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Geologic age dating sequence

That last, pink Precambrian column, with its sparse list of epochal names, covers the first four billion years of Earth's history, more than three quarters of Earth's existence. Paleontologists have used major appearances and disappearances of different kinds of fossils on Earth to divide Earth's history -- at least the part of it for which there are lots of fossils -- into lots of eras and periods and epochs.

Geologic age dating sequence

In the time since the previous geologic time scale was published in 2004, most of the boundaries between Earth's various geologic ages have shifted by a million years or so, and one of them (the Carnian-Norian boundary within the late Triassic epoch) has shifted by 12 million years.

When you talk about the Precambrian, Paleozoic, Mesozoic, and Cenozoic on Earth, or the Noachian, Hesperian, and Amazonian for Mars, these are all relative ages.

Relative-age time periods are what make up the Geologic Time Scale.

On other solid-surfaced worlds -- which I'll call "planets" for brevity, even though I'm including moons and asteroids -- we haven't yet found a single fossil.

Something else must serve to establish a relative time sequence. Earth is an unusual planet in that it doesn't have very many impact craters -- they've mostly been obliterated by active geology.

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