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Land Placoderm

Context

This creature right here is one of the decedents of placoderms that that transitioned from rivers to land. The got onto land during the Middle Devonian and remained isolated from all other land animals. The only non-placoderm descended creatures are arthropods, amphibians, and a few species of birds.

Anatomy

They have and spine, skull, a few ribs, flexible stringy organ support structures, and ball and socket joints for their exoskeleton. The exoskeleton is all over the outside except for in the joints and supports most of the creature. The joints are all endoskeletal and ball and socket joints covered with muscle and leathery skin. They have six legs and walk similarly to insects buy having three legs on the ground while walking. They don't molt their exoskeleton expands like sea urchins do and they don't even develop the exoskeleton until they at least halfway don't with growing to their full size. They are about 30% larger than a Paraceratherium ans weigh in at 45-57 tons.

Question

Based on how this creatures anatomy works could it physically sustain the weight and height it is at, and if not what are the minimal changes I can make to allow it to be at this size and weight?

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    $\begingroup$ Is that a 5th leg? Or are you just happy to see me? $\endgroup$ – Willk Sep 15 '18 at 1:48
  • $\begingroup$ @Willk The reproductive organs are actually on the tail and are claspers $\endgroup$ – Amoeba Sep 15 '18 at 2:12
  • $\begingroup$ @Willk I'd run... $\endgroup$ – kingledion Sep 15 '18 at 5:00
  • $\begingroup$ @Willk It looks like there are actually 6 legs, one on the other side hidden by two of the legs on the closer side. $\endgroup$ – John Locke Sep 15 '18 at 16:43
  • $\begingroup$ Exoskeletons don't exist on vertebrates for good reason. It looks to me like what you describe as an exoskeleton is actually just armor plating (as seen on Placoderms), which is an important distinction. $\endgroup$ – Zac Walton Sep 26 '18 at 20:45
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Looking at Other Massive Animals

The one of the largest land animals to ever live was the Argentinosaurus which could weigh in at a maximum of about 90 to 100 tons (though sizes of about 60 to 70 tons was probably more common.) Paleontologists utilized digital reconstruction of musculature and joints and estimated such an animal probably had a viable top speed of about 5 miles per hour. Bigger generally means slower. With extremely large animals joints and ligaments tend to be major structural weak points.

Additionally, titanic sized dinosaurs actually didn't have very big young, the massive Argentinosaurus laid eggs which were only about 22cm in diameter. Many such larger dinosaur species had to grow 10,000 to 20,000 times their birth weight to reach adulthood, predation was heavy and it is theorized that not many of the titanic dinosaurs ever reached adulthood. It is estimated that animals like the Argentinosaurus took 15 to 20 years to grow to the size they did. Something else you might include could be gastroliths, which are stones which the animal has swallowed to assist in grinding its food for more efficient digestion.

As far as your 6 leg design, there is a reason the only animals we see with limb configurations like that only tend to be very small like insects. An extra set of redundant legs burns a lot of extra calories without really adding much to the creatures ability to move. I understand that basic common sense makes it seem like 6 legs would be more stable and powerful than 4, but actually adding in an extra pelvis, spine, then another pelvis creates structural weak points as well. When dealing with moving weight in the tens of tons it is always better to minimize points of failure, and evolution seems to show us this. Across hundreds of millions of years no creature has ever evolved to have two pelvic bones and two spines. Its just too much complication and added points of failure when a stronger standard body layout is much more efficient.

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  • $\begingroup$ They don't have pelvises, the legs are connected by a ball and socket joint that connects to the exoskeletal carapace of the leg and the body. Does that change or solve the problems with six legs? $\endgroup$ – Amoeba Sep 15 '18 at 17:45
  • $\begingroup$ An exoskeleton that large would collapse upon itself under stress of movement. That's one of the main reasons insects are small, exoskeletons cannot support very much weight. $\endgroup$ – TCAT117 Sep 16 '18 at 6:55
  • $\begingroup$ @TCAT117 the exoskeletons of placoderm fish - which are vertebrates - are not their structural support, just an external layer of armour plating. Worth noting that Paraceratherium, which the OP mentioned in his question, has armour plating too. $\endgroup$ – SealBoi Sep 16 '18 at 18:33
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Given the ancestral creature is actually a fish, and that fish are the ancestors of all modern animals from frogs to elephants and beyond, the question is actually moot: they can evolve to whatever size is required to successfully fill an environmental niche.

enter image description here

placoderm skeleton

Like other animals, their skeletons will change and adapt, they may evolve structures analogous to hips or shoulders to deal with the weight and other adaptations might take place as well (large sauropod dinosaurs had many ligaments "strung" through their spinal columns and tails, creating a sort of composite structure using tension to provide a great deal of strength without a lot of weight.

Of course, the reason you chose this ancestral fish was likely the "cool" factor of the armoured head and jaws, but over the millions of years of evolution on land, you will need to explain what advantages this has in their current ecological niche. The picture seems to indicate a herbivore, so an armoured head and jaws don't seem to add much, and indeed might be an impediment, extra mass for little or no gain. Real creatures will either drop that adaptation if not needed, or shrink it to vestigial size.

So the short answer is this is an animal descended from fish, so like any self respecting fish (or animal) it will have a fully formed internal skeleton, not an exoskeleton, and thus have all the necessary adaptations and parts to successfully fill a niche on land.

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  • $\begingroup$ The reason for the exoskeleton is to store vital minerals and chemicals like calcium and ATP, to protect against falling scale trees, flesh eating insects, and larger predators... $\endgroup$ – Amoeba Sep 16 '18 at 5:34
  • $\begingroup$ Animals use all kinds of different ways to accomplish all of these things (storage of minerals, hormones and chemicals for life is generally done via internal systems like bones and glands), while protection can also be accomplished by sheer size, thick leathery skin or a highly aggressive attitude. A smaller herbivore in an environment with large, aggressive predators will also benefit from protection (like an armadillo or ankylosaur in different eras). $\endgroup$ – Thucydides Sep 17 '18 at 0:09

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