The Tertiary period was previously one of the major divisions of the geologic timescale, from the end of the Cretaceous period about 65.5 million years ago to the start of the Quaternary period about 1.6 million years ago. Its use has been widespread and continues, however the International Commission on Stratigraphy no longer endorses this term as part of the formal stratigraphic nomenclature. Instead the Paleogene and Neogene periods are recommended as the primary subdivisions of the Cenozoic era. In common usage, the Tertiary has included five geologic epochs -- the Paleocene, Eocene, Oligocene, Miocene and Pliocene.

The Tertiary covers roughly the time span between the demise of the dinosaurs and beginning of the most recent ice age. At the beginning of the period mammal replaced reptiles as the dominant animals. Each epoch of the Tertiary was marked by striking developments in mammalian life. The earliest recognizable hominoid relatives of humans, Proconsul and Australopithecus, appeared. Modern types of birds, reptiles, amphibians, fishes, and invertebrates either were already numerous at the beginning of the period or appeared early in its history. Modern families of flowering plants evolved. Marine invertebrates and non-mammal marine vertebrates experienced only modest evolution.

Continental drift was modest. Gondwanaland finally split completely apart, and India collided with the Eurasian plate. South America was connected to North America toward the end of the Tertiary. Antarctica -- which was already separate -- drifted to its current position over the South Pole. Widespread volcanic activity was prevalent. Climates during the Tertiary slowly cooled, starting off in the Paleocene with tropical-to-moderate worldwide temperatures and ending up with extensive glaciations at the end of the period.

The term Tertiary was first used by Giovanni Arduino in the 1700's. 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, later the use of mollusks was abandoned from the definition and the epochs were renamed and redefined.