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Looks difficult for a two-limbed creature to evolve the powered flight, specially the part of the blastoff, considering that pterosaurs had a quadrupedal blastoff which is good, but birds a bipedal blastoff which is more problematic or have less efficacy, so to have less limbs could be problematic

Also I just can think in not many real life species with just two limbs, one of them are the birds which became completly terrestrial and flightless, the question is a little bit strange, the birds which returned to a fully terrestrial life-style have lost their flight features and have reduced its wings till be so athrophyed that hardly can become useful again and even some completly dissapeared its wings.

Examples or birds like this are cassowary, tinamu, kiwi, emu and some other extinct island birds (a bird even turned its wings to rigid clubs).

The moa really lost its wings.

So if a bird with just two limbs could and should become flying again, how would do this?

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  • $\begingroup$ those birds still had wings, loosing a limb entirely is really rare in evolution. also the most likely answer is they can't since none of them are climbers. $\endgroup$
    – John
    Aug 19 '21 at 14:00
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    $\begingroup$ @John It's not entirely implausible that you'd have some flightless bird becoming something like a mountain goat, filling that niche. However, at that point, I'd expect their vestigial wings to redevelop into some sort of useful claw, suitable for holding onto a hold in a cliffside. One needs to go beyond Earth morphology/biology for this question to make sense. Some evolutionary paths may be closed entirely though, and if they are, this is a good candidate. $\endgroup$
    – John O
    Aug 19 '21 at 14:35
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    $\begingroup$ @JohnO yes this is probably a closed path, keep in mind birds can't do the splits, their hip joint is very different than ours. $\endgroup$
    – John
    Aug 19 '21 at 15:37
  • $\begingroup$ @JohnO . Oh yeah, that's a very a very interesting look at the limitations of birds, none have evolved adaptations to the scale because they normally fly and the only one that came close to developing that kind of adaptations I think was the highland moa. $\endgroup$
    – Drakio-X
    Aug 19 '21 at 16:54
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    $\begingroup$ Consider the hummingbirds, which have nearly vestigal legs. They use them only for perching (and scratching), not for walking or launching themselves: birdwatchingbuzz.com/do-hummingbirds-have-feet And @ John O: If you've ever had a parrot, you may have noticed that they climb very well with beak and feet. Likewise woodpeckers and similar birds. $\endgroup$
    – jamesqf
    Aug 19 '21 at 17:20
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How could two-limbed creatures evolve flight?

If an animal with no legs can do it then it's very hard to reason that an animal with two legs can't.

Flying snakes aren't there yet of course but they clearly have the precursors, one can easily imagine their ribs extending further over time & even the muscles for some of their ribs adjusting to provide better attitude control & (eventually) flapping for powered flight.

Your two legged ground birds are a poor example for what you're asking though as they'd be more likely to re-evolve flight exactly how the bipedal animals the birds evolved from in the first place did & simply regain their wings over time than any other way.

Practically no ground birds have lost their wings after all, they've simply become vestigial.

The cassowary you mention as an example of a wingless bird do in fact have wings (I don't know about the others but I suspect the same applies) just very small useless ones & any that have lost them most likely still have the genes for them just switched off.

