Geologic Time is ... went extinct around the same time, that might be marked as a change in the period on the time scale, such as the transition from Jurassic to Cretaceous 145.5 million years ago.

Prehistoric time line, geologic time scale, photos, facts, maps, and more from National Geographic. Humans have walked the Earth for 190,000 years, a mere blip in Earth's 4.5-billion-year history.

A Timescale is a scale ... events not on the class list. I have my students use a ruler to space years accordingly. An Index fossil is any animal or plant preserved in the rock record of the Earth ...