fossil changes that mark the transition from 1 time period to another?

Answer 1

Not sure what your question is but it is important to remember that many of the changes in time period occurred over spans of time that are way longer than the recorded history of humankind. I have always felt that a scale drawing of the earth's timeline was a very useful tool to help comprehend what geologic time entails. good luck

Answer 2

Fossil changes that mark transitions from one period to another are often associated with mass extinction events.