Biochronology

Collections of land mammal ages have been defined for every continent except Antarctica, and most are correlated with each other indirectly through known evolutionary lineages.

A combination of argon–argon dating and magnetic stratigraphy allows a direct temporal comparison of terrestrial events with climate variations and mass extinctions.

[7] The committee tried to make the definitions unambiguous by providing multiple criteria such as index fossils, first and last occurrences, and the relation to a particular formation.

For example, the Chadronian Land Mammal Age in the late Eocene was defined by the boundaries of the Chadron Formation in Nebraska as well as the co-occurrence of Mesohippus, an early horse, and titanotheres, a family of rhinoceros-like animals.

[1]: 240  Since NAMLs are not tied to stratigraphic sections, they are not true chronostratigraphic stages, so some authors place quotes around "Ages".

[5]: 11  In Africa, sequences of fossils (including those of primates) have been determined and some land mammal ages designated, but not yet formally defined.

Land-vertebrate "ages" (LVAs) based primarily on dinosaur faunas have been proposed for the late Cretaceous in western North America.

[12] The most widely utilized pre-Cenozoic tetrapod biochronology system involves Land vertebrate faunachrons (LVFs).

The LVF system was originally designed by Spencer G. Lucas to correlate terrestrial faunal assemblages of the Triassic period.

[14] Although LVFs are a common method used to date Triassic terrestrial sediments, their reliability is more heavily debated than that of Land Mammal Ages.

[17]: 10 Paleontologists have moved towards finer zonation of terrestrial fossils, with the potential to divide the Cenozoic into time intervals of 300,000 years or less.

[6]: 16  However, the fossil record remains discontinuous even in North America, and Woodburne speculates that "mammal age correlations provide results that are satisfactory to their users.