Tertiary Sub-Era, informal term designating a time span of almost 63 million years that forms part of the Cenozoic Era of the geological timescale. It extends from about 65 million years ago to 1.8 million years ago, from the end of the Cretaceous Period of the Mesozoic Era to the beginning of the Quaternary Period (sometimes referred to as Sub-Era) of the Cenozoic, and encompasses the Palaeocene (oldest), Eocene, Oligocene, Miocene, and Pliocene Epochs.

The term “Tertiary” was introduced by the French chemist and mineralogist Alexandre Brongniart in 1810, originally for the strata overlying the chalk in Europe, although it subsequently gained wider usage. Until recently the Tertiary was treated as the earlier of two sub-eras of the Cenozoic Era (the Quaternary being the later one). However, it was recently discarded by the International Commission on Stratigraphy as a formal chronostratigraphic unit and superseded by the Palaeogene (66 to 23.8 million years ago) and Neogene (23.8 to 1.8 million years ago) periods, although the name Tertiary will continue to be used as an informal term.

The Tertiary was a time during which the world as we now know it today progressively took shape: plate tectonic movements saw continents gradually assume their familiar outlines, topographies, and positions. Following the end Cretaceous extinction or decline of major animal groups such as the dinosaurs and ammonites, and plant groups such as the cycads, there was plenty of opportunity for the proliferation of the modern fauna and flora, most notably the mammals and angiosperm (flowering) plants, which diversified enormously in the Tertiary. After an initial period of global climatic warming, a significant reduction in palaeotemperatures in the Oligocene was followed by an irregular but persistent cooling trend throughout the remainder of the Tertiary. This trend, accompanied by a corresponding drop in global sea-level as more and more water was locked up as ice, reached its apex in the subsequent Quaternary Period, during the so-called “Ice Ages” of the Pleistocene Epoch.