Back in the late Pleistocene, when I was in school, we were told that the large Jurassic herbivorous dinosaurs, the ones with barrel bodies, tree-trunk legs, small heads and long tails and necks, were aquatic. One was called "Brontosaurus", although I understand there have been some changes in the nomenclature since, and they were simply too big and massive to have walked easily on land and probably spent most of their time in the water where it could help support their weight. This made sense, modern day elephants are already approaching the maximum practical size for a land animal and weight increases as the cube of the linear dimension. The strength of bone only increases as the square of the linear dimension. Strong corroborating evidence came from the fact that some of these big saurians had nostrils on the top of their skulls, so they could breathe when almost totally submerged.

Today this reconstruction has been replaced with one of a fully terrestrial creature, nimble and quite at home on land, and even capable of standing on its hind legs to nibble the tops of tall trees. They are depicted walking with both necks and tails fully extended, as if for balance. Supposedly, the tail could even be used as a weapon, like a great whip. The main reason for this new interpretation is that a creature that size would not be able to breathe when its body was fully submerged due to water pressure on its chest cavity, i.e., it couldn't use its neck as a snorkel.

Personally, I find it difficult to believe these creatures weren't aquatic, although I realize the idea of them walking on the bottom of a deep lake with their neck extended to the surface like a periscope just can't work. Their lungs would be compressed by water pressure and they wouldn't be able to breath. But with some modification of their lifestyle, they could still be lake or ocean dwellers. They could use their long necks to browse on deep benthic weeds (while holding their breath). Or alternatively, they could safely eat vegetation on shore using their long necks, while their body was safe from predators, standing in deep water. The long tale would not be for balance, but for bouyancy. By forcing it in or out of the water they could quickly alter their overall bouyancy to help them walk or swim. From the safety of deep water they could browse the shallows, too. and even in shallow water they could easily keep the entire neck and head submerged and invisible. The massive size and long neck/tail, plus the over-the-head nostril position of at least one of these guys suggests to me these animals lived more like hippos than giraffes.

Can any of you straighten me out, here?