Index fossils and relative dating
The K-Pg extinction is thought to have been caused by the impact of a large extraterrestrial body (10 km to 15 km across), but it is generally agreed that the other four Phanerozoic extinctions had other causes, although their exact nature is not clearly understood.
By the late Carboniferous, trees had evolved from earlier plants, and reptiles had evolved from amphibians.
The major groups of organisms that we are familiar with evolved between the late Proterozoic and the Cambrian (~600 Ma to ~520 Ma).
Plants, which evolved in the oceans as green algae, came onto land during the Ordovician (~450 Ma).
Some well-studied groups of organisms qualify as biozone fossils because, although the genera and families lived over a long time, each species lived for a relatively short time and can be easily distinguished from others on the basis of specific features.
For example, ammonites have a distinctive feature known as the suture line — where the internal shell layers that separate the individual chambers (septae) meet the outer shell wall, as shown in Figure 8.3.3.
In this diagram, the coloured bar represents the time range during which each of the four species (A – D) existed on Earth.