Below that is a bone called the sacrum, which is attached to the pelvic bone on either side, called the ilium, and where they meet is the sacroiliac joint.
The sacrum is, in turn, also fused or attached to the ilium depending which animal you're looking at and this whole structure, of course, is designed to support the great weight of the Tyrannosaur.
Because of the unique combination of features on the hips, for example a split in the shelf on the ilium, we can begin to see that this is something different than what we've seen before.