O2 Levels Have Been Very Important

He notes that atmospheric oxygen rose sharply at the end of the Silurian period about 415 million years ago, to reach a level of about 22 percent of the atmosphere, similar to today's oxygen content. But 55 million years later, atmospheric oxygen levels sank to 10 percent to 13 percent. The level remained low for 30 million years -- during which Romer's Gap occurred -- then shot up again, and vertebrates and arthropods again began moving from the sea to land.

"It matches two waves of colonization of the land," Ward said. "In the first wave the animals' lungs couldn't have been very good and when the oxygen level dropped it had to be hard for the vertebrates coming out of the water. I wonder if there is a minimum level of oxygen that has to be reached or nothing could ever have gotten out of the water."

Dinosaurs first appeared in the last part of the Triassic period, about 230 million years ago. That was during one of the lowest ebbs of atmospheric oxygen content of the last 500 million years, but he speculates that it took some time, until oxygen levels rose appreciably, before dinosaurs grew to their familiar gargantuan sizes.

I really need to get ahold of that book before I can comment too much.

Two interesting thoughts though.

1. O2 dip -> related to Late Devonian Mass Extinction? Something at sea (where most of the O2 production would be happened and then wiped out the land critters? Or are we seeing a sampling bias that might just mean that we don't have the right fossil beds found yet for that timeframe? It has been suggested that the Triassic Extinction is a statistical artifact rather than a real mass extinction...if so, this points to a problem in this research.

2. This research seems to beg the question then...why didn't the birds/archosaurs come out on top in the Paleogene? If they're that much more efficient, then even in low altitude ecosystems they ought to have thumped us. There's a reason why swimmers prefer to train at very high altitudes and why Los Alamos had at least some teams train there (when I lived there we had the Italian Olympic (IIRC) team come train there...we were at over 7K ft/2.1km altitude).

Like I said though, I need to read the book.