Answer:
Scientists mark the beginnings and the ends of eras by observing extinctions and appearances of species in the fossil record. The appearance of various species of ammonites, belemnites, trilobites, foraminifera, nannofossils and magnetic events also mark the boundaries of many of the defined eras.
Explanation:
Answer:
Scientist usually mark the end of a geologic era and the beginning of the next by mass extinction or drastic changes to the earth.
Explanation:
hope this helps
