Palaeogene Earth – The Rise of Mammals

Palaeogene Earth

66 to 23.03 Million Years Ago

Common mammals of the Palaeogene include many now-extinct orders such as hyracodonts, condylarths, nimravids and entelodonts.

Like the devastating Permian-Triassic mass extinction long before it, the boundary between Mesozoic and Cenozoic eras was marked by a catastrophic event that wiped out most life on Earth. The dinosaurs were gone, and a new era dawned upon a devastated world, but a world that was still abundant with opportunity. In Part 13 of the “Journey through the History of Earth” series, we’ll be exploring the rise of mammals and how they came to fill the ecological niches left behind by the dinosaurs.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

The world transformed profoundly at the close of the Cretaceous Period. The non-avian dinosaurs, which had ruled over every terrestrial ecosystem on the planet for 165 million years, had gone. Similarly, gigantic marine reptiles no longer prowled the Earth’s oceans, and pterosaurs no longer fleeted through the skies. Ecological niches the world over were left empty, rife for exploitation as the global climate recovered. Now was the time of the mammal.

Marking the beginning of an entirely new geographic era, the Palaeogene Period was the first of the Cenozoic Era, the era that persists to this day. Meaning ‘ancient-born’, the Palaeogene comprises three periods, the Palaeocene, Eocene and Oligocene. Spanning these epochs, the world started to resemble that which we know today. Most modern mammal genera evolved during this time as well, including our own primate ancestors.

Highlights of the Palaeogene

  • Major radiation of mammals
  • Evolution of the first primates
  • Himalayas form
  • Grasslands expand globally
  • First permanent polar ice sheets form
  • Major impact event in North America

Only the Strongest Survive the Cretaceous-Palaeogene Impact Winter

Titanoides was a pantodont, an extinct mammalian suborder whose ancestors survived the Cretaceous-Palaeogene extinction event. It was one of the largest mammals of its time when it lived during the Late Palaeocene.

Following the impact of a city-sized rock from outer space, our world was left reeling in the shadow of death and destruction. The boundary between the two eras is partly defined by the presence of large amounts of iridium in the rock strata of that time. This element is incredibly rare on Earth, but it is abundant in certain asteroids. Combined with the discovery of the Chicxulub Crater in the Yucatan, the evidence for what happened on that fateful day is overwhelming.

A study by the Geological Society of America suggests that the first few decades of the Palaeogene Period were marked by a devastating impact winter that saw global temperatures plummet as dust kicked up into the atmosphere blocked much of the sunlight.

There’s little doubt that the world was a desolate place at the start of the Palaeocene Epoch (not to be confused with the Palaeogene Period, of which the Palaeocene is a part). Almost nothing larger than a sheep survived, and as the last of the dinosaurs (and numerous other species) disappeared, only the smallest and most resilient scavengers and highly adaptable creatures managed to avoid succumbing to starvation or freezing to death.

Life has proven its incredible capability to adapt time and again throughout its near 4 billion years of existence on Earth. The first few million years of the Palaeocene were no exception. While animals remained, for the most part, small during this time, some important new orders evolved. Among these were the bizarre mesonychids and pantodonts and various new lineages of birds. Life also started to recover in the oceans, particularly among molluscs and reef-building organisms.

Slowly but surely, life recovered in the form of a profound proliferation of mammals and birds as the climate warmed once again and sea levels dropped to create vast swathes of virgin land.

Mysterious Animals Radiate into Vacated Ecological Niches

Ptilodus was a multituberculate, a highly successful group of mammals that evolved alongside the dinosaurs and survived the mass extinction.

Throughout the Palaeocene Epoch, mammals remained small for the most part. As such, relatively little is known about them, due to the fact their fossil remains are scarce. Nonetheless, many important new mammalian orders evolved during this time, including several that are now long extinct and quite unlike anything alive today. Additionally, many mammals of the time were monotremes or marsupials, which today are only found exclusively in Australia and Papua New Guinea.

