Harry Harrison 
West of Eden 
Eden Book 1

In this brilliant acclaimed novel dinosaurs still rule the Earth. The cosmic catastrophe that ended the Age of the Dinosaurs 65 million years ago never happened. The evolution of the great reptiles continued, climaxing with the Yilan , the most intelligent and advanced race on Earth. But when the onset of a new Ice Age forces them to cross the ocean to explore and colonize a vast contingent, they clash violently with a savage new breed of mammal that they have never before encountered. Mammals that walk erect, hunt and use crude stone tools – and weapons . . .