Is placoderm-like armor (semi-exoskeleton) viable for giant arthropods?

Question

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.

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?

Answer 1

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.

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