Mesozoic

The Mesozoic is the geological era from about 251.902 to 66.00 million years ago. It contains the Triassic, Jurassic and Cretaceous periods and precedes the Cenozoic. The chart gives uncertainty on the older numerical boundary.^[ref]

The Cretaceous is the youngest Mesozoic period, approximately $143.1$ to $66$ million years ago in the dated chart used here.^[ref] A numerical boundary estimate is a calibration of the geological time scale, not an assertion that every local geological process changed simultaneously. Stratigraphy establishes correlations; radiometric dating supplies numerical age constraints.

The Jurassic is the Mesozoic period between the Triassic and Cretaceous, approximately $201.4$ to $143.1$ million years ago.^[ref] Its position in a rock sequence can be established by correlated stratigraphic evidence even where no directly datable volcanic layer occurs. Numerical calibration and recognition of the interval in local strata are therefore distinct tasks.

The Triassic is the earliest Mesozoic period, approximately $251.9$ to $201.4$ million years ago.^[ref] It precedes the Jurassic. In a local sedimentary rock sequence, a missing part of this interval may reflect nondeposition or later erosion; the absence of a named interval in one section does not imply that no time elapsed there.