What killed the dinosaurs? Ok, ok, we know an asteroid did it, roughly, but that made a comparatively small hole in the ground — what actually killed them? We've heard lots about nuclear winter, global wildfires, all sorts of poisonous gases, and combinations thereof. But what if it was something far more mundane. What if it was ozone?

At the start, it seems hard to make a potent mass murderer out of a gas that is just three little oxygen atoms bounds together. And we all know about the ultraviolet-shielding effect of the ozone layer high in the stratosphere. Very helpful for life, that is.

But problems start when you bring O3 down near the surface. Mix together a soup of hydrocarbons, nitrogen oxides, and sunlight (the first two make up your basic car exhaust) and presto, we can find ourselves swimming in the stuff, especially in urban areas. No one knows what safe levels are, but medical studies suggest that lung tissue gets inflamed and damaged quickly at or around 100 parts per billion of O3.

What's that got to do with dinosaurs? Maybe a lot.

A new study in the journal Paleogeography, Paleoclimatology, Paleoecology puts forth the idea that the Chicxulub impact, long blamed for the extinction of the dinosaurs at the end of the Cretaceous era 65 million years ago, could have done them in by flinging huge amounts of ozone precursor chemicals — nitrogen oxides, methane, and other hydrocarbons — into the air.

According to the researchers' simulation, the impact could have produced enough ozone to raise concentrations in the atmosphere to over 1,000 parts per billion (or 1 part per million), about 10 times the dangerous level for people).

Now, I'm not sure we know enough about dino physiology to say for certain that these high O3 levels were enough to cause them all to keel over in a fit of wheezing and respiratory distress. But it's an interesting idea. The researchers suggest it may explain why the extinction was selective — it killed off 50 percent of all land animals, but spared large crocodiles, frogs, and mammals. That kind of picking and choosing doesn't seem to make a whole lot of sense in the "global firestorm" and/or "nuclear winter" hypothesis.

Still, there is at least one major hole in the authors' theory: not all dinosaurs died out, of course. Avian dinosaurs — birds — made it. As the researchers note, birds store outside air in their hollow bones. They exchange old air for new by exhaling, effectively passing poisonous air through their lungs twice. You'd think they'd be just as susceptible to high O3 levels as non-avian dinosaurs, most of which also had hollow bones.

If this finding turns out to hold up under scrutiny, it doesn't bode well for humans — studies suggest that ground-level ozone is already responsible for tens of thousands of deaths around the world each year. And ozone does well in warm weather. So — you guessed it — models suggest that as global warming eases onto the scene over the next century, people may start succumbing to the poisonous gas with more and more frequency.

Source: Paleogeography, Paleoclimatology, Paleoecology

Image: NASA