The Geologic TimeScale 2012 program involves over 40 geoscientists who are integrating paleontology, radio-isotopic dating, cycle stratigraphy, geochemical trends, and other stratigraphic information. The numerical scale for the Mesozoic primary scale of ammonoid zones merges several methods of extrapolation. Most other stratigraphic scales are assigned ages according to their calibration to those ammonoid zones.

The Early Triassic timescale is well constrained by U-Pb zircon ages and cycle-magnetostratigraphy. The Middle Triassic has fewer radio-isotopic constraints, therefore relative durations of ammonoid zones in some intervals are are scaled according to their number of subzones. Even though the Late Triassic contains an extensive cycle-scaled magnetostratigraphy from the Newark lacustrine formations and there are precise radio-isotopic ages for the Triassic-Jurassic boundary, the correlation of the marine-based Norian lacks a consensus among stratigraphers. For example, it is debated if the uppermost stage, the Rhaetian, spans 3 or 8 million years.

A linear decrease in Strontium isotope ratios through the Early Jurassic supported by radio-isotopic ages and cycle-stratigraphy provides durations of the ammonite zones. Middle Jurassic ammonoid zones are scaled by cycle stratigraphy from boreholes and outcrops or correlation to Pacific M-sequence magnetic anomalies. Cycle-scaled magnetostratigraphy indicates a progressive slowing of spreading rates for the M-Sequence from Callovian through Barremian.

Aptian-Albian zones are scaled via cycle stratigraphy that is constrained by radio-isotopic ages. Late Cretaceous (Cenomanian-Campanian) ammonite zones of the Western Interior have numerous Ar-Ar and U-Pb radio-isotopic ages. The magnetic polarity time scale for the latest Cretaceous (Campanian-Maastrichtian) is cycle scaled in ocean drilling cores, thereby providing numerical ages relative to the base-Cenozoic radio-isotopic age of 66.0 Ma.

The main time intervals lacking definitive radio-isotopic ages with unambiguous biostratigraphic calibrations are the Late Triassic and nearly all of the Middle Jurassic through early Cretaceous.