The Phanerozoic (occasionally phaenerozoic) Eon is the period of time during which abundant animal life has existed. It covers roughly 545 million years from the present back to the time when diverse hard shelled animals first appeared. Time previous to the start of the Phanerozoic is called Precambrian. The exact time of the boundary between the Phanerozoic and the Precambrian is slightly uncertain. In the 19th Century, the boundary was set at the first abundant metazoan fossils. But several hundred taxa of Precambrian metazoa have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the PreCambrian-Phanerozoic boundary either at the classic point where the first trilobites and archeocyathids appear; at the first appearance of a complex feeding burrow called Phycodes pedum; or at the first appearance of a group of small, generally disarticulated, armored forms termed 'the small, shelly fauna'. The three different dividing points are within a few million years of each other.
The Phanerozoic is divided into three Eras -- Paleozoic, Mesozoic, and Cenozoic. In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists. The term seems to be falling into disuse in more modern literature.
The time span of the Phanerozoic includes the rapid emergence of a number of animal phyla; the evolution of these phyla into diverse forms; the emergence of plants onto land; the development of complex plants; the evolution of fish; the emergence of animals onto land; and the development of modern faunas. During the period covered, continents drifted about, eventually collected into a single landmass known as Pangea and then split up into the current six continental landmasses.