Dinosaurs!

I learned, last night, why dinosaurs where able to get so big.
It's like this.

Throughout geological history, there have been large variations in the amount of oxygen in the atmosphere. The points with higher levels of oxygen have been found to correspond to periods when there have been large diversification of species, and the points with lower levels of oxygen have been found to correspond with mass extinctions.

Early small dinosaurs where one of the survivors of one of the low periods of oxygen. Then when the oxygen levels rose, they already had superbly efficient lung systems.
Since they where able to get lots of oxygen out of the air, very efficiently, they were able to grow very large during the following period of wide diversification.

Modern bird have similar lung structures, which is why they are able to breath easily while flying at very high altitudes in thin air.

Isn't that coo!l?!