By far the largest and most successful order of mammals throughout the Paleogene Period were the multituberculates. These strange rodent-like creatures, having evolved during the Jurassic, were contemporaries of the dinosaurs and appear to have been completely unaffected by the Cretaceous mass extinction. An incredibly diverse order, more than 200 individual species have been known, ranging from the size of a mouse to the size of an otter. They started declining during the middle of the Palaeogene Period (the Eocene Epoch), mysteriously disappearing entirely around 35 million years ago.

The Palaeocene also saw the evolution of the first placental animals which, of course, continue to dominate to this day. Among them were the very first primates, hoofed animals and proboscideans (elephants, mammoths and mastodons). There were also several other now-extinct orders of placental mammals, such as the bizarre cat-like condylarths.

GastornisTim Bertelink

The gigantic gastornis was a carnivorous flightless bird that lived 45 to 56 million years ago in what is now Western Europe. 6 species have so far been identified. Depicted here is gastornis gigantea, which grew up to 6.6 feet (2 metres) high.

Mammals weren’t the only animal class exploiting the environmental niches left empty by the demise of the dinosaurs. Birds also started undergoing an unprecedented period of diversification throughout the Palaeocene. Among the most impressive were the huge flightless carnivorous birds, such as gastorinis, which lived in Europe until around 45 million years ago. Meanwhile, in South America, the ten-foot-tall (3 metres) phorusrhacids, also suitably known as terror birds, ruled. These monstrous birds thrived for over 60 million years, only disappearing 1.8 million years ago well into the Quaternary Period.

Early Ancestors to Many Common Mammals Appear

HyracodonHeinrich Harder

Hyracodons filled a similar niche to that of modern-day horses, although they were not closely related, instead forming a separate and now long-extinct order.

The Eocene is the second of the three epochs that make up the Palaeogene Period. Lasting from 56 to 33.9 million years ago, this is the time when many modern groups of mammals started appearing. Of particular significance were the appearance of perissodactyls (the order includes includes horses, rhinoceroses and tapirs) and artiodactyls (the order that includes giraffes, pigs, deer, sheep and cattle).

These early ancestors to common mammals were quite different to what we’re familiar with today for the most part, but you could probably still recognize the similarity. For example, the first horse, hyracotherium, was a dog-sized grazing animal that lived some 56 million years ago.

Other perissodactyls that joined the earliest equine ancestors of the Eocene were the hyracodons, a mammalian family of hornless rhinoceroses that went extinct in the early Neogene Period. Fulfilling a similar ecological niche to that of horses and other grazing animals, these creatures enjoyed a vast geographic range spanning the entirety of Eurasia.

Ambulocetus, the ‘walking whale,’ an early cetacean, illustrates how whales evolved from other artiodactyls, a group of land-dwelling mammals that include camels, llamas, giraffes, sheep, goats and cattle.

Another highly significant evolutionary event occurred during the Eocene, and that was the migration of certain mammals to the oceans. The first whales evolved from artiodactyls, an order of herbivorous hoofed animals. These cetacean ancestors started life exclusively on the land, eventually turning to coastal waters for sustenance. While the first ‘whales’ walked on four legs, their forelimbs turned to flippers to better adapt to an aquatic lifestyle. The earliest of these strange creatures, such as pakicetus, didn’t exactly resemble a modern whale but, by the end of the Eocene, cetaceans had become fully aquatic.

Global Cooling Forms New Environmental Niches

The Eocene was a time of global warming when scenes like this could be found as far south as the Antarctic Peninsula. The large animal in the background is a brontothere, a distant relative of the horse.

While the Eocene Epoch was a time of global warming and vast swathes of subtropical forest, the climate started to change dramatically in the Oligocene. So profound were the changes in fact that it lead to a mass-extinction event. Though minor compared to that which claimed the dinosaurs, the Eocene-Oligocene extinction event was no doubt greatly accentuated by global cooling.

