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The ascent of mammals and primates after the Dinosaurs disappeared

Posted in Animals, Dinosaurs, Prehistory, Wildlife on Wednesday, 18 December 2013

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This edited article about prehistoric animals first appeared in Look and Learn issue number 497 published on 24 July 1971.

Oligocene age animals, pcture, image, illustration

Animals from the Oligocene Age

Everything seemed to go haywire on earth 60 or 70 million years ago. During this broad span of time, the sea which split North America into two islands drained away.

A tremendous surging from the bowels of the earth erupted as glowing, crimson lava which cooled to become the enormous range of mountains we know as the Rockies.

North America was still joined to the British Isles by way of Iceland. Truly, the map makers would have had some amazing changes to record.

But the most amazing changes would have been noted by the zoologists. They would have had an unsolved mystery to ponder over. For, in this period, the lumbering, extraordinary dinosaurs, that had been the kings of the earth for over a hundred million years, rose to the height of their power and variety . . . and then, suddenly they became extinct!

Perhaps the shake-up the world had undergone created conditions in which they could no longer exist. Whatever the reason, their end brought the Mesozoic era to a close and opened the way for the New-life (or Cenozoic) era, which has come to be known as the age of mammals.

Palaeontologists (people who study extinct life) have divided the Cenozoic into several epochs, the last of which is the Holocene, the times in which we now live.

For many millions of years before the great reptiles became extinct, the ancestors of the mammals had been in existence.

They were mostly small animals, not much larger than shrews or rats. Living under the feet of the savage reptiles helped to sharpen their wits, enlarge their brains and (since they were warm-blooded and able to stand greater changes in temperature than were the cold-blooded dinosaurs), they could live in conditions of cold that the reptiles were unable to stand.

So it came about that when the dinosaurs died out, the mammals were fitted to take their place and expand into every corner of the earth.

By the time of the Palaeocene Epoch, which began about 63 million years ago, most of the ancestors of modern mammals had already come into existence.

The forerunners of the two great orders of Ungulates (Hoofed-mammals) were the Condylarths, while the Creodonts were the ancestral carnivores. Both groups were squat, sheep-sized animals and had five blunt claws on their fore and hind legs.

By the Eocene there were ancestral tapirs, rhinoceroses and a terrier-sized four-toed horse, Eohippus, the “Dawn-horse”, forerunner of all the mighty tribes of horses and their near relations, that evolved in North America.

The primates were represented by Lemur-like forms that had developed from little tree-living insectivores.

The Oligocene Epoch saw a great variety of rhinoceroses and one giant, the largest mammal ever to have lived in North America, the Brontotherium.

This epoch was marked by the wearing down of the Rockies, the debris being carried away and spread out across the continent to build great plains and park-like savannahs. Here grew the first grasses, the coming of which produced such ideal conditions for the herbivorous mammals, that they thrived and expanded in untold numbers.

Living on the plains favoured longer limbs and running on tip-toe rather than on the flat of the foot. This tended to shorten the side digits and throw the weight on to the centre toe. The abrasive stems of the grasses had an effect on the teeth which grew higher crowns and stronger enamel, thus changing the feeding habit from that of browsers on soft leaves and plants to tough grazers.

The most numerous of the grazers, in this and the following epoch (if one can judge by numbers of fossils left behind) was an extinct group of sheep-sized, long-tailed herbivores called Oreodonts. These, together with ancestral camels, three-toed horses, the first peccaries and primitive rabbits all shared the life-giving grass.

The carnivores also flourished in the shape of Hyaenodon, a heavily built hunter, who no doubt preyed on the Oreodonts, and Hesperocyon, a slender fox-like animal that probably ran down rabbits.

The epoch that followed, the Miocene, saw the peak of the mammal expansion. Never before had so many numbers and varieties of mammals roamed across the world.

Ancestral elephants, camels, dogs and smaller carnivores abounded, while vast arrays of strange – and now extinct – branches of mammals came into existence, flourished and passed away, leaving only their fossilized bones to give us some small glimpse of the extraordinary richness of prehistoric life.

Towards the end of the Pliocene Epoch, many of the old type mammals had died out and modern ones had taken their place.

With the Pleistocene Epoch, the time had arrived for man to make his appearance. His evolution however, did not take place in the Americas, but in the land masses of Africa and Eurasia.

About a million years ago, great ice sheets crept down from the North Pole to cover Canada and the northern part of America. In places, the ice was up to ten thousand feet thick. Four times at least, the ice advanced and retreated over the land, the last regression being close to eleven thousand years ago. Animal life moved with the changing climate or adapted to it.

One of the most spectacular lines of evolution was that of the elephants for, though they originated in Africa, they spread into North America during the Pliocene and so prospered that, in the next few million years, there were at least four species of elephants roaming across the continent, while in the north, majestic Mammoths kept company with the Woolly Rhinoceroses on the Arctic tundra.

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