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Phanerozoic
The Phanerozoic (British English Phanærozoic) is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers million years to the present, and goes back to the period when diverse hard-shelled animals first appeared. Its name derives from the Ancient Greek words (fanerós) and (zo̱í̱), meaning ''visible life'', since it was once believed that life began in the Cambrian, the first period of this eon. The time before the Phanerozoic, called the ''Precambrian'' supereon, is now divided into the Hadean, Archaean and Proterozoic eons. Plant life also appeared from early in the Phanerozoic eon.
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 and development of complex plants; the evolution of fish; the emergence of insects and tetrapods; and the development of modern faunas. During this time span tectonic forces caused the continents to move and eventually collect into a single landmass known as Pangaea, which then separated into the current continental landmasses.
==Proterozoic-Phanerozoic boundary==
The Proterozoic-Phanerozoic boundary is at million years ago. In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils. But several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since systematic study of those forms started in the 1950s. Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef building animals (archaeocyatha) such as corals and others appear; at the first appearance of a complex feeding burrow called ''Treptichnus 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.
In the older literature, the term ''Phanerozoic'' is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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