The Tertiary Period represents the first major division of the Cenozoic Era, the current and most recent era in our planet’s history. Understanding the duration of the Tertiary Period requires examining the boundaries defined by immense global events and subsequent changes in geological classification. This period marks the transition from a world dominated by reptiles to one characterized by the rise of mammals.
Establishing the Time Frame and Duration
The Tertiary Period began approximately 66 million years ago (Ma) and concluded around 2.6 million years ago, lasting about 63.4 million years. The start of the Tertiary is marked by the Cretaceous-Paleogene (K-Pg) boundary, a catastrophic mass extinction event. This event wiped out roughly 75% of all species on Earth, including the non-avian dinosaurs, thereby clearing the ecological landscape for new life forms.
The end of the Tertiary Period is defined by the transition into the Quaternary Period. This boundary is set at the onset of major global cooling and the beginning of widespread Northern Hemisphere glaciation. The time frame of the Tertiary encompasses five distinct epochs, ranging from the Paleocene to the Pliocene, all of which witnessed significant environmental and biological changes.
The Shift in Geological Terminology
While the term “Tertiary Period” remains widely recognized, it is now considered obsolete by the International Commission on Stratigraphy (ICS). The name originated from an outdated four-part classification scheme—Primary, Secondary, Tertiary, and Quaternary—that was developed in the 18th century to categorize rock layers. Modern geology has moved away from this historical naming convention to establish a more scientifically precise and consistent global time scale.
The time interval previously covered by the Tertiary is now formally divided into two distinct periods: the Paleogene and the Neogene. The Paleogene Period covers the earlier part of this span, from 66 Ma to 23.03 Ma. This period includes the Paleocene, Eocene, and Oligocene epochs, representing a time of significant biological recovery and major climatic shifts.
The Neogene Period follows the Paleogene, beginning at 23.03 Ma and extending to the start of the Quaternary at 2.58 Ma. The Neogene includes the Miocene and Pliocene epochs. Separating the time span into the Paleogene and Neogene allows for a clearer distinction between the major geological and biological events that occurred in the early and later parts of the Cenozoic Era.
Defining Characteristics of the Tertiary Timeframe
Following the K-Pg extinction event, the planet recovered and transformed. This timeframe is often characterized as the “Age of Mammals,” due to the rapid diversification and expansion of mammalian species that filled the ecological niches left vacant by the dinosaurs. Mammals evolved from small, shrew-like creatures into a wide variety of forms, including the ancestors of modern bats, whales, and primates.
Continent movement continued to reshape the planet, with landmasses drifting toward their current positions. Significant mountain-building events, such as the formation of the Alps in Europe and the Himalayas in Asia, occurred during this period as tectonic plates collided. These geological processes profoundly influenced global climate and local ecosystems.
In terms of climate, the early Paleogene began with a very warm, “greenhouse” world, with tropical jungles extending far into northern latitudes. A major cooling trend, however, took hold by the Oligocene Epoch and continued through the Neogene. This gradual drop in temperature ultimately led to the development of extensive ice sheets, marking the transition to the “icehouse” world that defined the Quaternary Period. The cooling favored the spread of grasslands and savannas, which in turn drove the evolution of grazing mammals.