I'm not sure about Phanerozoicums but, the PhanerozoicEonis the current eon in the geological timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name is derived from Greek and means make life appear, since it was once believed that life began in the Cambrian, the first period of this eon.