How hard were ancient arthropod exoskeletons?

Submitted by TrillCozbey t3_ycjdk0 in askscience

So from the human perspective, modern arthropod exoskeletons are quite weak. I can crush even relatively large insects without much effort. However, we know that hundreds of millions of years ago there existed giant arthropods. How hard would their exoskeletons have been? If I was transported back to the carboniferous and faced a giant centipede would I be able to do anything to its "armor?"

I'm assuming there is a relationship between the volume of the creature and the thickness of the chitin, like the whole square-cube law thing, but I don't know nearly enough about it.

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IronSmithFE t1_itom1vl wrote

i was told a long time ago that there is a physical limit to the size of a creature with an exoskeleton because of weight constraints among others. i don't remember exactly what that constraint was but i believe it was something like the size of a football.

according to my source, the internal skeleton model allows for much larger construction. i don't know how this applies exactly but i imagine, if true, that larger creatures with exoskeletons might have needed thinner shells just to remain mobile.

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Diligent-Jackfruit45 t1_itowg7w wrote

IIRC its not so much the rigid structure that determines the size of an insect but the fact that they rely on diffusion to oxygenate their cells. Get too big and the oxygen requirements of the creature grow too large to overcome without specialized organs like lungs.

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IronSmithFE t1_itp0rkn wrote

now that you mention this, he had stated something to that effect as well. of course, supposing a creature had only an exoskeleton but also had lungs, it still seems like it would be limited on size because of rigidity and weight.

the expert, i think, was arguing against the bugs in starship troopers as impossible creatures.

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Madeforbegging t1_itoqo9a wrote

The fossil records contain ARTHROPOD fossils like centipedes almost 6 feet long.

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IronSmithFE t1_itottx8 wrote

a 6-foot-long centipede could curl up in an area smaller than a football depending the centipede's flexibility and short diameter. so, i don't think that disproves the assertion.

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daysofbreeze t1_itoyb4u wrote

They are significantly harder than in modern arthropods. This is due to the fact that the exoskeletons of ancient arthropods were typically made of chitin, a long-chain molecule made of sugar and protein. (found in the cell walls of fungi and the exoskeletons of insects, crustaceans, and other arthropods). Chitin is a natural polymer a much harder material than the exoskeletons of modern arthropods, which are typically made of chitin and protein.

One of the largest ancient arthropod exoskeletons found to date are those of the millipede-like creature Pulmonoscorpius kirktonensis, which could reach up to 2.5 meters (8.2 ft) in length. But Some of the largest known arthropods could have had exoskeletons quite larger.

Chitin is a tough, insoluble material and therefore difficult to break. The average force required to break chitin is reported to be around 4,000 newtons. which would be enough to snap a human spine in half.

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PlaidBastard t1_itqf0vm wrote

How is that chitin resisting the 4000 Newtons, in terms of the orientation of the forces being exerted on the material? Tensile strength? Compressive? Resistance to buckling? Stiffness?

I sincerely doubt that value is a fair or meaningful comparison to a human spine any more than, for example, me letting everyone know that chicken eggs have an ultimate strength of 14,000 Newtons if you apply the load properly.

I'm perfectly willing to believe that the rest of what you say is true, but that specific bit really stuck out and cast doubt on all what you said, for me. A solid block of calcium carbonate would be impossible to crush with your hands, but, well, eggs are like eggs are, because of how thin that incredibly hard shell is.

If those paleoarthropods had thicker exoskeletons relative to their other proportions, or if we had some numbers backing up that chitin with less protein in its matrix has a higher tensile/compressive strength, I think maybe some of these 2.5m creatures could resist a guy with reasonably good boots on, but I don't think you've shown any of that to be true yet.

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