For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life.Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.The following four timelines show the geologic time scale.
In Ancient Greece, Aristotle (384-322 BCE) observed that fossils of seashells in rocks resembled those found on beaches – he inferred that the fossils in rocks were formed by living animals, and he reasoned that the positions of land and sea had changed over long periods of time.
For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.
The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.
Apart from the Late Heavy Bombardment, events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets.
Construction of a time scale that links the planets is, therefore, of only limited relevance to the Earth's time scale, except in a Solar System context.