Eli5. – How were prehistoric species able to be so much larger than current species?

Question

I feel that there must be some physiological stressors or environmental factors that inhibit modern organisms of all domains from reaching the size, of say, a megaladon or brontosaurus.

In: 18

Answer 1

If I remember correctly from science class there was a lot more oxygen back then because plants had been around for millions of years pumping O2 into the atmosphere.

So nature needed a way to use up all that O2 and make CO2. Hence really big dinosaurs with really big lungs sucking up lots and lots of Oxygen to make carbon dioxide for the plants.