what is the primary basis for differentiating the eras?

Answer 1

Since this is not my area of expertise, I searched Google under the key words "differentiating geological eras" to get these possible sources:

http://www.google.com/search?client=safari&rls=en&q=differentiating+geological+eras&ie=UTF-8&oe=UTF-8

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.