Ok, so, some quick background... I'm not an evolutionary biologist or anything, but I'm helping a friend with some alien design ideas, and at least one of them she designed and wanted help to somehow explain was a group that basically falls into the classic Star Trek "Humans but Kinda Weird-Looking" milieu... and while I was initially willing to handwave their existence with "Boy, space sure is wacky, what are the odds that it keeps making humans!", I kept thinking about it more and more, and I actually really liked the idea of something we might consider a "functional vertebrate" with seemingly mammalian- even HUMANOID- characteristics actually being a case of something we would possibly classify as an Invertebrate undergoing some Seriously Bizarre Convergent Evolution that took a completely different path from ours... Yet ended up with a Weirdly Similar Result?
So we ended up coming up with a pretty solid model for how this race evolved, why they have the adaptations that they do, and we have them pretty solidly fleshed out... Except that there's one thing that I'm not 100% sure about, and it's kind of a Big Freaking Deal if we want an alternatively-evolved "Vertebrate": given that the ancestor we envisioned for them was something akin to what might have happened if Cephalopods or other Molluscan Critters had somehow developed into Primitive Chordates, the way something like the Lancet did, How Would They Evolve A Skeletal Structure, and How different would it be from Ours?
So I looked back at something like an Ammonite, and wondered if it was possible for something that still had a vestigial mollusk-style shell after somehow developing an early Notocord to sort of... Use that as a springboard for developing Vertebrae to protect it, and the rest of the skeleton following suit? Could the "chambers" in the shell maybe... start to diverge into separate nested "bands to allow for freedom of movement, or something like that?