The period since the Cambrian explosion is known as the Phanerozoic and is divided into three eras: the Paleozoic, the Cenozoic, and the Mesozoic.  These eras are further subdivided based on discontinuities in the fossil record (i.e. extinctions).  In class today, we will examine the most recent mass extinction event, which marks the boundary between the Mesozoic and Cenozoic periods.