Pterosaur flight might also be partially explained by a denser atmosphere. Thicker air would provide more lift for an airfoil.

I've always wondered about those high O2 concentrations; wouldn't that make the forests of that time real tinderboxes, even more so if
the pressure was higher than our present 15 lbs/in^2. Still, that would explain how big insects were in the olden days.

Does O2 concentration at a contemporary level in an atmosphere at a higher pressure provide extra oxidation potential for organisms without
increasing the fire hazard? I don't know enough chemistry to answer that question.

A cursory Google search yielded this:

Concentration.

To quote this NASA report on flammability tests in Oxygen rich environments:

Flammability characteristics ... show a strong dependence on oxygen concentration with little relation to total pressures above 41 kPa (6 psi). Below 41 kPa (6 psi), MOCs and required oxygen partial pressures show increased dependence on total pressure.

and

The conclusion drawn from these data is that lower concentration/higher pressure data (e.g., 21% O2, 101.4 kPa (14.7 psia)) cannot be conservatively applied to higher oxygen concentration/lower pressure environments (e.g., 30%, 70.3 kPa (10.2 psia)) despite equivalent partial pressures.

In other words, there are more things flammable at 30% O2 / 10.2 psi than there are at 21% O2 / 14.7 psi, despite identical partial pressures of Oxygen.