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.