Answer:
-mass extinctions.
Explanation:
Mass extinction may be defined as the sudden decrease of species on earth. The earth has experienced five mass extinctions.
According to the study of fossil records, five major mass extinction took place in earth's history that can be determined by identifying traces, left in fossil records.
Mass extinctions destroyed more than half species of earth and thus, lowered the variety of organisms present in today's earth.
Thus, the correct answer to fill in the blank is 'mass extinction.'
