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 it sank back-first into the ocean floor. Minerals seeped into the skin and armor and supported its back which immortalized its form as tonnes of rock piled on top of it.