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.
tbf that’s sorta true, they didn’t have “mini-brains” but stuff like severe pain signals get processed in the spine, since every millisecond matters when you’re absentmindedly stepping onto smouldering coal. That’s why reflexes are reflexive, the brain isn’t even involved at all.