If eras are marked by major changes in life forms, what distinguishes one period from the next

Periodic large-scale mass extinctions have occurred throughout the history of life; so, it's on this basis that the earth science eras were initially established."

Explanation:
The geological timeScale is that the history of the planet counteracted into spans of your time marked by varied events. There are alternative markers, just like the forms of species and the way they evolved, that distinguish just one occasion from another on the geological timeScale.