|Comparison of saruopod dinosaur necks (right side) to necks of non-dinosaurs (left). Credit: Taylor and Weidel, PeerJ 2013 )|
Such a huge neck on a mammal would be extremely heavy because of the bone weight. In the case of sauropods, the bones were not solid, but were penetrated with air sacs, so their relative weight was reduced compared to modern large mammals. These air sacs may have also contributed to breathing efficiency by making sure fresh air intake was optimized (the long necks would have made breathing like trying to breathe through a straw).
The paper also discusses the possible unique muscle anatomy of these dinosaurs. The weight (despite all the air sacs) and length of these huge necks required strong muscular attachements to lift and move. Muscles are rarely preserved in fossils, but the bones have residual marks, such as ligament scars, which can give clues about where the muscles ran. Taylor and Weidel speculate that the some of the sauropods had bones and muscles working together like a cantilevered beam, with a very strong lifting support muscle group and specializations to stabilize the neck from side-to-side. For others such as brachiosaurus, there may have been other anatomical ways to achieve the forces necessary to operate their necks.
A really fascinating look at the mechanics demanded by a really unusual physical attribute.