Many believe that, absent the giant asteroid strike, dinosaurs would still dominate the earth. But, in the natural history of this planet, there have been five mass extinction events, and most did not require extraterrestrial intervention. A new paper argues that dinosaurs were already headed for extinction and that the asteroid impact 66 million years ago was more of a coup de grâce.
Before any asteroid hit the earth, the late Cretaceous period witnessed radical change. The earth’s climate has always oscillated between cooler and warmer periods, and it appears that in the millions of years before the dinosaurs went extinct, it was cooling rapidly. (Well, rapidly in geologic terms.) Most of this period had been comparatively warm, with volcanic activity creating greenhouse gases that raised temperatures. But then the earth began cooling.
(...) This hit dinosaurs pretty hard. Dinosaurs likely had poor thermoregulatory controls (being “mesothermic,” a halfway point between cold- and warm-blooded), which meant that they relied in part on external temperatures to keep their bodies warm enough to function properly. If the temperature did drop as scientists now think, the larger dinosaurs especially would have had no way to regulate their temperature and would have gradually weakened and died.
(...) Of course, climate change also would affect vegetation and food availability. The fossil record shows that just before the extinction event, tropical vegetation was giving way to woodland plants. Such a change in flora would mean that a very particular subgroup of dinosaurs would flourish, at the expense of others. And the ones who were predominantly the winners in this were the “hadrosaurs”.
Hadrosaurs are basically very large herbivores, and they took to this new environment with gluttonous glee. But, food chains are incredibly complex, and hadrosaurs played a crucial role. Out-eating and out-competing other herbivores, like the triceratops, meant that not only did triceratops die out but so did their carnivorous predators, like tyrannosaurs. No species is an island, and the presence or absence of a keystone species can cause the entire network to break down.
In short, changing climate meant changing vegetation, which in turn caused chaos in the very complicated and sensitive food web. The late-Cretaceous period was great for hadrosaurs, but most other species of dinosaur were likely on the road to extinction already.
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