<<<<br />insofaras large animals, a possibility, too, is that the earth's gravity was a bit less than it is today. allowing for larger animals to proliferate. under today's gravity, creatures of such sizes as the largest dinosaurs may not have been able to walk, stand, pump blood, anything. <br /> />>><br /><br />I don't think there's any evidence that the earth's gravity has changed to any material degree in a mere 65m years (which is after all only 1.4% of the earth's entire age), and we still have some large animals around today, not least the cetaceans.<br /><br />There are some theories about large animal evolution, the details of which I forget for now. However, if I recall, it's something to do with the metabolic rate of an animal being lower in proportionate terms to the animal's overall surface area. So there is some efficiency gained with size, provided there is fuel for the system (ie. food), and large animals also tend to live longer than small animals. In any case, there may be climatic reasons and food reasons why there are conditions which predispose evolution to larger animals at certain eras (eg. tropical conditions across the world and large amounts of plant growth).