There seems to be a growing consensus that the number of dinosaur species was already in decline before the great asteroid impact that ended the Cretaceous era 66 million years ago. As Science News reports, as of about 50 million years before the mass extinction the number of new dinosaur species was being eclipsed by the number going extinct and dinosaur diversity was decreasing. Duck-billed and Triceratops-type dinosaurs were doing well until the end of dinosaur days as was a group of small toothed raptors. But ultimately, only avian dinosaurs – the birds – survived.
Why did the number of dinosaur species decline over time and why did only avian dinosaurs survive? The dinosaur decline might have been due to climate change perhaps brought on by continental drift and the resulting land-form, rainfall and ocean current alterations from the late Jurassic onward. Perhaps only birds survived the long “nuclear-type” winter after the impact because they could eat carrion and seeds, of which there might have been much. Some small non-avian dinosaurs also could have been able to do the same but they might not have been able to travel long distances. Perhaps only a small number of birds – even just a few species – made it through on remote islands and as the earth recovered, they could spread. The land-bound non-avian dinosaur survivors – if any – might not have been able to reach places where their numbers could then rebound.
But what if there was no impact or somewhere creatures like the small raptors made it through? Carnivorous tyrannosaur- and velociraptor-type dinosaurs (theropods) were doing well at the end of the Cretaceous. Indeed, it may be that the hundred million year-plus competition between carnivores and herbivores had led to the evolution of a lesser number of species but ones ever more evenly matched. Some of the largest herbivores and carnivores ever were alive at the end. And it may have been that the carnivores were getting smarter, perhaps even hunting in packs. (The herbivores apparently had long been herd animals.) Seems the smaller theropods – like Troodon – were the (relatively) smarter ones. It is interesting to speculate how earth's evolutionary processes might have played out differently if at least some of these non-avian theropods had survived the great impact. With another 66 million years of evolutionary competition, might they have gotten even bigger brains, as primitive primates eventually did. Or perhaps I was just too impressed at an early age with the Gorn captain forced into combat with Captain Kirk.