Mammals inherit a warming world
Paleogene
Cenozoic Era · 66 – 23.03 million years ago
The Paleogene Period lasted from 66 to 23.03 million years ago, a span of about 43 million years that began in the wreckage of the dinosaur extinction. In that time mammals grew from mostly small survivors into horses, whales, bats, primates and the ancestors of many groups alive today.
It was also a period of climatic extremes. The Paleogene opened with some of the warmest conditions of the past 66 million years, including a sudden burst of global warming 56 million years ago, and ended with ice sheets spreading across Antarctica for the first time in this era.
Key facts
- Began
- 66 MaMillion years ago
- Ended
- 23.03 MaMillion years ago
- Duration
- 43 million years
- Era
- Cenozoic“Recent life”
- Global temperature
- ~23°CRoughly 18–28°C · today 14.5°C
- Plants on land
- Flowering plants
- Atmospheric CO₂
- ~1,000+ ppm early; much lower (~500–800) by the Oligocene
- Oxygen
- Close to today's ~21%
- Sea level
- Higher than today; fell several tens of metres ~34 Ma
- Continents
- India hits Asia; Antarctica isolated at the South Pole
- Signature life
- Early primates, Gastornis, early horses, first whales
- Began with
- Aftermath of the end-Cretaceous mass extinction
- Ended with
- No mass extinction; a brief glaciation ~23 Ma
Ages follow the International Chronostratigraphic Chart (ICS). Global temperatures are rough reconstructions after Scotese et al. (2021); ancient CO₂, oxygen and sea-level values carry large uncertainties.
Epochs of the Paleogene
- Paleocene66 Ma – 56 Ma
- Eocene56 Ma – 33.9 Ma
- Oligocene33.9 Ma – 23.03 Ma
Continents and oceans
The map of the Paleogene looks broadly familiar, but with important differences. The Atlantic was narrower, Australia and South America were still linked to Antarctica, and India was an island continent closing fast on Asia across the Tethys Ocean.
Around 50 million years ago India began colliding with Asia, the start of a collision that would eventually raise the Himalaya and the Tibetan Plateau. Africa pushed north into Europe, beginning to squeeze the Tethys and build the Alps.
In the south, Australia drifted away from Antarctica and the Drake Passage opened between South America and the Antarctic Peninsula, though the exact timing of both is still debated. These gaps eventually allowed a current to circle Antarctica, helping to isolate it from warmer waters.
Climate and atmosphere
About 56 million years ago, at the Paleocene–Eocene boundary, thousands of billions of tonnes of carbon were released into the oceans and atmosphere within a few thousand years. This Paleocene–Eocene Thermal Maximum (PETM) warmed the planet by an estimated 5–8 °C, acidified the oceans and lasted roughly 150,000–200,000 years, making it a key natural analogue for modern climate change.
After the warmth of the early Eocene, the world cooled in fits and starts. At the Eocene–Oligocene transition about 34 million years ago, falling carbon dioxide and changing ocean gateways helped a large ice sheet grow on Antarctica. Sea levels dropped, and the planet moved from a greenhouse into an icehouse state that, in one form or another, has continued ever since.
Life in the Paleogene
With the large dinosaurs gone, mammals and birds diversified rapidly into empty ecological niches. Most of the major groups of modern mammals appear in the fossil record during the Paleogene, often first as small, generalised animals.
Giant birds and snakes
For a while, birds and reptiles rivalled mammals as large land animals. Gastornis, a flightless bird around 2 metres tall, roamed Europe and North America, and Titanoboa, a snake perhaps 13 metres long, lived in the tropical rainforests of what is now Colombia.
Early primates and horses
The first undoubted primates appear around the PETM, alongside early horses and the ancestors of deer and cattle. Many of these animals were small; early horses such as Eohippus were roughly the size of a fox.
Whales take to the sea
Whales evolved from land-dwelling hoofed mammals related to hippos. Pakicetus, about 50 million years old, lived on land near water, while by about 40 million years ago Basilosaurus was a fully marine predator up to around 18 metres long with tiny hind legs.
Giants of the Oligocene
In the cooler, drier Oligocene, forests gave way to more open woodland in many regions. Here lived Paraceratherium, a hornless rhinoceros relative and one of the largest land mammals ever, which may have weighed around 15–20 tonnes.
Timeline of key events
- 66 Ma
Life recovers after Chicxulub
Mammals, birds and flowering plants begin to refill ecosystems emptied by the end-Cretaceous extinction.
- 56 Ma
PETM global warming
A massive release of carbon warms the planet by roughly 5–8 °C and marks the start of the Eocene.
- 50 Ma
India collides with Asia
The collision begins that will eventually build the Himalaya, while early whales such as Pakicetus live along the shores of the Tethys.
- 47 Ma
Messel lake fossils form
A volcanic lake in Germany begins preserving early horses, primates and birds in extraordinary detail.
- 40 Ma
Basilosaurus rules the seas
Fully marine whales such as Basilosaurus hunt in the warm Tethys Ocean.
- 33.9 Ma
Antarctic ice sheet forms
At the Eocene–Oligocene transition, global cooling allows a continental ice sheet to grow on Antarctica.
- 23.03 Ma
Paleogene ends
A brief glaciation marks the passage from the Oligocene into the Miocene and the Neogene Period.
How the Paleogene ended
The Paleogene did not end in catastrophe. Its close, 23.03 million years ago, is defined by a reference point in rocks in northern Italy and roughly coincides with a short-lived expansion of the Antarctic ice sheet. Life passed into the Neogene with mammals already established as the dominant large animals on land.
Where to see Paleogene fossils
The Messel Pit near Frankfurt, a UNESCO World Heritage Site, preserves 47-million-year-old early horses, primates, bats and birds, sometimes with fur, feathers and stomach contents intact. The Green River Formation of Wyoming, including Fossil Butte National Monument, is famous for its countless fossil fish, and the London Clay of the Isle of Sheppey yields tropical seeds and fruits.
In Egypt, Wadi Al-Hitan, the Valley of the Whales, holds hundreds of skeletons of Basilosaurus and other early whales in the open desert. Badlands National Park in South Dakota showcases Oligocene mammals, and Riversleigh in Queensland records Australian life from the late Oligocene onwards.
Surprising facts about the Paleogene
- During the warm Eocene, forests of dawn redwoods grew on Arctic islands such as Ellesmere and Axel Heiberg, home to turtles, alligator relatives and early primates.
- About 49 million years ago, the freshwater fern Azolla appears to have blanketed the surface of the then partly enclosed Arctic Ocean, and some scientists think it helped draw down carbon dioxide.
- Whales' closest living relatives are hippos — a link supported both by DNA and by distinctive ankle bones in early whale fossils.
- The Messel Pit has preserved a pair of turtles fossilised while mating, as well as fossil insects that still show their original iridescent colours.
- Around 34 million years ago, many European mammals disappeared and were replaced by immigrants from Asia, a turnover so abrupt that it is known as the Grande Coupure, or 'great break'.
- Gastornis was long pictured as a fearsome predator, but studies of its beak and bone chemistry now suggest it was more likely a plant-eater.
Where it sits in deep time
The globe beside this article shows Earth 50 million years ago, reconstructed from the PALEOMAP paleo-elevation models. Step to another period and watch the continents drift into place.