The period since
the Cambrian explosion is known
as the Phanerozoic and is divided into three eras:
the Paleozoic, the Mesozoic, and the Cenozoic.
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.