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The Phanerozoic - literally "visible life" - includes most of the history of complex life on Earth, as shown by the fossil record. Unlike the Precambrian it remains a valid time unit in contemporary geology and paleontology.
The present very brief unit is really just used as a node or junction point between a number of other converging timescale perspectives. On the one hand there is the exponential and logarithmic deep time scale. So the Phanerozoic is only about one tenth (well, one ninth) the entire history of the Earth, but is itself again almost nine times the length of the Cenozoic. The Cenozoic is thirteen times as long as the Pliocene + the Quaternary. So there is an approximate powers of ten relation, which gives as an understanding of how vast deep time really is.
On the other hand, in terms of the Geological Timescale, the phanerozoic is one of four equal eons of Earth history (the other three being the Hadean, Archean, and Proterozoic. In view of the fact that life has another half billion of reasonable potential time on earth before the aging and heating sun makes conditions impossible, we could add a further aeon, although what form this will take depends on the outcome of our present age.
Making up the Phanerozoic itself are three progressively shorter eras, the Paleozoic, or age of ancient life (invertebrates, fish, amphibians and reptiles, and spore-bearing and early seed plants on land), the Mesozoic or Middle Life, the age of reptiles, and the Cenozoic or recent life, characterised by birds, mammals, flowering plants, and modern types of invertebrates. In the following units, each of these eras is explored in some detail. MAK120709
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