Palaeogene Period

The Palaeogene Period lasted from 66 to 23.04 million years ago and opened the Cenozoic Era. It began at the K–Pg boundary, when all non-avian dinosaurs, ammonites and many other organisms disappeared. Its 42.96 million years then carried life from post-impact recovery through exceptional Eocene warmth to a cooler Oligocene world with a major Antarctic ice sheet.

Mammals and birds expanded into newly available roles, but this was not an instantaneous takeover. Early Palaeocene communities contained surviving lineages, temporary ecological experiments and groups unlike any living fauna. Modern-looking ecosystems emerged through repeated radiations, extinctions, migrations and climate shifts.

Name, boundaries and nine stages

Palaeogene means “old-born” or “ancient recent”, contrasting these earlier Cenozoic faunas with younger Neogene ones. The obsolete Tertiary combined both periods. It remains useful in historical publications but is not a formal period on the current international chart.

The lower boundary is the globally recognisable K–Pg layer at 66 Ma. The upper boundary at 23.04 Ma is marked in marine sections near the first appearance of the planktonic foraminifer Paragloborotalia kugleri. The geological time guide explains why a named boundary is tied to a physical reference while its numerical age can be refined.

Palaeocene recovery

The first forests above the boundary were often dominated locally by rapidly spreading plants. More diverse vegetation returned over thousands to millions of years. In western North America, dated sections show mammals increasing in body size and ecological variety through pulses rather than one smooth radiation.

Birds survived as the only dinosaur lineage and diversified on land, in water and in the air. Crocodilians, turtles, amphibians and fishes persisted in freshwater systems. Their survival was selective, shaped by geography, food webs and life history rather than immunity to the crisis described in the extinction guide.

Continents and migration routes

The Atlantic widened, India moved into Asia, and connections between northern continents repeatedly opened and closed. Antarctica remained linked to South America and Australia early in the period, then became progressively more isolated as southern ocean gateways developed.

These changes controlled dispersal. Mammal groups could spread between Europe, Asia and North America during warm intervals, while long isolation produced distinctive South American and Australian faunas. Similar-looking ecological roles on separate continents do not imply close relationship.

From greenhouse warmth to Antarctic ice

Global temperature rose through much of the Palaeocene and peaked in the Early Eocene. Forests grew at high latitudes, although polar darkness still imposed strong seasonality. Long-term carbon dioxide decline, ocean-gateway changes and feedbacks in the carbon cycle later drove cooling.

The Palaeocene–Eocene Thermal Maximum

At 56 Ma, a rapid release of carbon caused the Palaeocene–Eocene Thermal Maximum, or PETM. Carbon-isotope records show a major disturbance, while deep-ocean temperatures rose and ocean acidification dissolved carbonate on parts of the seafloor. On land, rainfall patterns shifted and some mammal lineages temporarily became smaller.

The initial carbon source probably involved several reservoirs and feedbacks. Volcanism around the North Atlantic is important, but methane, organic carbon and permafrost-like stores may also have contributed. Exact proportions remain debated.

Eocene forests and mammal radiations

Early representatives of perissodactyls, artiodactyls and primates spread during the Eocene. Their first appearances in a region record immigration as well as evolution. Large flightless predatory birds occupied important roles in some continents, while bats were already capable of powered flight and echolocation in the Early Eocene record.

Exceptional sites such as Messel preserve fur outlines, stomach contents and articulated skeletons. Green River lake deposits preserve fishes, plants, reptiles, birds and mammals. Such Lagerstätten reveal fine details but sample particular lakes, seasons and burial processes.

How land mammals became whales

Early cetacean evolution is documented by a sequence of anatomical mosaics. Forms such as Pakicetus retained weight-bearing limbs but have characteristic ear structures. Ambulocetus combined terrestrial and aquatic locomotion. Protocetids became increasingly marine while retaining hind limbs.

By the Late Eocene, basilosaurids were fully aquatic. Their tiny hind limbs could not support walking, the spine and tail powered swimming, and nostrils had shifted backwards. Wadi Al-Hitan in Egypt preserves numerous skeletons in their geological setting.

The Eocene–Oligocene transition

Around 33.9 Ma, oxygen-isotope records show major cooling and Antarctic ice growth. Falling carbon dioxide was a central driver, while opening southern gateways helped establish circumpolar circulation. Ice expansion lowered sea level and reorganised habitats.

Oligocene environments

Cooler, drier and more seasonal conditions favoured open woodland and shrubland in many regions, although forests remained extensive. Grazing-adapted faunas did not instantly replace browsers. Teeth, pollen and soils record a mosaic of habitats.

Seven fossil windows

Direct observations include bone shape, tooth wear, pollen and layer position. Relationships, body mass and climate are inferences tested with multiple datasets. Colour, expression and the exact arrangement of animals remain artistic choices.