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.

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.

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. However, the ages that stretch into the Cretaceous are sometimes referred to as "North American land vertebrate ages" to reflect the fact that mammals, while still abundant, were not the dominant form of terrestrial life during the Mesozoic.

Ages

Cenozoic land mammal ages

Some land mammal ages have recognized biochronological subdivisions.

  • Saintaugustinean: Lower boundary 0.004Ma. Upper boundary Present.
  • Santarosean: Lower boundary 0.014Ma. Upper boundary 0.004Ma.
  • Rancholabrean: Lower boundary 0.21Ma. Upper boundary 0.014Ma.
  • Irvingtonian: Lower boundary 1.4Ma. Upper boundary 0.21Ma.
  • Blancan: Lower boundary 4.7Ma. Upper boundary 1.4Ma.
  • Hemphillian: Lower boundary 9.4Ma. Upper boundary 4.7Ma.
  • Clarendonian: Lower boundary 12.5Ma. Upper boundary 9.4Ma.
  • Barstovian: Lower boundary 16.3Ma. Upper boundary 12.5Ma.
  • Hemingfordian: Lower boundary 18.5Ma. Upper boundary 16.3Ma.
  • Arikareean: Lower boundary 30.0Ma. Upper boundary 18.5Ma.
  • Whitneyan: Lower boundary 32.0Ma. Upper boundary 30.0Ma. Wh2: Lower boundary 31.4 Ma. Upper boundary 30.0 Ma. Wh1: Lower boundary 32.0 Ma. Upper boundary 31.4 Ma.
  • Orellan: Lower boundary 33.7Ma. Upper boundary 32.0Ma. Or4: Lower boundary 32.5 Ma. Upper boundary 32.0 Ma. Or3: Lower boundary 33.1 Ma. Upper boundary 32.5 Ma. Or2: Lower boundary 33.4 Ma. Upper boundary 33.1 Ma. Or1: Lower boundary 33.7 Ma. Upper boundary 33.4 Ma.
  • Chadronian: Lower boundary 37.0Ma. Upper boundary 33.7Ma. Ch4: Lower boundary 34.7 Ma. Upper boundary 33.7 Ma. Ch3: Lower boundary 35.7 Ma. Upper boundary 34.7 Ma. Ch2: Lower boundary 36.5 Ma. Upper boundary 35.7 Ma. Ch1: Lower boundary 37.0 Ma. Upper boundary 36.5 Ma.
  • Duchesnean: Lower boundary 40.1Ma. Upper boundary 37.0Ma.
  • Uintan: Lower boundary 46.3Ma. Upper boundary 40.1Ma. Ui3: Lower boundary 42.8 Ma. Upper boundary 40.1 Ma. Ui2: Lower boundary 45.7 Ma. Upper boundary 42.8 Ma. Ui1: Lower boundary 46.3 Ma. Upper boundary 45.7 Ma.
  • Bridgerian: Lower boundary 50.1Ma. Upper boundary 46.3Ma. Br3: Lower boundary 47.0 Ma. Upper boundary 46.3 Ma. Br2: Lower boundary 49.2 Ma. Upper boundary 47.0 Ma. Br1b: Lower boundary 50.0 Ma. Upper boundary 49.2 Ma. Br1a: Lower boundary 50.1 Ma. Upper boundary 50.0 Ma.
  • Wasatchian: Lower boundary 55.0Ma. Upper boundary 50.1Ma. Wa7: Lower boundary 52.4 Ma. Upper boundary 50.1 Ma. Wa6: Lower boundary 53.0 Ma. Upper boundary 52.4 Ma. Wa5: Lower boundary 53.4 Ma. Upper boundary 53.0 Ma. Wa4: Lower boundary 54.1 Ma. Upper boundary 53.4 Ma. Wa3: Lower boundary 54.4 Ma. Upper boundary 54.1 Ma. Wa2: Lower boundary 54.7 Ma. Upper boundary 54.4 Ma. Wa1: Lower boundary 54.9 Ma. Upper boundary 54.7 Ma. Wa0: Lower boundary 55.0 Ma. Upper boundary 54.9 Ma.
  • Clarkforkian: Lower boundary 56.2Ma. Upper boundary 55.0Ma.
  • Tiffanian: Lower boundary 60.9Ma. Upper boundary 56.2Ma.
  • Torrejonian: Lower boundary 63.8Ma. Upper boundary 60.9Ma.
  • Puercan: Lower boundary 66Ma. Upper boundary 63.8Ma.

Cretaceous land vertebrate ages

  • Lancian: Lower boundary 67 Ma. Upper boundary 66 Ma.
  • Edmontonian: Lower boundary 73 Ma. Upper boundary 67 Ma.
  • Kirtlandian: Lower boundary 75 Ma. Upper boundary 73 Ma.
  • Judithian: Lower boundary 79 Ma. Upper boundary 75 Ma.
  • Wahweapian: Lower boundary 82 Ma. Upper boundary 79 Ma.
  • Aquilian: Lower boundary 84 Ma. Upper boundary 82 Ma.

Other continental ages

See also