You have discovered a fossil-bearing layer in East Africa and you wish to estimate its age. There are abundant fossils of many savanna species, and you have identified most of them to species or genus. You know that the genus Equus makes its first appearance in Africa at 2.3 mya, and the species Connochaetes gnou first appears at about 1.07 mya. 1. If your site includes both Equus and Connochaetes gnou, - It is probably younger than 1.07 mya. - The younger species is the key here. The site cannot be older than 1.07 mya if the species did not exist before that time.
2. If your site contains neither Equus nor Connochaetes gnou, - You cannot determine the relative date of this site using these species. - Biostratigraphic dating relies on the existence of certain species at sites for comparison against index species.
3. If your site includes Equus but not Connochaetes gnou, - It may date to between 1.07 and 2.3 mya. - The lack of the younger species indicates that the site is likely older than 1.07 mya.