Age of Mammals

Age of Mammals

Mammals scurry among a dinosaur's bones.Mammals scurry among a dinosaur's bones. By the time the dinosaurs became extinct, several different groups of mammals had already evolved around the world. The first mammals evolved during the Late Triassic Period. They were furry, warm-blooded animals that probably laid eggs. But while dinosaur predators were about, most mammals remained tiny, shrew-like animals, venturing out of their burrows only at night. The extinction of the dinosaurs enabled mammals to become the dominant land animals. The period after the extinction of dinosaurs, known to scientists as the Palaeogene Period, is sometimes known as the Age of Mammals—although birds also developed into a wide variety of species, including some terrifying predatory giants.

Geological time scale

Geological time scaleGeological time scaleAfter the dramatic extinctions at the end of the Cretaceous Period, 66 million years ago, the Earth entered what is known as the Palaeogene Period. Until recently, geologists referred to the time after the Cretaceous as the Tertiary Period, but this has now been divided into two and renamed the Palaeogene Period, comprising the Palaeocene, Eocene and Oligocene epochs, and the Neogene Period, comprising the Miocene and Pliocene epochs.

The Quaternary Period runs from about 2.6 million years ago to the present. The Pleistocene Epoch occupies all but the last 12,000 years of it. The most recent epoch, the Holocene, began about 12,000 years ago. 

Cretaceous (1), Eocene (2) and Quaternary (3) worldCretaceous (1), Eocene (2) and Quaternary (3) world

The break-up of the continents

During the Triassic Period—at the time when the first mammals appeared—all the continents had been locked together in one huge “supercontinent” called Pangaea. But the continents were on the move again at the beginning of the Jurassic, about 200 million years ago, and have continued their drift ever since. This meant that the very early mammals, which had spread out simply by walking from continent to continent, became stranded as the world split up into several large landmasses separated by seas. So the mammals started to evolve into different groups in isolation.


Evolution of mammals

Prehistoric mammalsPrehistoric mammalsThe mammals started adapting to the new environments in which they found themselves, so by the time the dinosaurs disappeared off the face of the Earth at the end of the Cretaceous, 66 million years ago, several different groups of mammals had already evolved worldwide. At this time, no large carnivorous mammals existed. Over the course of the Palaeogene and Neogene periods, thousands of different mammal species came into existence, including a wide variety of carnivores. 

Ambulocetus, an Eocene whaleAmbulocetus, an Eocene whaleWhales and dolphins, for example, lost the fur and four legs of their ancestors and developed tails, fins and flippers to move more easily through the water. Many ungulates developed efficient digestive systems to get the maximum goodness from tough grasses. Giraffes evolved very long necks to browse from the tallest trees
which other mammals could not reach. 

Evolution of birds

Gastornis preys on a small horseGastornis preys on a small horseBirds as well as mammals flourished in the Palaeogene. After the dinosaurs died out, some became large predators, standing up to 2 metres (6.6 feet) tall. They were flightless, their small wings acting only to balance their heavy bodies. Fearsome-looking creatures such as Andalgalornis and Gastornis may have seized mammals—including even small horses—in their powerful jaws (although some scientists think they might have been plant-eaters, their beaks adapted to crush nuts and seeds). Other birds, such as Argentavis, a vulture with a 7-metre (23-foot) wingspan, were scavengers.

North Africa at the end of the Eocene EpochNorth Africa at the end of the Eocene Epoch

Life in the forests

During the Palaeocene (66–56 million years ago) and Eocene (56–34 million years ago), world climates were warmer than they are today; tropical rainforest was widespread, even at the poles. Mammals that were best suited to moving amongst dense trees dominated.

Forest-dwelling mammals included a number of species that look unfamiliar to us today. Some roamed the forest floor seeking plant stems or roots. Others, including the rodents and primates, browsed for leaves and fruits in the trees themselves. Andrewsarchus was an early carnivorous giant that lived during the Eocene. 

Meanwhile, in the oceans, some fish-eating carnivores developed the ability to spend more time in the water, and evolved into the group we call the cetaceans: whales and dolphins.

Andrewsarchus feasts on a carcassAndrewsarchus feasts on a carcass

Hapalops, a ground sloth Hapalops, a ground sloth

Changing environments

During the Oligocene Epoch (34–23 million years ago), the climate began to cool. Ice caps formed at the poles and many dense forests became more open woodland. The forest-dwellers gave way to much larger mammals that thrived in the new conditions. They included both leaf-browsing forest herbivores and carnivores that preyed on them. Among the browsers were Paraceratherium, a massive rhinoceros from Central Asia.

A scene from Oligocene Central AsiaA scene from Oligocene Central Asia


Life on the grasslands 

By the Miocene (23–5 million years ago), great swathes of grassland had replaced forest in many parts of the world. Grass is very difficult to digest, so mammals evolved in order to be able to feed on it. Some mammals, for example, developed tough, grinding teeth and a very long digestive system to help break the grass down and release its nutrients.

Grass-eating mammals such as horses and antelopes thrived, but they needed to be quick on their feet because the lack of tree cover made them highly visible: easy prey for powerful carnivores such as wolves, hyenas and huge sabre-toothed cats. Massive elephants, adapted to feeding from trees on grasslands, evolved at this time as well. 


Mammal globetrotters

In the drier climate of the Miocene, sea levels fell everywhere, land “bridges” started to link the island continents, and mammals, once again, were able to cross from one continent or island to another.

Rodents and monkeys for example, travelled from Africa to South America, "hopping" from island to island that emerged from the sea in between the two continents, still lying relatively close together.

The Great American Interchange

During the Pliocene Epoch (5–2.6 million years ago) continental drift brought North and South America together. The two continents finally collided at the narrow Isthmus of Panama about 2.5 million years ago to form the single land mass that exists today. Mammals from North America could now travel overland south across the narrow Panama “land bridge” into South America, while mammals from South America could work their way northwards into North America. 

Thylacosmilus, a marsupial sabre-toothed predatorThylacosmilus, a marsupial sabre-toothed predatorBoth groups found themselves competing for food and territory with unfamilar “relatives”. They also faced new predators. Some kinds died out altogether as a result: they included all the South American ungulates and marsupial carnivores.

ConsultantChris Jarvis

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Paraceratherium was the largest land mammal of all time. This prehistoric rhinoceros measured 8 m (about 25 ft) from head to tail and stood around 5 m (16 ft) tall at the shoulder.

Armadillos, opossums and porcupines that live in North America today are all descendants of animals that travelled from South America during the Great American Interchange and survived. Most southern species failed to do so. Camels, tapirs, horses, elephants, cats, dogs and bears all travelled into South America from the north.

Giant, flightless, predatory birds also evolved in South America. They are known as "terror birds", and lived from 62–2 million years ago.

The largest flying bird the world has ever seen, the condor-like Argentavis, soared in the skies of late Miocene South America, about 6 million years ago. It had a wingspan of about 7 m (23 ft).

During the Palaeogene and Neogene periods, several previously isolated landmasses drifted together. India collided with Asia (50 million years ago), Africa linked to Asia (30 million years ago) and North America and South America joined at the Isthmus of Panama (3 million years ago).

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