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hawkwings t1_j8i1bqh wrote

An armadillo has armor, but it also has an internal skeleton that it uses for running. Reconfiguring muscle attachments to attach to external armor instead of the internal skeleton would require a huge amount of evolution. I don't think that any creature has done it.

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Trips-Over-Tail t1_j8j0nff wrote

The skeleton on echinoderms is classed as an endoskeleton because it is enclosed by the dermis. But it takes the outside shape of the animal rather than an internal structure. The plates themselves exist within the skin, the skin tissue existing above, below, and between the plates.

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GeriatricHydralisk t1_j8j41wp wrote

Like everything in biology, there's one, unified answer: It depends.

Specifically, how do you define "exoskeleton"?

If you define it broadly enough to simply be a hard, outer layer, then basically everything with armor fits the bill: turtles, armadillos, oysters, etc. Even a variety of protists like diatoms.

If you say it has to support body weight, it gets more tricky, but there a few examples that squeeze in here: turtles again, crocodiles have specialized muscles to "brace" against their armored hide when walking, etc.

If you specify that it has to be involved in turning muscle shortening into body motion, like in arthropods and with our endoskeleton, nematodes may fit the bill, as they have a collagenous "cuticle" which inertnal muscles attach to. But it's not truly "rigid" the way an arthropod exoskeleton is (think of it like stiff rubber).

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Evolving_Dore t1_j8uggnq wrote

Replying to extra2002 (#1,807,187)

Turtles sort of cheat their way into having an "exoskeleton", but it's not a true one. The carapace is comprised of modified vertebrae (neural bones) and ribs (costal and peripheral bones). The plastron was once thought to be a modified sternum, but it actually seems to be composed of the clavicles and gastralia, a structure found only in reptiles.

Turtles have all the same bones as other vertebrates, and their internal organs are still housed within a body cavity protected by bone. They've just gone a little overboard with the level of protection. One can jokingly refer to their shell as an exoskeleton, but it is in reality a highly modified endoskeleton.

Turtle morphology is endlessly fascinating and has baffled researchers for decades, both in terms of how they relate to one another and how they relate to other reptiles. We're finall starting to parse them out based on genetics, but even still it's tricky and involved. I don't deal with genetics though, I just put my faith in the geneticists.

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