Back when I was in grade school, we were taught large dinosaurs had to live in water as they were too heavy to walk on dry land. And those that did walk on dry land walked upright and dragged their tails along the ground.
IT WAS SETTLED SCIENCE! back then.
All brontosauruses are thin at one end, much MUCH thicker in the middle, and then thin again at the far end. That is the theory that I have and which is mine, and what it is too.