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For what I know birds have an much lower superior size limit for flying forms than pterosaurs because they use two different groups of muscles to locomotion, wings for fly and legs for walk in difference with pterosaurs which the same muscles to fly and walk, this cause that the legs turn to dead weight folded under the body when birds are flying, generating flight drag and requiring more effort by the wings.

The biggest flying birds were notoriously littler than the biggest pterosaur.

Both groups were part of archosauria, they have hollow bones, air sacks and a super efficient respiratory system, so the limitations looks like are caused by the locomotion muscles distribution.

That brings me to my problem with hexpod dragons with similar characteristics to archosaurs, since the point of having a dragon is that at first glance it seems that it is a perfect combination between air and land displacement maneuverability by having six limbs one pair. dedicated to flight and two pairs to ground movement, however taking into account what they mention about the size of birds and pteoresaurs, the fact that a dragon has two pairs of limbs seems that it will make its maximum size noticeably smaller than birds bigger.

So considering that, what would be the largest size (maybe considering lenght and weight) that a dragon could reach? And in case it's really small, how could it be bigger?

For other details, these dragons will be very similar to archosaurs, without any strange material as wildcard like graphene (although I could elaborate on this in case the dragons are too small). And principally based on the body shape of this pseudosuchio, just with the wings and modifications to look like a dragon.

I ask about because in questions about realistic dragons this detail isn't enough considered

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  • $\begingroup$ the size limitation that limited pterosaurs is still there, if anything the weight of extra limbs will force your creatures to be smaller. so your still not getting anything bigger than Quetzalcoatlus. and that's about as good an answer as you can get with limited information, we don't even know how much Quetzalcoatlus with much accuracy. the real thing you have to think about is where to fit flight muscles and a set of front limbs. $\endgroup$
    – John
    Jul 27, 2021 at 4:53
  • $\begingroup$ @John I'm almost completly sure that Quetzalcoatlus is the limit size for all flying creatures in the Earth, I dont want a dragon bigger than that, just some kind of realist animal which comparable with the classic dragon shape. $\endgroup$
    – Drakio-X
    Jul 27, 2021 at 5:07
  • $\begingroup$ I get that but it comes down to weight more than size, you need a wingspan to account for mass not volume of the body, but again we have a large margin of error on Quetzalcoatlus mass estimates. which is why you are not going to get anything better than not bigger than Quetzalcoatlus any kind of more detailed analysis would be hugely dependent on minutia of the creatures anatomy. the first thing you need to figure out is were to put the flight muscles, because they will be huge, they should take up about half the volume of the torso. $\endgroup$
    – John
    Jul 27, 2021 at 5:13
  • $\begingroup$ keep in mind the size of these animals can be deceptive. Quetzalcoatlus northropi is the size of giraffe but weights about as much a horse or less. If you want to play it safe use Quetzalcoatlus sp. as your limit instead of Q. northropi and you are probably good, just be aware your ground limbs need ot be very thin and gracile since you are essentially splitting the mass of a single pair of normal limbs between them. basically you need to build your bodyplan around the size you want then check and see if the bodyplan works. $\endgroup$
    – John
    Jul 27, 2021 at 5:32
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    $\begingroup$ bats have other issues though bats are mammals are mammals are awful at being big our respiratory and musculoskeletory system plus live birth all add extra costs or problems with being big. if you are really worried use the largest birds, birds are less efficient flyers than bats so you should be fine. your real issue is you still need the same amount of the body dedicated to wings and flight muscles, so you need to steal the mass for the extra limbs and tail from other parts of the body. $\endgroup$
    – John
    Jul 29, 2021 at 4:32

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Don't let the naysayers get you down. In 2019, no one would have been surprised to hear about a newer, larger jet plane that could pack in even more rows of cramped customers side by side. They could do so at any time - they just don't. Biology is just another type of engineering, and the same is true here. The most relevant practical limit on both systems is the same: a lack of ample prey in range to feed these mighty beasts. While it is true that some taxa (insects) seem to have upper size limits rooted in physics, that is in a situation where they compete with other taxa better organized and adapted to lifestyles as large animals. For a time before the K-T impact the surviving precursors of mammals (cynodonts) appeared doomed to thrive only as very small organisms, despite having an evolutionary history and warm-blooded metabolism more appropriate for larger bodies. Biology can't always outcompete itself, but when posed with a hard practical obstacle and a lack of competition, there is always the potential to invent something new. There's no need to suppose that the limits on wing loading are an insurmountable law of nature when they may simply mean that biology preferred to evolve more fuel-efficient ways to increase size to the extent that has so far been needed.

Your dragons carry a heavy "payload", but if the additional limbs mean that they are able to take a wider range of prey more often and with better efficiency, they can still be worth having.

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    $\begingroup$ except flight imposes some pretty hard limits itself, the square cube law is unforgiving, it really is insurmountable. Also your second link has nothing to do with the question, $\endgroup$
    – John
    Jul 29, 2021 at 4:36

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