The Tertiary Period represents a vast span of geologic time that immediately followed the catastrophic extinction event which ended the age of dinosaurs. It marks the beginning of the Cenozoic Era, often informally called the “Age of Mammals.” This time was defined by significant biological change, the establishment of Earth’s modern continents, and evolving climate systems. Life recovered and flourished, moving the planet from a dinosaur-dominated world to one where mammals and flowering plants became the dominant forms.
Defining the Tertiary Period
The Tertiary Period traditionally covers the time from approximately 66 million years ago to about 2.58 million years ago. It begins precisely at the Cretaceous–Paleogene extinction event, which eliminated the non-avian dinosaurs, and concludes just before the onset of the major Quaternary glaciations.
However, the term “Tertiary Period” is no longer formally recognized by the International Commission on Stratigraphy (ICS). In formal geology, the period was replaced and split into two distinct periods: the Paleogene Period and the Neogene Period. Although technically obsolete in professional stratigraphy, “Tertiary” remains widely used in general literature and historical references to describe this combined time interval.
Major Climatic and Tectonic Shifts
The period began with a warm and wet global climate. Early in the period, a brief but intense global warming event, known as the Paleocene-Eocene Thermal Maximum (PETM), marked one of the warmest intervals in Earth’s history. Following this peak warmth, the overall trend for the remainder of the Tertiary was long-term, gradual global cooling and drying.
Tectonically, this was a time of intense mountain-building activity as the continents drifted toward their current positions. The collision of the Indian subcontinent with the Eurasian plate between 50 and 40 million years ago resulted in the massive uplift of the Himalayas. The Alps and the Rocky Mountains also experienced significant periods of formation and uplift.
The separation of Australia and Antarctica allowed for the development of the Antarctic Circumpolar Current. This current thermally isolated the continent, leading to the formation of the first major Antarctic ice sheets during the Oligocene Epoch. Later, the closure of the Isthmus of Panama dramatically altered global ocean circulation and contributed to Northern Hemisphere glaciation near the period’s end.
The Rise of Mammals
The defining biological feature of the Tertiary Period was the adaptive radiation of mammals. With the removal of large terrestrial reptiles, surviving small, shrew-like mammals rapidly exploited the newly available ecological niches. Within the first 10 million years, over 20 new orders of mammals evolved.
This diversification led to the emergence of nearly all modern mammalian groups, occupying diverse environments. Early primates appeared, adapting to arboreal habitats, while other lineages became the ancestors of modern whales, adapting to aquatic life. The earliest ancestors of ungulates (hoofed animals) and modern carnivores also evolved and diversified.
As the climate cooled and forests gave way to grasslands, the evolution of grazing mammals accelerated. Horses and rhinoceroses evolved specialized teeth and limb structures to process vegetation and travel across the new plains. This biological turnover cemented the Tertiary Period as the “Age of Mammals.”
Subdivisions The Five Epochs
The Tertiary Period is subdivided into five traditional epochs:
- The Paleocene Epoch (66 to 56 million years ago) was the initial recovery phase following the extinction event.
- The Eocene Epoch (56 to 33.9 million years ago) saw the planet experience peak warmth and the first appearance of most modern mammalian orders.
- The Oligocene Epoch (33.9 to 23 million years ago) was characterized by significant global cooling and the formation of the first major ice sheets on Antarctica.
- The Miocene Epoch (23 to 5.3 million years ago) featured extensive tectonic activity, the expansion of global grasslands, and the diversification of early apes.
- The Pliocene Epoch (5.3 to 2.58 million years ago) saw the formation of land bridges, continued cooling, and the emergence of the earliest human ancestors, such as Australopithecus.