It used to be thought that they were aquatic, like hippos, and that water offered them support and protection. But modern paleontologists point out that they couldn’t breath if their body was fully submerged–water pressure would not allow their lungs to inflate with air properly if their body was completely under water.

My guess is that they they were like hippos, walking in the shallows, while their long necks could reach down into deep water, or high up on shore, to forage while their bodies were safe in a relatively deep lake or river. Hippos achieve neutral buoyancy, so they can run around on the bottom, but they must still swim up to the surface to breath. But that still doesn’t explain the long tail and neck, hippos don’t have one, and they hold their breath and swim down to deep water to pick up vegetation from the bottom. Maybe the dinosaurs could raise or lower their tail out of the water to help balance them when their neck was out of, or deep under, water.

Modern reconstructions always show them traveling around in herds on land, but they look just too big and clumsy, and their necks and tails too long and heavy for them to be fully terrestrial creatures. I think modern elephants are pretty much the maximum size and weight for a land animal, and they spend a lot of time in the water. Elephants are good swimmers, and use their trunks like a snorkel, but must be floating on the surface for their lungs to be able to operate against the water pressure. I think these big reptiles lived the same way, but spent most of their time in the water, like modern hippos.

Anybody have any ideas?