Survival of the fittest

whereas birds had develop insulating feather from the had scales of their dinosaur ancestors grew, mammals grew fur from soft, porus skin. This was just as well because the world's climate grew progressively cooler over next 50 million years. Mammalian evolution also saw experiments with size and ferocity. There were giant rhinos measuring 4 m at the shoulder and weighing 15 tonnes, and huge deer, such as the Irish elk. Sabre-tooth tigers and their marsupial equivalents lived along side a host of other mammalian monters, such as the wolly mammoth of Europe and the mastodons of North America.