Age of Mammals
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 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.
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 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 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 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.
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.
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.
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.
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 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.
Consultant: Chris Jarvis
See also in Prehistoric
See also in Earth