North American land mammal age
The North American land mammal ages (NALMA) establishes a geologic timescale for North American fauna beginning during the Late Cretaceous and continuing through to the present. 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.
The utility of the system led to its expansion into the Cretaceous (formalized 1986) and the Holocene (formalized 2014). These additions have been used in research related to the Cretaceous–Paleogene extinction event and the ensuing recovery, and to the Anthropocene debate, respectively.
Cenozoic land mammal ages
- Saintaugustinean: Lower boundary 0.4 ka. Upper boundary Present.
- Santarosean: Lower boundary 0.014 Ma. Upper boundary 0.4 ka.
- Rancholabrean: Lower boundary ~0.24 Ma. Upper boundary 0.014 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 Ma. Upper boundary 63.3 Ma.
Cretaceous land mammal ages
- Lancian: Lower boundary 70 Ma. Upper boundary 66 Ma.
- Judithian: Lower boundary 84 Ma. Upper boundary 70 Ma.
- Aquilian: Lower boundary 86 Ma. Upper boundary 84 Ma.
Other continental ages
- Wood, H. E.; Chaney, R. W.; Clark, J.; Colbert, E. H.; Jepsen, G.L.; Reeside, J. B.; Stock, C. (1941). "Nomenclature and correlation of the North American continental Tertiary". Geological Society of America Bulletin. 52 (1): 1–48. doi:10.1130/gsab-52-1.
- Woodburne, Michael O., ed. (1987). "A prospectus of the North American Mammal Ages". Cenozoic mammals of North America : geochronology and biostratigraphy. Berkeley: University of California Press. pp. 285–290. ISBN 978-0520053922.
- Lillegraven, J. A.; McKenna, M. C. (1986). "Fossil mammals from the" Mesaverde" Formation (late Cretaceous, Judithian) of the Bighorn and Wind River basins, Wyoming: with definitions of late Cretaceous North American land-mammal" ages"". American Museum Novitates (2840): 1–68.
- Barnosky, A. D.; Holmes, M.; Kircholtes, R.; Lindsey, E.; Maguire, K. C.; Poust, A. W.; Stegner, M. A.; Sunseri, J.; Swartz, B.; Swift, J.; Villavicencio, N. A. (2014). "Prelude to the Anthropocene: Two new North American land mammal ages (NALMAs)". The Anthropocene Review: 1–18. doi:10.1177/2053019614547433.
- Woodburne, Michael O., ed. (2012). "Mammalian Biochronology of the Latest Cretaceous". Late Cretaceous and Cenozoic mammals of North America biostratigraphy and geochronology. New York: Columbia University Press. pp. 21–43. ISBN 9780231503785.
|Preceded by Proterozoic Eon||Phanerozoic Eon|
|Paleozoic Era||Mesozoic Era||Cenozoic Era|