How Did It Get There?
Scientists speculate that the unlucky dinosaur died and ended up being swept into the river by a big flood. It was kept afloat by gases in its body which washed its belly-up carcass far out to sea.
After a week or so, the carcass burst and sank back into the ocean floor. Minerals seeped into the skin and armor and supported its back, immortalizing its form as tonnes of rock piled on top of it.