Determining the ages of dinosaurs is accomplished by a mixture of biostratigraphy, litho-stratigraphy, and geochronology. These allow paleontologists to date rocks and fossils in relation to each other, as well as to obtain estimates of their ages in years before present. Using these techniques, geoscientists have constructed and refined a geological time scale for the entire history of the Earth. The timescale is hierarchically divided into successively more refined time intervals: Eras, Periods, and Epochs.
The earliest dinosaurs appeared during the Late Triassic, a time in which the Earth's continents were united into a single supercontinent called Pangaea. Since then, the continents have separated, moving to their present positions.
The presence of a single supercontinent had major implications for climates, which were strongly seasonal. These mitigated throughout the Jurassic, although the Late Jurassic appears to have had strong seasonality, at least in North America. By mid-Cretaceous time, high sea levels, melted polar ice, and high levels of atmospheric CO2, appear to have acted synergistically to produce global warming and greenhouse conditions.
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