What was the Tertiary Period?

Tertiary is a historical name for the interval from the end of the Cretaceous, about 66 million years ago, to the beginning of the Quaternary, about 2.58 million years ago. It is no longer a formal period on the international geological time scale. The same span is now divided into the Paleogene and Neogene periods, followed by the Quaternary.

The old term remains useful when reading older books, maps and museum labels. For current dates and comparisons, naming the Paleogene, Neogene and their epochs is more precise. Those units track a long, changing history rather than one uniform age of mammals.

The interval at a glance

• Paleogene: about 66 to 23.03 million years ago, comprising the Paleocene, Eocene and Oligocene epochs.
• Neogene: about 23.03 to 2.58 million years ago, comprising the Miocene and Pliocene epochs.
• Quaternary: began about 2.58 million years ago and continues today. It is not part of the modern definition of the Tertiary.

This is why a modern source should not label the entire Cenozoic as “Tertiary”. A concise overview of the Cenozoic Era gives the larger frame, while the guides to the Paleogene, Neogene and Quaternary follow the current divisions.

After the Cretaceous extinction

The interval began after the mass extinction at the Cretaceous–Paleogene boundary. Non-avian dinosaurs, pterosaurs, ammonites and many marine organisms disappeared. Birds, mammals, crocodilians, turtles and other lineages survived. The loss of some dominant groups changed ecological communities, but it did not turn the whole planet into an empty landscape: recovery differed among regions and habitats.

During the Paleogene, many mammal and bird groups appeared or became widespread. Early Cenozoic climates were generally warmer than today, followed by a long-term cooling trend. Primates, hoofed mammals and carnivorous mammals diversified. One mammal branch returned to the sea: fossils document the transition of early whales from land-associated ancestors to increasingly aquatic forms.

Neogene landscapes and animal communities

Across the Miocene and Pliocene, many regions became cooler and drier, and open grassy habitats expanded. This was not a single worldwide shift at one date. Local vegetation responded to geography, rainfall and changing circulation. In several lineages, teeth and limbs changed alongside the spread of more open environments. Horses, bovids and other animals adapted to life in open country became prominent in some regions, while forests and woodlands remained important elsewhere.

Many living families of birds and mammals took shape during the Neogene. Fossils show when particular forms occur; explanations linking a trait to a specific climate or habitat are interpretations that must be tested against anatomy, dating and evidence from other regions.

Continents, climate and animal exchange

Plate movements altered ocean circulation and the routes available to migrating animals. The formation of the Isthmus of Panama eventually allowed many terrestrial animals to move between North and South America. This Great American Biotic Interchange reshaped both faunas. It involved movement in both directions and different outcomes among groups, rather than a simple victory by one continent's animals.

Climate, vegetation and geography changed together but not in lockstep. A fossil from one basin records a local community within a wider history. It cannot, by itself, show that the same habitat or climate covered every continent during an entire epoch.

Why the word still appears

“Tertiary” persists in older geological maps, literature and popular writing. Recognising it helps readers interpret historical sources, but current scientific descriptions normally use the Paleogene, Neogene and Quaternary. The change is more than a new label: splitting the old interval makes comparisons easier to connect to defined boundaries and a finer chronology.

The period guides above place these divisions in sequence. The mass-extinction overview explains the boundary event that opened the Cenozoic, while the Ice Age animal catalogue collects profiles from much later Quaternary ecosystems.