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North American land mammal age
The North American land mammal ages (NALMA) establishes a geologic timescale for prehistoric North American fauna beginning 66.5 Ma during the Paleocene and continuing through to the Late Pleistocene (0.11 Ma). These periods are referred to as ages or intervals (or stages when referring to the rock strata of that age) and were established using geographic place names where fossil materials were obtained.
==System==
The North American land-mammal-age system was formalized in 1941 as a series of provincial land-mammal ages. 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. The system was revised into a formal chronostratigraphic system. 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.
The basic unit of measure is the first/last boundary statement. This shows that the first appearance event of one taxon is known to predate the last appearance event of another. If two taxa are found in the same fossil quarry or at the same stratigraphic horizon, then their age-range zones overlap.
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