Fossil record
Extinction occurrences, as reflected in the fossil record, have fluctuated throughout earth's history. Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times.
Source
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"OpenStax College, The Biodiversity Crisis. October 17, 2013."
http://cnx.org/content/m44892/latest/Figure_47_01_04.jpg
OpenStax CNX
CC BY 3.0.
