So I built a stegosaurus model kit, which included some factoids in the instructions. One of these factoids was that stegosaurs are not believed to have had a secondary brain in the hips to help them control their rear half after all. That was wild to me, since the whole stegosaurs and sauropods with their tiny heads needing a secondary brain for their huge bodies was commonly accepted back when I was a kid. So I looked it up, and indeed, the current hypothesis is that the cavity that the second brain was thought to occupy is used for a thing called a glycogen body. But what exactly does a glycogen body do? We’ll get back to you on that, apparently.
Glycogen is an energy storage molecule that is more easily accessible (easier to convert into glucose, what your cells actually burn) by the body than fat is. I would assume this organ was a dedicated store for it.
Edit: ah that’s what I get for commenting before reading the damn article. Scientists haven’t confirmed that it’s definitely an energy storage organ, but it’s one of the leading theories.