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Due to similar answers in the multiple questions I have asked about giant arthropods, seems like the presence of an exoskeleton is a bigger limitation for giant arthropods than their cardiovascular or respiratory system.

So the problem is how arthropods could get/evolve an endoskeletal system, with teh required integument over, muscle, skin and others?

Things that could happen is internalize the exoskeleton or develop new internal structures that can be called a skeleton in their own right, but I really don't know how it could happen.

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  • $\begingroup$ What is the difference between a vertebrate and an arthropod with an internal skeleton, other than the relative position of the main blood vessels, the digestive tube and the main nerve chord? (Fun factoid: tetrapods have a similar number of articulated segments in their limbs as insects.) $\endgroup$
    – AlexP
    Aug 9, 2021 at 11:21
  • $\begingroup$ "Due to similar answers in the multiple questions I have asked about giant arthropods, seems like the presence of an exoskeleton is a bigger limitation for giant arthropods than their cardiovascular or respiratory system." // having had a glance at your prior questions I don't see where you got that from, care you give your reasoning,& how it's a problem for your goals? $\endgroup$
    – Pelinore
    Aug 9, 2021 at 11:40
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    $\begingroup$ ^ OK having made a careful read through of your other questions I think I've identified the concern, & I think you've misunderstood the problem, a large animal that moults would eventually reach a size where having no skeleton (endo or exo) post-moult to hang it's flesh on will die, it's organs crushed by it's own weight, you can fix this by rather than moulting splitting the shell along a seam so it's still got something to hang it's flesh on (the 2 halves), expanding to fill the gap, then hardening new shell in the gap. $\endgroup$
    – Pelinore
    Aug 9, 2021 at 14:35

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It's gonna be very difficult, borderline with impossible: switching from exoskeleton to endoskeleton means a massive redesign of all joints and muscles, with related nerves.

Considering that evolution happens by random mutations, it would require a sheer luck to get all the needed mutations at the same time and in the right spot. More or less like buying the first 50 winning tickets at the new year lottery.

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    $\begingroup$ Plus of course once their done evolving they don't look anything like arthropods, couldn't by any stretch of the imagination still be called arthropods & in fact won't be arthropods any more so why even bother going through the process of trying to rationalise plausible ways they could change & not just start with something that's already there // I suspect the OP has got himself stuck in some recursive thinking & lost sight of his original goal. whatever it was :) $\endgroup$
    – Pelinore
    Aug 9, 2021 at 12:42
  • $\begingroup$ Yeah I know how random is the evolution, but how is the solution to the problem? $\endgroup$
    – Drakio-X
    Aug 10, 2021 at 4:59
  • $\begingroup$ @Pelinore .Originally I was very focused about the use of giant arthropods and some based in science explanations for them, so I made multiple questions trying to deglose each problematic point for get them, but when I gave an answer to the question about the viability of ambiskeletons, I realized that my own "model" was defective. So reconsidering what interestong features might persist even with endoskeleton, looks like other things can be interestingly used. $\endgroup$
    – Drakio-X
    Aug 10, 2021 at 5:11
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    $\begingroup$ @Drakio-X Hmm, OK, was it the "Arthropod skeletons already account for a greater proportion of the organism's weight than those of vertebrates" coupled with "double an animal's size, its weight will increase eight times, but its weight-bearing capacity only quadruples and muscular strength only doubles" line? // I don't think that's going to be a problem in & off itself, after all "The largest Ankylosaurus specimen ever found was 20.5 feet (6.25 meters) long, 5.6 feet (1.7 m) tall at the hips and 4.9 feet (1.5 m) wide", with its armour plus bones it still got about just fine. $\endgroup$
    – Pelinore
    Aug 10, 2021 at 16:11
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    $\begingroup$ @Drakio-X ^ the larger end of the megafauna scale (i.e. the biggest dinosaurs ever) may be problematic due to the greater percent of bodyweight of these exoskeleton but the small to mid range of it is available I'm sure., wouldn't like to hazard a guess at where the drop off is though // at least in respect of the weight to strength ratio issue. $\endgroup$
    – Pelinore
    Aug 10, 2021 at 16:23
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Endo-Exoskeleton:

To evolve what you are looking for, you need a transitional state that can exist without a functioning skeleton at all, or a very soft, flexible one, so an ecological niche would be needed where the organism could exist for a time without a lot of support. But there are lots of organisms (worms, cephalopods) that function without any skeleton, so this isn't so outrageous to imagine. Remember, we're talking millions/billions of years of evolution to play with, and a lot of strange things happen in that kind of time. And since arthropods already molt and frequently have only minimal support, an organism that finds itself in a protected niche for a few million years (with gradual pressure to evolve endoskeleton over time) could certainly achieve this.

An arthropod that thickens some areas of their exoskeleton to provide better structural support for weight and thins other areas of their exoskeleton for better flexibility will very quickly start looking like they have an endoskeleton. Many bones tend to be near the surface, and your hybrid organism isn't so different. A skull and rib cage are in many ways an endoskeleton trying to become an exoskeleton.

