The biostratigraphy of dinosaur remains in the Upper Jurassic Morrison Formation and related beds was studied throughout the Western Interior in an effort to place as many dinosaur localities as possible in their relative chronostratigraphic positions. First, we established a regional stratigraphic framework for the Morrison Formation throughout the Western Interior. Three primary stratigraphic markers in the formation aided in regional correlations. An important marker in about the middle or upper middle part of the formation, known as the clay change, marks the abrupt transition from predominantly non-smectitic clays below to predominantly smectitic clays above. This surface, as well as the J-5 unconformable to conformable surface at the base of the Morrison and the K-1 unconformity at the top of the Morrison, comprise the basis for most of the correlations. After the stratigraphic framework was established, a total of 128 dinosaur sites were placed in the stratigraphic framework and then were correlated to a primary reference section (DQW) near the Carnegie Quarry at Dinosaur National Monument in northeastern Utah. Where the clay change is not evident (Black Hills and Montana), the correlations are more tenuous so the 13 localities in these areas are treated separately, even though other evidence from the regional stratigraphy and calcareous microfossils help relate these sites to the primary reference section.

The biostratigraphic distribution of the dinosaurs allows the Morrison Formation and related beds to be divided into four biozones, numbered one through four from oldest to youngest. The zones are based on the stratigraphic (vertical) distribution of long-ranging taxa (mostly genera and species), which are taxa that extend through two or more different stratigraphic positions in the formation. Single-site taxa that are only found at one locality in one stratigraphic level are not used in the zonation. Dinosaur Zone 1 extends from the base of the formation to the middle of the Salt Wash Member. Dinosaur Zone 2 extends upward to about 30 ft (9.1 m) above the clay change in the primary reference section