The classic opening of the Phanerozoic is marked by the appearance of trilobites and archaeocyatha, while fossil finds have pushed the first stratigraphic appearance of the 'small shelly fauna' back a few million years.
The Phanerozoic is subdivided into:
● Paleozoic Era, 542-251 Mya
● Mesozoic Era, 251-65.5 Mya
● Cenozoic Era, 65.6 Mya to present
Each era is further subidived into periods and epochs.