All vertebrates have atlases! The C1 vertebra was thusly named by ancient Greeks because it supports the weight of our cranium (“world”) just like the Titan, Atlas, held up the world.
I would thinking that a mammoth atlas would be similar in design and robustness as that of a Gomphothere since they are related. As for the comparative anatomy of a T-Rex/alligator/bird atlas, I too would like to see examples of them!