Elephants and their relatives have big heads. They’re big animals to begin with, but they also have strong, heavy jaws and teeth, as well as big brains. Then they throw in a massive trunk and huge heavy tusks (four of them, in the case of some extinct species). If you stick all these heavy features onto your skull, there are going to be consequences.
Two things we very rarely see together in one animal are long necks and large heads. In some ways the head and neck act as a third-class lever, which actually increases the force necessary to move the head. The longer the neck the greater the necessary force. Significantly, that also applies to the force needed to stop the head from moving once it’s started; without enough force, the head may keep moving independent of the neck and the rest of the body, with unfortunate consequences. What this boils down to is that animals with big, heavy heads will almost always have short necks.
The image at the top is the third cervical (neck) vertebra from a mastodon, seen in anterior view, and below is the same vertebra in posterior view:
The vertebra has both epiphyses attached, which suggests that it was not a particularly young animal (we can’t say that it was fully mature, since the cervical epiphyses fuse at a relatively young age). There are some hints of osteoarthritis. This could indicate that the animal is older, but I wonder if this is common in elephant neck vertebrae, given all the stresses on the neck.
The right lateral view is especially interesting:
This vertebra is amazingly short. If I had found it in a marine deposit I would have probably initially thought it came from a whale – another large-headed animal with short neck vertebrae.
In mastodons and other proboscideans, the third through seventh cervical vertebrae are all short like this. A string of these short vertebrae is not going to be very flexible, so one of the trade-offs in having a big, heavy head is having a short, inflexible neck. In most whales and dolphins some of the neck vertebrae are actually fused together making them completely immobile. This hasn’t happened in elephants, but they still have very limited neck mobility. As an example, check out videos such as this one in which elephants are facing off with possible threats. Notice that as the elephant in the video looks in different directions, he’s actually turning his whole body. His head is staying in almost the same position relative to his back the whole time. Elephants can still nod their heads and rotate their heads around the neck, because these motions are largely controlled by the first two neck vertebrae. But they have very little ability to point their head in a different direction than the rest of the vertebral column, a limitation that they share with their mammoth and mastodon relatives.