This time scale was devised as a relative time scale, based on index fossils. The numerical ages were added much later using absolute dating. Although the age of the earth is 4.6 billion years, the sediments on the earth s surface generally only account for about 570 million years. Of all the divisions, remember the eras. Reptiles were the dominant life form.
Mammals are the dominant life form.
Absolute dating A numerical value is assigned to the age of something. The spontaneous transformation of the nucleus of an atom to another element by the capture or ejection of subatomic particles. We ll cover these in detail next Wednesday.