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.
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.
- Rancholabrean: Lower boundary ~0.24 Ma. Upper boundary 0.011 Ma.
- Irvingtonian: Lower boundary 1.8 Ma. Upper boundary ~0.24 Ma.
- Blancan: Lower boundary 4.9 Ma. Upper boundary 1.8 Ma.
- Hemphillian: Lower boundary 10.3 Ma. Upper boundary 4.9 Ma.
- Clarendonian: Lower boundary 13.6 Ma. Upper boundary 10.3 Ma.
- Barstovian: Lower boundary 16.3 Ma. Upper boundary 13.6 Ma.
- Hemingfordian: Lower boundary 20.6 Ma. Upper boundary 16.3 Ma.
- Late Hemingfordian Lower boundary 20.4 Ma. Upper boundary 16.3 Ma.
- Arikareean: Lower boundary 30.8 Ma. Upper boundary 20.6 Ma.
- Geringian: Lower boundary 30.8 Ma. Upper boundary 26.3 Ma.
- Whitneyan: Lower boundary 33.3 Ma. Upper boundary 30.8 Ma.
- Orellan: Lower boundary 33.9 Ma. Upper boundary 33.3 Ma.
- Chadronian: Lower boundary 38 Ma. Upper boundary 33.9 Ma.
- Duchesnean: Lower boundary 42 Ma. Upper boundary 38 Ma.
- Uintan: Lower boundary 46.2 Ma. Upper boundary 42 Ma.
- Bridgerian: Lower boundary 50.3 Ma. Upper boundary 46.2 Ma.
- Wasatchian: Lower boundary 55.4 Ma. Upper boundary 50.3 Ma.
- Clarkforkian: Lower boundary 56.8 Ma. Upper boundary 55.4 Ma.
- Tiffanian: Lower boundary 60.2 Ma. Upper boundary 56.8 Ma.
- Torrejonian: Lower boundary 63.3 Ma. Upper boundary 60.2 Ma.
- Puercan: Lower boundary 66.5 Ma. Upper boundary 63.3 Ma.