Were dinosaurs already headed for extinction before the asteroid hit?

By Graham Templeton 

One question that often occurs to people studying evolution is this: if evolution is so great, and so powerful, how come virtually no dinosaurs survived the asteroid impact? Sure we’ve still got crocodiles and modern birds, but none of the monsters we think of as dinosaurs were able to make it through the asteroid-induced global climate disaster. Sure, they were big and suited to a hot environment — but why were there so few real survivors from among the dinosaur population? New research published in the Proceedings of the National Academy of Sciences suggests that the answer could be a perfect storm for dinosaur extinction. When the asteroid hit, it may have simply accelerated an already oncoming apocalypse for the dinosaurs.

The question of whether dinosaurs truly “ruled the earth” right up until the asteroid impact some 65 million years ago is an old one, and controversial. It’s difficult to judge based solely on fossil evidence, since you often can’t be sure that you have more variety of skeletons from one era because that era had more variety of life or simply because more skeletons happen to survive. But after so many years of collecting and cataloging dinosaur remains, some very complete family trees have been constructed. This team set out to take the latest such dinosaur family tree and do some statistical analysis to see the rate of new dinosaur introduction.

Their findings indicate that by the time the asteroid hit, dinosaur species were already dying out much faster than they were being replaced with new ones. In fact, they’d already been in that state for more than 24 million years by that point. Times were tough and diversity was down, so as a result the group’s overall ability to deal with a major disaster was lowered.
Actually, the researchers break down the fates of different dinosaur by type, and found that in spite of the overall trends the so-called duck-billed and horn-faced dinosaurs were doing quite well. They suggest that the development of flowering plants could have provided a new food source that allowed them to maintain their diversity, while others died out. Sauropods like diplodocus were in a virtual freefall, while therapods like the legendary T-Rex were doing better, but still declining.

Now, this doesn’t necessarily mean that the number of dinosaurs went down in this way, just the number of species. So if all sauropods but diplodocus died out, but this led to an explosion of the diplodocus population to some 20 billion individuals, we would still count that as a horrifying loss of diversity. Looking at the overall fate of a group of species, that’s the better measure to use.

Why were they declining, seemingly having lost their ability to form new species fast enough to offset the loss of species to the natural passage of time? There are many theories. Competition for resources with the newly emerging order of mammals is one possibility, as they spread across the world with their demanding but rugged warm-blooded metabolism. Another possibility is continental drift, slowly moving more and more of the world away from a dino-centric climate. As a younger planet, the Earth was also undergoing rapid natural climate change due to sustained volcanic activity, and changing sea levels.

In other words, the dread asteroid of the Chicxulub impact may just have been a final, fatal kick in the ribs to an evolutionary clade that had been down for a very long time before.