There could be a gene duplication, allowing one set of genes for exoskeleton to mutate into a small, hard degenerate form (endoskeleton) slowly internalized, and a softer, frequently shed exoskeleton allowing growth. Or their soft internal tissues increasingly grow on the outside of their rigid exoskeleton, with only a paper-thin exoskeleton "skin" on the outside to cover them. By some estimations, 80-90% of arthropod deaths may be associated with vulnerability during molting, so a process that reduced this vulnerability would have a huge evolutionary advantage to the organism that evolved it.

A new process then evolves where instead of molting their endo-exoskeleton externally, they start internally scavenging these "bones" and re-growing them larger. This could eventually become semi-continuous, or still follow a molting pattern of periodic softening and re-hardening. Now the bones are free to stay inside of tissue permanently. Harder materials (like calcium) start getting deposited (biomineralization, like crustaceans already do), making the "bones" more bone-like.

The external "skin" exoskeleton is always free to harden again in evolution (like a turtle) in some or all parts of the organism, so there can still be all the fun arthropod features. some things (like eyes) would be highly conserved and remain on the outside exoskeleton. but other functions would be free to mutate and form new functions.

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It might be possible...

No arthropod has ever evolved from an exoskeleton to endoskeleton body plan, but that is not the same as saying it could not happen given the right circumstances. There is one group of exoskeleton based organism that have made this transition which at least suggests it might be possible: Coleoidea.

Coleoidea are a sub-class of cephalopods where the exoskeleton has been moved inside of the the body. This group includes octopuses, squid, and cuttlefish. When you look at their ancestry they basically evolved from worms to snails to nautaloids to coleoidea. So, while the hard shell seems like it would have been there to stay, being that a protective shell should be better than no shell, its niche called for flexibility above all other features.

Likewise, an arthropod which evolves under enough pressure to become more flexible may also loose its shell. At first I would suppose that the arthropod would develop a progressively softer and softer shell for squeezing though hard to reach places until its shell becomes so soft, it is almost skin like, and your arthropod would move more like an octopus than a bug. However, it may want to retain bits of harder shell in selective places: mostly the mouth for chewing food and close to vital organs to protect it from accidental death.

Over time the size, shape, and position of these selective plates will be optimized. And for an organism that values squishiness, the most optimal configuration for these plates is as small and close to the vital bits as possible. So, like the Coleoidea, your "softopods" would pull their bony bits inside of the body over time. Once its outsides have become its insides, future generations would become able to start using those plates as attachment points for musculature that could eventually become a proper endo-skeleton.

But this is probably not useful to you

The evolutionary path to get from an arthropod to an endoskeletal creature would be very long; likely hundreds of millions of years. In this amount of time, it would not just be the skeleton changing, but everything about the creature would change. By the time you get from point A to point B, the new species will be so far removed from arthropods that is will not even be recognizable as such. You will probably have a creature that has convergently evolved to be far more coleoidea like than arthropod; so, the simpler conclusion is to not go down the the whole arthropod evolutionary track at all, but instead plan to start from somewhere else.

My guess is that your goal is really just a terrapod like animal with the extra appendages. Believe it or not, this is easiest done by just starting with a terrapod and adding the extra appendages. While the extra appendage thing has never been successful enough to catch on, the mutation does happen often enough in nature that it could catch on given the right evolutionary pressure.

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For large terrestrial creatures, limbs are a big investment in weight and resources; so, going from 4 to 6 or 8 is a huge sacrifice that does not benefit most niches, but there is one niche that has only recently evolved where this sort of mutation could be helpful: Tool Users. While a frog, cat, dog, lizard, or bear would only be slowed down by extra appendages, there are 4 basic body plans in works of fiction where a tool using species would do very well to have 6 appendages instead of 4 making it possible for a random mutation resulting in extra limbs to catch on:

Gegeinoid: This body plan is where a bipedal tool user has extra hands to work with. While humans have gotten along just fine with 2 arms, we often find ourselves limited by our lack of hands. Many things we try to do would simply be much easier with extra arms; so, if such a mutation were to happen in a stone-aged civilization, the advantage could be enough for that person to advance to the top of his clan giving him preferential matting rights allowing the mutation to spread to a whole population over time.

Centauroid: This body plan is where a bipedal tool user gets an extra set of feet to work with. While these feet would not automatically be bovine as often depicted, quadrupeds can generally move much faster than bipeds; so, if this mutation where to happen in a place like the Asian Steppes where horseback riding became such a way of life, this mutation could take horses out of the equation and allow tool users to directly be able to move more quickly.

Dragonoid: Many birds are basic tool users, but they lack the dexterity to manipulate things with their talons. This is largely because their talons need to remain specialized enough to be feet that they can not also be fully specialized hands. If a bird were to sprout an extra set of legs, it could then split their functionality to have separate hands and feet improving their potential as tool users. Especially if their new hands are far enough forward to be able to better visualize what they are holding.

Veloxi: I'm not sure what else to call this one since the only place I can think of seeing this body plan is from the old 1980's game called Starflight. The veloxi were a mantis like race described as having one set of appendages that were exclusively hands, one set that were exclusively feet, and a middle set that could function as both, in theory giving it advantages similar to both the Gegeinoid and Centauroid body plan.

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