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    $\begingroup$ @Drakio-X but vestigial limbs can easily redevelop anyway, a fast running bird might begin using it's stubby useless wings for balance, becoming bigger with more surface area to provide more leverage against wind for faster turning & braking could then prove advantageous, (& that's like as not how it first started in birds ancestors as well) a population isolated in mountains might then begin leaping across gaps etc etc, that's how evolution works, there are a million & one ways it could conceivably happen. $\endgroup$
    – Pelinore
    Aug 19 '21 at 17:49
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    $\begingroup$ I'm not entirely sure the moa has lost it's wings the way a snake has & weren't just really teeny (I might go check later)? but if it had lost them like snakes have lost their limbs & our ancestors that came out of the water lost their dorsal fin then there's always atavistic genes, on occasion you get a calf or sheep with a third limb growing from their back (the fin gene switching back on) or a snake with legs, this can be genetic mutation making it hereditary, if the mutated animal achieves some sort of reproductive isolation you have yourself a population with the wings again. $\endgroup$
    – Pelinore
    Aug 21 '21 at 11:29
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    $\begingroup$ ^ that would be your first step if it had lost it's limbs then you'd go from there (the initially re-established limb isn't likely to be fit for purpose) // if it's an animal that never had limbs then you're likely stuck with modifying the ribs or some other bone like the snake & some lizards have their ribs into pseudo-wings & one or more of those eventually developing into a full limb or wing of some sort over a much greater timeframe. $\endgroup$
    – Pelinore
    Aug 21 '21 at 11:38
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    $\begingroup$ Oh my!! an idea just popped into my head for sheep with an arm on their backs,, first the atavistic leg then selective breeding to morph it into an arm & hand, :)) why!! why would I even think that!! & why? what use could it possibly be to anyone doing it? $\endgroup$
    – Pelinore
    Aug 21 '21 at 11:55
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    $\begingroup$ also entirely new bone might be developed & conceivably evolved into new skeletal structure that eventually join up fully with the rest of the skeleton given enough time & appropriate interim stage evolutionary usefulness // it happens Osteoderm is an example of that (of new bone, not of the full development into a new limb of course). $\endgroup$
    – Pelinore
    Aug 21 '21 at 12:08
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One possibility to consider is that, having effectively lost their wings to evolution (the way snakes and a few lizards lost their legs), your "flightless" birds developed their legs into wings. They'll run along the ground like the cartoon roadrunner, and when they're going fast enough, they'll hop into the air and (possibly with a muscle-driven reconfiguration of their pelvis) spread their legs to the sides and begin to flap and glide like any ordinary bird.

Balance, as with many birds and most pterosaurs, is handled mostly with varying wing sweep, likely aided to some point by the feathered tail they haven't had any reason to delete.

These will look odd to our eye; instead of the highly developed pectorals bonded to the cartilaginous keel found in most flying (even partially flying) birds, musculature of similar size and power would need to be bound in some way to the pelvis (along with evolving the hinges and locks needed to reconfigure the pelvis from walking to flying).

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    $\begingroup$ I've upvoted this, because it's a smart answer. However, I find the notion a little dubious... though I doubt it'll ever happen, I've love to see someone's formal writeup proving or disproving that this could work. If it can work, I'm certain it limits the flyer's size even more so than that for typical birds. $\endgroup$
    – John O
    Aug 19 '21 at 14:58
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    $\begingroup$ legs hat let you run around will not make good wings. a limb can only do so many jobs well functionally. keep in mind bird cannot splay their legs, their hip joint is rod and socket not ball and socket like mammals. $\endgroup$
    – John
    Aug 19 '21 at 15:35
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    $\begingroup$ @John "bird cannot splay their legs," -- hence why I state that they'd need to be able to reconfigure their pelvis "on the fly" as it were. Not impossible -- snakes do something similar with their jaws (and some have teeth in their esophagus to break eggs that were swallowed whole). Not likely -- but then how likely is a fish that can discharge 600V? FWIW, IIRC there was at least one pterosaur that used its wing "wrist" joints as auxiliary feet. Whale, seal, and sea lion flippers started as legs. Limbs doing stuff they don't do well is just a matter of how long you're willing to wait. $\endgroup$
    – Zeiss Ikon
    Aug 19 '21 at 16:02
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    $\begingroup$ @John Yet gliding obviously served some function for the earliest distinct ancestors of birds (there was even one that had a tandem wing setup, flight feathers on front and rear limbs, that's believed to have used both for gliding -- and seems to have managed enough leg splay for that). $\endgroup$
    – Zeiss Ikon
    Aug 19 '21 at 18:13
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    $\begingroup$ @ZeissIkon actually no, early birds did not have a gliding stage, they maintain high performance running throughout the development of flight. those birds with "wings" on the legs are just using them as control surfaces, held vertical not horizontal. Artists often get it wrong, if you see a fossil with splayed legs you will also se the femur is snapped in half to achieve that position. $\endgroup$
    – John
    Aug 19 '21 at 18:17

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