TL;DR - The Giants of Utah

    Scientific American, March 2012 -- "Dinosaurs of the Lost Continent," Scott Sampson

    Sampson details paleontological work that he and others have done in western North America, stretching from Alberta to Mexico, that suggests that in the Cretaceous era there were multiple (~ 17-20) species of giant (>1 ton) dinosaurs cohabitating.  Why this is surprising is because at the time, these dinosaurs were sharing a continent--Laramidia--that was significantly smaller than present-day North America, and which, based on understandings of current species, should have been too small to sustain that number of species.  As a comparison, current-day Africa is only able to sustain about 6 giant species.  Two main theories have been advanced to explain this: either the dinos were much more efficient than present-day mammals (possibly being cold-blooded, or maybe lukewarm-blooded), or the flora of the time was much more productive (the climate would have supported a hothouse environment at mid latitudes).