Why were dinosaurs so massive in size?

Why did giant dinosaurs like the Amphicoelias evolve such long necks and such a large size?

• I mean the neck didnt always grow tall or upwards like a giraffes to reach the tree tops they just grew very very long almost horizontally!?

• Answer:

  Good question. Some paleontologists do believe that sauropods stood upright on their hindlegs, and use their long neck to reach for vegetation that are out of reach of most dinosaurs. This idea is so deeply held by some that when a set of bipedal sauropod tracks were found, some researchers suggest that the baby sauropod that made them were "spooked" into running bipedally. My own interpretation of the bipedal tracks is that they were made under water. If the water was deep enough, then the sauropod would need to rear up on its hindlegs to be able to keep its neck above water as it walked along the bottom. The Wikipedia shows a silhouette comparison of the various sauropod dinosaurs. From this, one can see an interesting trend. The bigger the dinosaur, the longer its neck and tail are. One possible reason for this sort of body plan is the relationship between length and suface area. If we take two objects of the same volume but different lengths, then the longer object has greater surface area. The greater the surface area, the faster the rate of heat exchange. The body parts of biological organisms, including our own, conform to this principle. Human beings living in hot climates tend to be taller with longer limbs, whereas human beings living in colder climates tend to be shorter, with shorter limbs to minimize the surface area. Since sauropod dinosaurs lived in a warm Mesozoic environment, they need to lose body heat quickly to avoid heat strokes if they are large, since large objects have proportionally smaller amounts of surface area. Therefore, one way to grow big without overheating easily is to grow some parts of the body long. As to the reason for growing big, one of the advantages of being big is that there are few predators that can eat you. The African elephant has few natural enemies. Only a few lion prides are able to feed on elephants. OTOH, a small antelope can be eaten by anything from medium sized cats, African wild dogs, cheetahs, leopards, to hyenas and lions. http://en.wikipedia.org/wiki/File:Longest_dinosaurs1.png