What triggered these great upheavals? scientific thinking about the various factors that might have been responsible for such replacements has undergo some signiticant changes of its own. times, for example scientists have suggested that mammals caused the extinction of the dinosaurs by eating their eggs or by competing for the same food resources. Now, however, most people are convinced that the mammals played only a minor role, if any (largely because 15 both groups had lived side for millions of years), and that, instead, environmental change was primarily responsible The mammals apparently sat around for 150 million years in the Mesozoic undergrowth until, with the extinction of the dinosaurs, they had their opportunity to radiate into the available niches.