Which dinosaur was the most common?

From the fossil record of today the most abundant types of dinosaur known seem to be the hadrosaurs or 'duck-billed' dinosaurs of North America. In one area of Montana the archaeologist Jack Horner has discovered a layer of rocks which measures several kilometres square and is packed with the remains of the dinosaur hypacrosaurus. He has estimated that there may be between 10 and 40,000 Hypacrosaurs in this area alone, and has suggested that they may have been part of a herd that may have been overcome by poisonous fumes given off by a volcano in the vicinity.