NOTE: The time scale was initially devised based on paleontologic evidence, with each period and epoch representing a significant paleontologic change. Each of the epochs can be further subdivided (e.g., the Cenomanian age that is in the Cretaceous Period, with ages ranging from about 97.5 to 91 million years (m.y.) ago).

^a The relative numerical ages, based largely on radiometric determinations, are mostly from the Decade of North American Geology (1983) time scale issued by the Geological Society of America, with more recent modifications for the Cenozoic part of the record and for the Cambrian-Precambrian boundary reconstruction. Diverse new techniques have also fostered progress—improved methods for dating strata, for example, and new techniques for studying rates of evolution and extinction, as well as innovative ways of using isotopes to evaluate changes in environments, biological activity, and biogeochemical cycles.