Question

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

Answer 1

The correct answer is periodic large-scale mass extinctions.  

If eras are signified by the prime modifications in life forms, then periodic large-scale mass extinctions separates one period from the next.  

The geological time scale refers to the history of the planet counteracted into the spans of the time signified by various events. There are alternative markers, just like the species form, and the manner in which they have developed, which distinguish just one occasion from another on the geological time scale.  

Answer 2

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.