The abrupt shift in the evolution of Earth’s climate at the end of the Eocene may have been caused by volcanic activity or one or more impact events or, perhaps, even a combination of the two. One compelling piece of evidence for the impact theory is the existence of a 39-million-year-old impact crater in the remote Devon Island of the Canadian Arctic. Another suspect is the 35.5-million-year-old Chesapeake Bay impact crater off the eastern coast of the U.S.

Until the Oligocene Epoch, Antarctica was largely free of any permanent ice. In fact, until about 52 million years ago, the Antarctic Peninsula was decidedly warm, so much so that subtropical plants like palm trees could grow there. However, after the intense phase of global warming in the early Eocene, the global climate started to cool rapidly. For the first time in hundreds of millions of years, the South Pole froze over, and permanent ice sheets covered Antarctica.

As Antarctica changed from a lush forested land into an icy desert, the oceans cooled globally which, in turn, would have led to a significant drop in global temperatures. Consequently, grasses, which are more tolerant of lower temperatures, expanded across much of the globe effectively forming the first savannahs and steppes.

Open Landscapes Herald a New Land of Giants

Paraceratherium, also known as indricotherium, was the largest terrestrial mammal that ever lived.

Throughout the Oligocene, mammals dominated almost every terrestrial environmental niche on Earth. Sprawling open landscapes, similar in many ways to the steppes found in Central Asia and Mongolia today, formed an entirely new biome that had never existed before. This allowed mammals to grow larger and more diverse than they ever had. A new land of giants formed among the wide-open spaces of Eurasia, North America and beyond.

It was during the Oligocene that the largest terrestrial mammal that has ever walked the Earth evolved. Paraceratherium was a hyracodont that dwarfed even an African elephant. This colossal titan was distantly related to modern-day rhinoceroses, though it stood well over three times higher and may have weighed up to 30 tonnes. Like most hyracodonts, it didn’t have horns, instead relying on its immense size for protection.

EntelodonHeinrich Harder

Popularly known as ‘hell pigs’ entelodons actually formed a separate order of their own. They were omnivorous predators that lived across Eurasia.

While enormous, paraceratherium was just a peaceful grazing animal found across Eurasia. Some of its contemporaries, however, were every bit as terrifying as the carnivorous dinosaurs. One of the most successful apex predators of the Oligocene was the aptly-named hell pig. Formally known as entelodonts, they were artiodactyls but not closely related to pigs. The largest of this diverse order was the formidable daeodon, meaning ‘dreadful teeth’. The hulking boar-like omnivore stood up to six feet (1.8 metres) tall at the shoulder.

Despite their informal title of ‘bear dog,’ amphicyons were not closely related to either, instead forming their own family within the carnivoran order of mammals.

Another family of predators that thrived during the Oligocene were amphicyonids. Also known as bear dogs, the largest weighed up to 1,300 lb (600 kg). As the name suggests, they superficially resembled something of a cross between a dog and a bear. However, while they share the same order, they were not otherwise related, instead forming a now long-extinct family of carnivores.

Proailurus, which appeared in the late Oligocene Epoch, is the earliest known true cat. It was likely a primarily arboreal animal.

The first true cats also appeared towards the end of the Oligocene. Proailurus, meaning ‘first cat’, lived some 25 million years ago throughout Eurasia, and was little larger than a domestic cat. It evolved down a separate path to that of the closely related and now-extinct nimravids, or false sabre-tooth cats.


By the end of the Palaeogene Period, modern ecosystems were starting to take over. Having undergone extensive global cooling throughout the Oligocene, the following Neogene Period would see the world become remarkably like the one we know today. Our own human ancestors would soon evolve from other primates, and animals would start to resemble their modern counterparts. Next week, we’ll be exploring the rise of the hominids and the proliferation of modern animals across the globe.

Part 14: Neogene Earth – Human Ancestors

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