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?

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    $\begingroup$ You are looking at organisms which are way too evolved, too specialized. For an idea of where vertebrates come form, look at the amphioxus. Fun factoid: among the inverterbrates, our closest relatives are the echinoderms (starfish, sea urchins, sea cucumbers, crinoids and their allies). Tunicates (salps, sea squirts) are some weird chordates. $\endgroup$ – AlexP Jan 4 at 22:09
  • $\begingroup$ Yeah, that's what I was worried about too... Though I guess since it's an alien world the Proto-Thing would just have to be LIKE something instead of being "The Thing But ____" XD $\endgroup$ – BonnetBee Jan 4 at 22:12
  • $\begingroup$ Though I guess the question I'm trying to ask is less "Can this Happen" and more like "Ok, assume These Things have Already Happened, and that somehow we now Have An Organism that has both a Cuttlebone/Gladius-Style Thing and a Notocord-ish Thing, what are some ways in which the One might start to develop into shielding for the Other?" Sorry for the confusion, I guess I didn't state that very clearly :( The Amphioxus thing is pretty helpful though, THEIR spines had to come from Somewhere, right? Maybe they'll give me some ideas, thanks! :D $\endgroup$ – BonnetBee Jan 4 at 22:22
  • $\begingroup$ The vertebral column does not sheath the notochord, it replaces it. The notochord does not need sheathing, it is a structural construction element. You are thinking of the neural tube, which is a separate structure. $\endgroup$ – AlexP Jan 4 at 22:36
  • $\begingroup$ Ohhhhh, that's actually really helpful! I only just started looking into how Spines actually work/where they come from and I might be in a little over my head here XD So the thing I'm looking to protect is the Neural Tube, not the Notocord... Ok, that might make researching a little easier, thanks! :D $\endgroup$ – BonnetBee Jan 4 at 22:42

Basal chordates lack a jointed vertebral column, and have instead a rigid cartilaginous "notochord".

A gladius is very similar to a notochord. If chordates could do it, so can cephalopods.

However, I doubt that squids would become terrestrial. They're pelagic specialists. Most people use octopuses or cuttlefish for their terrestrial cephalopod ancestors.

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    $\begingroup$ Yeah, we were actually thinking Cuttlefish, since her people often Radically Alter Their Skintone as a form of communication and expression :D $\endgroup$ – BonnetBee Jan 4 at 22:24
  • $\begingroup$ The entire point of the notochord is that it is not rigid. It is flexible and elastic. Its role is to store energy in the same way as the elastic limbs of a bow. $\endgroup$ – AlexP Jan 4 at 22:26
  • $\begingroup$ @AlexP Fair enough. I'll change it to "unbroken". $\endgroup$ – SealBoi Jan 4 at 22:33
  • $\begingroup$ Cool! So they could probably develop a Spine-Like thing? I'm assuming it might look a little different than ours, and would probably be made of something like Chitin and/or something like Aragonite, like a Cuttlebone... I'll look and see if anyone's asked about how that would affect them, maybe make a new question if no one has. :3 $\endgroup$ – BonnetBee Jan 4 at 22:49
  • $\begingroup$ @BonnetBee I think gladii are made of calcium carbonate. This paper explains why vertebrates use calcium phosphate in their skeletons rather than other, more common materials: ncbi.nlm.nih.gov/m/pubmed/28563613 It's not exactly in layman's terms, but you could probably figure out the consequences of a non-calcium phosphate skeleton from the abstract. $\endgroup$ – SealBoi Jan 4 at 23:08

how about this. Similar to how "whales had to wait until their noses migrated to the top of their heads" (quoting the book 'Sapiens' by Harari) in order to facilitate breathing while swimming, in your case it was the exoskeleton that migrated to the interior of the body, in order to more closely focus their protection on the nervous system. I can envision the soft tissues gradually "seeping through" to the outside of the exoskeleton in response to a shrinking exoskeleton, perhaps due to a shortage of calcium in the environment.

  • $\begingroup$ Hmmmm! That certainly helps add to the theme of "Looks like a familiar thing but evolved RADICALLY differently", I like it! $\endgroup$ – BonnetBee Jan 4 at 23:37

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