Why did they get so big?
Well, we still don't know for sure, but there is one prevailing idea that links food quality to size. The idea is that as animals get bigger, they require more food. This means they have to accept poorer quality (nutritionally speaking) food, and larger bodies are better equipped to cope with low quality food than smaller bodies.
Obviously, this reasoning has limited explanatory power, being completely circular, and, according to Marcus Clauss from the University of Zurich and his colleagues, has the added disadvantage of probably not being true.
There are other factors besides browse quality that could have led to giganticism. Martin Sander from the University of Bonn examined a number of possible ‘evolutionary cascade models’, including size and number of newly hatched babies, lack of food processing (no chewing capability), and even the type of lungs sauropods had to explain why they grew so large. Most likely a combination of many factors was ultimately responsible for the enormous bulk of these creatures.
What did sauropods do with those long necks?
Giraffes have long necks, but like nearly every other mammal, they only have seven neck vertebrae. Some birds also have long necks, but they can have over twenty neck vertebrae. This is why swans have far more flexible necks than giraffes. Despite their large number of neck vertebrae, sauropods were not nearly as flexible as birds.
Kent Stevens from the University of Oregon has found that the ‘neutral’ position of the head and neck (when the animal is resting comfortably) is most likely represented by position E in the graphic below.
While not as flexible as birds, they could sweep out fairly large swaths from ground level to high above their bodies, as shown in the next picture.
|Apatosaurus and Diplodocus are shown reaching down to ground level (A and B), and reaching as high as possible (C and D). While both necks sweep out a huge surface area, Apatosaurus could reach higher despite having a somewhat shorter neck than Diplodocus.|
How fast could these giants move?
Probably, not very. Here's a handy infographic, courtesy of the University of Manchester:
Even better, here's an animation.
Marcus Clauss, Patrick Steuer, Dennis W. H. Müller, Daryl Codron, & Jürgen Hummel (2013). Herbivory and Body Size: Allometries of Diet Quality and Gastrointestinal Physiology, and Implications for Herbivore Ecology and Dinosaur Gigantism PloS ONE DOI: 10.1371/journal.pone.0068714.
P. Martin Sander (2013). An Evolutionary Cascade Model for Sauropod Dinosaur Gigantism - Overview, Update and Tests PloS ONE, 8 (10) : doi:10.1371/journal.pone.0078573.
Kent A. Stevens (2013). The Articulation of Sauropod Necks: Methodology and Mythology PloS ONE DOI: 10.1371/journal.pone.0078572.
William Irvin Sellers, Lee Margetts, Rodolfo Aníbal Coria, & Phillip Lars Manning (2013). March of the Titans: The Locomotor Capabilities of Sauropod Dinosaurs PloS ONE, 8 (10) DOI: 10.1371/journal.pone.0078733