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I thought this model could possibly work as a possible way to get a functional system for giant arthropods, but I realized with problems when I was writing an answer to this question Are 'ambiskeletons' viable?.

The placoderms, a clade of extinct fishes in which almost the half of their body was covered by bone plates and for what I know, the soft tissue over the plates was minimum, this would allow to keep the exoskeleton-like appearence.

enter image description here

The placoderms (underwater) could reach bigger sizes than the biggest arhtropods like euripteryds of 2.5 meters long, with one of the biggest species having moderated estimations till to 6-7 meter long and the upper estimations with even to 10-11 meters.

Also the placoderms solved a very mentioned problem for giant arthropods of having a body with organs contained within a hard armor.

But also an specific group even got arthropod-like pectoral fins, extremly strange between vertebrates.

So the problems with which I realized are principally biomechanics when extrapolating to terrestrial giant arthropods:

  • These were acuatic creatures, so is probable that all the extra weight provided by the armor were sustented by water, which might cause problems on land for be so heavy. So would be necesary to determine how thick it could be before reaching a limit which it falls under its own weight and also just the half or their body covered by the armor.
  • The the biological mechanism by which these animals could grow within their armor without shedding as arthropods would, I have assumed that by a stacking of layers of bone dissolving into the lower ones as they grew. But I´m not how that worked and if tis possible for arthropods to use a similar mechanism.
  • The "semi" part, for much than placoderms could have an exoskeleton-like look, still having a layer of girded soft tissue over the bones plates in specific plates, so to use this on arthropods, appears the doubt about if arthropods could develop some "skin zones".
  • And the mechanic part, I don't know if the surface provided together by the "minimum" internal skeleton and exoskeleton would be enough to anchor a sufficient amount of muscle to be able to move on land. So, is that, to find how much surface muscles require.

Next a crude scheme with something like a giant generic insect, about how this could look, practically an internal rigid support shown in black which could be considered a spine or column as internal muscle anchor point, the column would be directly conected with the "braincase" and specific parts of the body in which some muscle and skin is over the exoekeleton together with the internal muscles to improve the strenght.

So the question is if this system is really viable and a solution for giant arthropods, considering the previous points with a biomechanical focus about the working?

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    $\begingroup$ Probably would work better with shorter legs $\endgroup$
    – Firestryke
    Aug 16 '21 at 22:00
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    $\begingroup$ Doesn't having muscle and skin over the "exoskeleton" defeat the purpose of an exoskeleton and make it a hollow endoskeleton with a different name? Isn't it part of the point of an exoskeleton's definition (apart from being on the outside) that it's repeatedly shed as the creature grows, should the creature therefore slit itself open , disgorge it's skeleton during development - it sounds like a major life-threatening set of wounds is being inflicted every season of growth. $\endgroup$ Aug 16 '21 at 22:01
  • $\begingroup$ @ARogueAnt. Probably you have reason in some points, but that's way I chose placoderms, at least for me images like this alchetron.com/cdn/… keep the arthropod esence with a visible segmented armor, and can be seen skin together with armor in differents parts of the body $\endgroup$
    – Drakio-X
    Aug 17 '21 at 6:12
  • $\begingroup$ @ARogueAnt. But now that you mention it, yes looks like the way in which the the armor could be melted is problematic. $\endgroup$
    – Drakio-X
    Aug 17 '21 at 6:14
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    $\begingroup$ Ah, I see now. Well, then I'd say the question deserves an upvote from me. $\endgroup$ Aug 17 '21 at 6:28
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As long as they have an endoskeleton under it it will work fine. there were real terrestrial vertebrates with something like an secondary exoskeleton. Ankylosaurs, glyptodons, even armadillo to a certain extent.

Do note this means its limbs also need an endoskeleton. Also note in all these groups there is still a layer of skin on the outside of the armor, if they don't they will need to periodically shed the exoskeleton.

Look at this section of a Doedicurus tail, you have a bone ring surrounding a vertebra, each vertebrae had its own ring. something similar to this could evolve, even for limb segments. It would not even need to be made if lots of individual scutes fused together, it could just be a few sections. keep in mind such armor needs a reason to stick around either its musculature must connect to it or maybe there is a developmental reason the outer sections cannot be lost. Maybe they are part of the essential signaling pathways for muscle development or segmentation. enter image description here

As a side note placoderms are not even the extreme for armored fish, look up kwikwi or armored catfish, armor from head to toe.

enter image description here

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  • $\begingroup$ Two doubts which persist are comparing ankylosaurus with glyptodonts and tortoises, ankylosaurus are notoriously bigger, which is the adaption that permited it? $\endgroup$
    – Drakio-X
    Aug 18 '21 at 7:14
  • $\begingroup$ And about the limbs requiring and internal support, the mentioned placoderm with arthropodo-like fins (Bothrilepis) probably had a reduced or almost lost internal cartilage skeleton, so is that enough support to extrapolate to arthropods (on land)? making possible maybe something like this reddit.com/r/SpeculativeEvolution/comments/or4pbb/… $\endgroup$
    – Drakio-X
    Aug 18 '21 at 7:19
  • $\begingroup$ @Drakio-X I never mentioned tortoises they are not a good model. ankylosaurs get larger because dinosaurs have several adaptation that make getting bigger easier than it is for mammals. And the placoderm limbs don't have to support a load. $\endgroup$
    – John
    Aug 18 '21 at 13:14
  • $\begingroup$ Placoderm limbs don't have to support the load, but if similar limbs should to, they would be able to? $\endgroup$
    – Drakio-X
    Aug 19 '21 at 2:22
  • $\begingroup$ A limb with an endo skeleton can just fine but a limb with just an exoskeleton will never handle large loads. limbs on large animals need large joint surfaces to disperse load transfer. $\endgroup$
    – John
    Aug 19 '21 at 2:25

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