Jesse E. McCall, age 11, of Montgomery, Ala., for his question:
Why is the Cenozoic era called the Age of Mammals?
The Age of Reptiles ended with the Mesozoic era of the history of life on earth, some 60 million years ago. It was replaced by the Age of Mammals, which opened with the Cenozoic era and still goes on. During this era, the mammals advanced from small, mousy creatures to the fascinating variety of warm blooded, fur coated animals that populate our modern world.
The story of life on earth began more than 2 billion years ago and advanced slowly, step by daring Step to the teeming plants and animals that throng the earth today. If the long story were reduced to a year, the mammals would arrive in the last day and a half. The first of them were small. And mousy, and in a world dominated by fantastic reptiles their chances may have Seemed slim. Nevertheless, those frail mammals bid goodbye to the bulky dinosaurs. They survived and advanced to dominate the world.
When the Cenozoic era opened, Some 60 million years ago, the mammals had the earth to themselves. The world climate was tropical, and the earth was clothed with lush forests and Weedy swamps. There were tender grasses and hosts of plants bearing flowers and fruit. The early mammals had toes, five on each foot. They had assorted teeth and keen senses, though their small brains had little gray matter.
Life at first was easy, but this was soon to change. The climate cooled and the tender tropical foliage became tougher. Finally the world was gripped by a Series of bitter Ice Ages. These changes took millions of years, so the mammals had time to adapt themselves to meet them. The little horse ancestors developed tougher teeth and learned to run on tip toe. They grew bigger in time and the running toe on each foot became a hoof.
The ancestors of the cats and dogs, the pigs and elephants also improved their teeth and grew bigger. Ancient rhinos developed horns and cattle ancestors developed horns and hooves. As the story unfolded, most mammals added more gray matter to their brains. But not all of them survived. In the Ice Ages, there were shaggy mastodons and saber toothed tigers, giant armadillos and 5 ton, 20 foot sloths. For these bulky fellows, the long struggle to improve ended in failure. But many others succeeded and became the most advanced of all creatures. They adapted themselves to meet the changes and challenges of the Cenozoic era. This era, which still goes on, rightfully belongs to them as the Age of Mammals.
The first mammals would be out of place in our world. We would have trouble recognizing the ancestors of the cat and the dog, the ape and the bear that lived 20 million years ago. We would be startled by some of the mammals that shared the Ice Age world of our own remote ancestors, for the first human braves hunted the great mammoth and defended their families from the fangs and talons of the saber toothed tiger.