Yup - I think you've hit the nail on the head here - these guys probably spent most of their time up to their shoulders in water - keeps predators away, support

Yup - I think you've hit the nail on the head here - these guys probably spent most of their time up to their shoulders in water - keeps predators away, supports a big chunk of their mass, eliminates the "fall over and you're dead" problem - which would be something of an evolutionary disadvantage if they were purely terrestrial, and would probably preclude their very existence were this the case. Also - it happily explains how it ended up in quicksand, at the bottom of a lake or what have you. Edit: further to this, this is also supported by the huge chevrons on this thing, as if you're a big-ass dinosaur who likes to paddle, you're going to want to use your tail for locomotion, which mandates big muscles, which mandates big anchors for your big muscles.