The Phanerozoic eon is the current eon on the geologic timescale. It is the one in which abundant animal life has existed. It covers about 541 million years, and goes back to the time when hard-shelled animals first appeared. It was once thought that life began in the Cambrian, the first period of this eon.
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The exact time of the boundary between the Phanerozoic and the Proterozoic is slightly uncertain. In the 19th century, the boundary was set at the first abundant metazoan fossils. But several hundred kinds of Proterozoic metazoa have been discovered in the Ediacaran. Study of the Ediacaran biota started in the 1950s. There are three dividing points at the Proterozoic-Phanerozoic boundary. It might be where the first trilobites and archaeocyatha appear; or at the first appearance of signs of burrowing; or at the first appearance of the 'small shelly fauna'. The three different dividing points are within a few million years of each other.
- the rapid emergence of animal phyla;
- the evolution of these phyla into diverse forms;
- the emergence of terrestrial plants;
- the development of complex plants;
- the evolution of fish;
- the emergence of terrestrial animals; and
- the development of modern faunas.