[1] The North American land-mammal-age system was formalized in 1941 as a series of provincial land-mammal ages.
[2] The system was the standard for correlations in the terrestrial Cenozoic record of North America and was the source for similar time scales dealing with other continents.
This approach is nominally justified by international stratigraphic codes; it holds that first appearances of individual species in particular sections are the only valid basis for naming and defining the land-mammal ages.
If two taxa are found in the same fossil quarry or at the same stratigraphic horizon, then their age-range zones overlap.
[5] These additions have been used in research related to the Cretaceous–Paleogene extinction event and the ensuing recovery,[6] and to the Anthropocene debate,[5] respectively.