The Earth's atmosphere was much denser
Millions of years ago the Earth's atmosphere was much denser and had more carbon dioxide. There's a theory that the dinosaur's main advantage was having avian respiration that allowed them to operate in a CO2 rich atmosphere. The higher atmospheric density also enabled the really big pterosaurs to fly. Much of that ancient atmosphere got turned into limestone and dolomite. It's debatable about how much was sputtered off by the solar wind. The sun's intensity and the nature of its radiation has changed significantly over the millenia.
Gary