The Quaternary period saw the global temperature continue to fall, culminating in what is known as the Ice Age. And even the glacial periods themselves are divided into colder phases and temperate interstadials . Biosphere During the Pleistocene the age of mammals was at its height. Mammals included both small forms and giant ones. The giants are referred to as the megafauna .
Evolution proceeds rapidly, stimulated by climatic and environmental change.
Curiously, the megafauna disappeared at the end of the Pleistocene and the start of the Holocene. The only way to think of how it was at the height of the age of mammals would be to imagine the magnificent wildlife of the Africa game reserves, but everywhere all over the globe. Exactly why such large animals and successful and highly diverse animals should all of died out at the end of the Pleistocene is a mystery. Another has them hunted to extinction by early man - a sort of stone age overkill. Which fails to explain how animals even more heavily hunted, like reindeer and horses, survived into the modern age.