300 million year history of the Paleozoic, atmospheric oxygen reached its present levels, generating the ozone shield that screens out ultraviolet radiation and allows complex life to live in the shallows and finally on land.
The geological time-scale is here used to define the major stages in the history of life on Earth.
Here the four and a half billion year history of planet Earth is divided into six segments, although this is semi-informal classification, mixing eons and eras. The name says it all; a hellish period lasting some 760 million years, when the Earth was subject to frequent bombardment by comets, asteroids, and other planetary debris.