[Lusis]

Another explanation I've read for the size of the creatures back then was that the atmospere may have contained much more oxygen (or some other gas), allowing bigger animals to exist.

[Swordmaker]

Another explanation I've read for the size of the creatures back then was that the atmospere may have contained much more oxygen (or some other gas), allowing bigger animals to exist.

Possible... but I doubt it. We know the power available in the chemical engines we call muscles. Hyperbaric experimentation has not shown any significant increase in muscle efficiency due to higher oxygen content of the atmosphere... endurance is increased but not strength.