...But why did mammals prevail and dinosaurs die? Doesn't this fact point to some intrinsic mammalian superiority? Not necessarily. We do not know the answer, but here is one plausible scenario for a partial explanation: the rules change in mass extinction, and adaptive advantages of the past may become dangerous deficits. Large populations provide a good hedge against extinction, all other things being equal. Dinosaurs, with their massive bodies, must have maintained species of small population size. The world must contain far fewer elephants than ants, far fewer brontosauruses than mouse-sized mammals. So perhaps mammals gained a crucial edge by large populations maintained as a consequence of small body sizes.
Now why were mammals small? Surely not because they knew that a comet would hit 10 million years down the road, and that large populations would then be useful. Presumably they were small for a negative reason in Darwin's immediate world of competition: because dinosaurs had usurped the ecological space of large terrestrial vertebrates, and relegated mammals to a periphery. Yet the reasons for relative failure in normal times may translate fortuitously to the crucial ingredient of success in prevailing through a mass extinction.
Stephen Jay Gould, The Confusion over Evolution , "The New York Review of Books" (1992)
Stephen Jay Gould "The Structure of Evolutionary Theory" (2002)
reviewed by Gert Korthof (updated 14 May 2006).
NSF: Ecosystems With Many Plant Species Produce More and Survive Threats Better