after reading parts of Michael Denton's Nature's Destiny I would speculate that God has designed every possible form of creature that could possibly exist within the confines of the laws of physics (and the laws of physics need to be exactly as they are to support human life as explained there). Denton says there that there is a limit to the size of an animal due to various factors such as the speed of nerve signal transmission which is determined by the diffusion rate of ions. the current values for these are crucial to life. He explains there that for an animal of 100 meters it would take roughly 4 seconds for certain nerve impulses to travel and that would be way too slow for the animal to be nimble. So perhaps the dinosaurs were a hashlama (completion) to the possiblle types of life forms that could exist on earth as part of the perfection of nature. Only that they needed to no co-exist with us for safety reasons.