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Essentially, I've been trying to make a world of arthropod-like creatures, but as a result of that, the more I researched arthropods, the more I understand the limits their structure has on size.

  • they're limited to a single kind of joint due to the nature of their exoskeletons (unless we start allowing for exposed muscles).

  • molting becomes increasingly more problematic the bigger you get.

  • the association of bones with a covering of soft tissue seems better overall at supporting weight.

Overall, it almost seems like the square cube law dunks on giant bugs harder than it does on Dragons. And so I started looking for a potential vertebrate-like approach to something that at least seems like it's covered in armor, and lo and behold, armored catfish.

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These fish, belonging to various families, have an internal skeleton as any fish, but are also almost entirely covered by bony plates. Such a structural system maintains the desired aesthetic of a creature that's almost arthropod-like and mostly encased in what's essentially an organic suit of armor while, at least at first glance, still reaping the benefits of an internal skeletal structure, mostly ridding the creature of problems like the limited types of joints possible and the matter of requiring to molt in order to grow without having to do like turtles and completely sacrifice the spine's ability to move in the process.

Based on these fish, I wished to have creatures with similar anatomical traits, having an internal skeleton handling most, if not all of the support function while having their bodies and limbs mostly covered by a group of segmented bony plates (ideally with some of them fusing into a form reminiscent of certain late medieval armors, with more uniform, larger plates along arms legs and torso regions while having a larger number of smaller, more flexible plates near the joint regions). My main worry however is if this could actually translate into larger animals on land or if such additional bony armor would only make it marginally superior to an exoskeletal supporting system, especially since these guys are the only vertebrates I know how be armored to this degree (the closest thing to it being turtles, placoderms and certain reptiles like komodo Dragons and crocodiles, which do have bony protection covering a reasonable portion of their bodies, and in an almost chainmail-like way in the case of komodo Dragons, but still not nearly as extreme as the plates in these fish).

That said, could a creature with most of its body (limbs included) covered by sets of bony plates under the skin in a way similar to armored catfish actually reach larger sizes on land better than something with an exoskeleton? the addition of likely consequences of growing larger while having this kind of bony structures in answers would be much appreciated.

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  • $\begingroup$ Land, sea or both? Not exactly an answer, but worth checking out glyptodonts - probably not exactly what you're after, but a general idea of what's possible. $\endgroup$ Nov 22, 2022 at 13:05
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    $\begingroup$ The tortoises shake their burly legs at you in indignant protest! "Look at that fin!" they grunt, pointing at the catfish (who pretends not to notice). "You call that armor?" $\endgroup$
    – Willk
    Nov 22, 2022 at 13:33
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    $\begingroup$ Dunkleosteus is a genus of placoderm fishes which are thought to have reached almost 9 meters (29 ft) in length and 4 tonnes in weight. They were not catfishes though, and not even similar to catfishes. $\endgroup$
    – AlexP
    Nov 22, 2022 at 13:41
  • $\begingroup$ Spiders have different body segments to other arthropods (they're fused) as well as hydraulicly operated legs. Are you sure your limitations need be fatal? $\endgroup$
    – user86462
    Nov 22, 2022 at 17:54
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    $\begingroup$ So that's why those carppy little bozons are so honking hard to kill.... $\endgroup$
    – JBH
    Nov 22, 2022 at 20:30

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You can absolutely do it.

osteoderms have evolved many times, there is however a trade off the more armor you have the less flexible you can be.

Osteoderms have evolved many times. knonw examples exist in crocodiles, xenarthrans including, ground sloths, armadillo, several lines of dinosaurs, several reptiles, multiple amphibians, ect. It is a reoccurring theme.

Many small sperate osteoderms is the most common but not the only option.

Armadillo is probably closest to what you want out of living animals.
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Xenarthrans are the only group to evolve large plates that can give you the segmentation look you want. below is a glyptodon tail segment with vertebra.

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This will be a very helpful paper for you, https://link.springer.com/article/10.1007/s11837-012-0301-9/figures/3

Note they all have either skin or keratin over the bone, you don't want naked bone exposed to the air for long.

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A giant pangolin is a real thing matching your description

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The giant pangolin is the largest of all pangolin species. While its average mass has not been measured, one specimen was found to weigh 33 kg (72.6 lb). Males are larger than females, with male body lengths about 140 cm (4.6 ft) and females about 125 cm (4.1 ft). Like all pangolins, the species is armored with large, brown to reddish-brown scales formed from keratin.

And they get larger than creatures with exoskeleton. Not to mention extinct creatures which were even larger than that, like the Doedicurus.

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  • $\begingroup$ This isn't quite what I'm searching for. I'm aware of pangolins and their scaly keratin armor, but right here I'm more interested in a more extreme version of osteoderm armor to a brigandine or plate-like level, with actual bony plates covering the body rather than a series of keratinous scales over the skin. $\endgroup$ Nov 22, 2022 at 13:32
  • $\begingroup$ @ProjectApex Giant sloth? $\endgroup$
    – DKNguyen
    Nov 22, 2022 at 14:34
  • $\begingroup$ @DKNguyen kinda, but more extreme. $\endgroup$ Nov 23, 2022 at 0:42
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Look at dinosaurs

Right, so the ankylosaurus is known to be a pretty well-armoured fellow weighing between 4.8 and 8 metric tons and being between 6 and 8 metres long.

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Their armour plates are osteoderms, basically meaning bone, and they give you a pretty good idea about what has been done to armour more significantly substantial animals. Their exact placement and the amount of those plates it had may be up for debate, but their weight alone should give you a good enough approximation of how large an armoured animal could get, though you may want to be a bit more conservative with its size if you want them to be as armoured as your catfish as far as weight concerns is concerned.

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