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Tertiary is an old unofficial but still widely used term for a geologic period from 66 million to 2.58 million years ago, a time span that lies between the superseded Secondary period and the Quaternary. The Tertiary is no longer recognized as a formal unit by the International Commission on Stratigraphy, its traditional span being divided between the Paleogene and Neogene Periods and the first stage of the Pleistocene of the Cenozoic Era.
The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start of the Cenozoic Era, and extended to the beginning of the Quaternary glaciation at the end of the Pliocene Epoch. The Tertiary also included the early Pleistocene.
The term Tertiary was first used by Giovanni Arduino during the mid-18th century. He classified geologic time into primitive (or primary), secondary, and tertiary periods based on observations of geology in northern Italy. Later a fourth period, the Quaternary, was applied.
In 1828, Charles Lyell incorporated a Tertiary Period into his own, far more detailed system of classification. He subdivided the Tertiary Period into four epochs according to the percentage of fossil mollusks resembling modern species found in those strata. He used Greek names: Eocene, Miocene, Older Pliocene and Newer Pliocene.
Although these divisions seemed adequate for the region to which the designations were originally applied (parts of the Alps and plains of Italy), when the same system was later extended to other parts of Europe and to America, it proved to be inapplicable. Therefore, the use of mollusks was abandoned from the definition and the epochs were renamed and redefined.
Climates during the Tertiary slowly cooled, starting off in the Paleocene with tropical-to-moderate worldwide temperatures and ending before the first extensive glaciation at the start of the Quaternary.