The four eras in the geological time scale are:
Precambrian era: The Precambrian era was the earliest era in the geological ages which was marked by different layer of sedimentary rocks. These rocks consist of permanent records of the earth's past, including the fossilized remains of the plants and animals buried in the sedimentary rocks. Precambrian era extends from 4.6 billion years ago.
Paleozoic era: The Paleozoic era began after Precambrian era and this era extends from 542 million years ago to 251 million years ago. In this era, life started to flourish in the seas.
Mesozoic era: Mesozoic era is the era of many mass extinction. Three to five largest mass extinctions in earth history took place in this era. The Mesozoic era extends from 245 million years ago to 65 million years.
Cenozoic era: The Cenozoic era only spanned about 65 million years ago it is also known as the age of mammals, because the largest land animals have been mammals during that time. This is a misnomer for several reasons.