Mesozoic Era: 251 - 65.5 mya. Cenozoic Era: 65.5 - 0 mya. Phanerozoic Eon. The phanerozoic is the most recent eon in geological terms.
It started about 542 million years ago, and continues to this day. The name "phanerozoic" (meaning visible or apparent life) reflects that this was the period when large organisms (visible to the naked eye) appeared in great profusion. In the early days of geology, this was the designation given to strata where metazoan fossils first started to appear. Today, this definition remains largely true, though we now know that in some places at least, metazoans appeared earlier than the start of the phanerozoic (in the Ediacaran or Vendian period of the preceding proterozoic era). Britannica Online Encyclopedia.