The classical view of dinosaurs presents a perplexing problem. The group of vertebrates which dominated the land before the rise of the dinosaurs were the synapsids, the mammal-like reptiles… Most paleontologists have believed that the locomotion and physiology of these mammal-like synapsids were more similar to those of active, warm-blooded mammals than to sluggish modern lizards or alligators. Surprisingly, though, when the first dinosaurs and their near relatives appeared in the Triassic period, the synapsids began to decline and soon became extinct. The dinosaurs then ruled the land unchallenged for over 100 million years while the early mammals, the surviving descendants of the synapsids, remained very small in size and number. Only after the dinosaurs suddenly disappeared about 70 million years ago did the mammals develop into the great variety of dominant land vertebrates we have today. The problem is this: if the later synapsids were such splendidly advanced animals with the improved physiology of mammals, and if dinosaurs were slow and sluggish, why were the mammal-like synapsids exterminated in competition with the first dinosaurs? And why didn't the mammals achieve a more significant diversification during the dinosaurs' reign?
One might expect that mammals would have taken over the land vertebrate communities immediately, but they did not. From their appearance in the Triassic until the end of the Cretaceous, a span of 140 million years, mammals remained smal and inconspicuous while all the ecological roles of large terrestrial herbivores and carnivores were monopolized by dinosaurs; mammals did not begin to radiate and produce large species until after the dinosaurs had already become extinct at the end of the Cretaceous. One is forced to conclude that dinosaurs were competitively superior to mammals as large land vertebrates. And that would be baffling if dinosaurs were "cold-blooded."
Without doubt the most dangerous devices for active defense among the Dinosauria emerged in Triceratops. The scene has been portrayed in paintings, drawings, and illustrations hundreds of times, but it remains thrilling. Tyrannosaurus, the greatest dinosaur toreador, confronts Triceratops, the greatest set of dinosaur horns. No matchup between predator and prey has ever been more dramatic. It's somehow fitting that those two massive antagonists lived out their co-evolutionary belligerence through the very last days of the very last epoch in the Age of Dinosaurs. Tyrannosaurus stood over twenty feet tall when fully erect, and a large adult was as heavy as a small elephant — five tons. No predatory dinosaur, no predatory land animal of any sort, had more powerful jaws. Withstanding a Tyrannosauruss attack required either tanklike armor – the approach taken by Ankylosaurus