I've seen some speculations about how dragons could have evolved, like this, but none of them ever seem particularly convincing. Swim bladders that collect hydrogen and help the dragon float while letting it breathe fire, blah blah blah...no. And the explanation of wings is even worse -- "A mutation gave lizards an extra pair of limbs, which evolved into wings" -- do they know how evolution works? A massive mutation to add an additional pair of limbs would almost invariably screw up a bunch of other stuff. Even if it didn't, the new limbs would have inadequate blood supply, useless musculature, and no spinal attachments. And even if this did somehow present a survival advantage, a massive mutation like that isn't going to be consistently passed on.
Coming up with a good speculative biology for the evolution of dragons requires more than "Oh, look, a mutation." There needs to be specific selection pressures to help each of the steps along. I was thinking something along these lines...
The Draco volans lizard from Asia is basically a dragon. Extra ribs, unattached in the front, that are pulled out by muscles to form gliding wings:
They can glide from tree to tree pretty well, flying-squirrel-style. Of course, they're quite small, and the placement of their ribs is not ideal for further developments. But some of its extinct cousins, like Kuehneosuchus, were larger and had higher-mounted rib-wings:
They fit their gliding-from-tree-to-tree niche well. But what environmental selection pressures could have turned something like this into a full-on dragon?
I was thinking that an aquatic component might spur things along. Gliding between trees is good and all, but imagine a population of kuehneosaurids branching out into a seaside or lagoon environment with lots of fish in the water and lots of high cliffs. Being able to launch from a high point and glide down to hit a fish would be a pretty neat hunting trick, and help support larger morphologies and a larger population. Cliffs offer longer glide times as well, meaning that improvements in glide control would be more rapidly selected for.
Anything that would help with glide path control would be a huge advantage -- it's not like fish can't change direction, after all. So it wouldn't be difficult to progressively evolve more musculature to control wing attitude and shape. The more muscle connections formed, the more finely-tuned gliding flight would become, and at some point the protodragons would be able to use the wings along with their whole bodies to add speed, perhaps just skimming over the surface of the water. Lepidosauromorphs like kuenhneosaurids (related to modern iguanas) have a primitive sprawling gait which allows for sinusoidal trunk and tail movements similar to fish and crocodiles, so full-body movements to add speed (and thus add range and lift) wouldn't be hard to develop.
Of course, when one of these protodragons did hit the water, its wings would only be in the way. So the ability to fold them up against the body would be a huge help, leading to advantages for ever-higher-seated rib sockets. More space between the wings and the back means more room for movement, which adds control and strength. True elbow joints probably couldn't evolve, but perhaps vertebral elongation would allow for a similar effect.
Over time, flapping action could evolve. It would still probably require a launch (at least at first), and the skeletal structure wouldn't be anything like bat or bird or pterosaur wings (which evolved from tetrapodal limbs), but that wouldn't keep it from becoming highly efficient. A steady diet of fish, small mammals, and smaller reptiles would support significant growth in size. Ease of takeoff is a huge advantage, of course, and so adaptation would move in that direction until you ended up with large, four-legged, fully-winged flying reptiles. Takes care of morphology.
Anything that actually proposes breathing fire is, of course, ridiculous. Methane or hydrogen bladders? Please.
But transitioning from arboreal life to an aquatic/terrestrial environment would surely open the population up to predation, particularly at small sizes. Depending on the time and environment, there could be any number of natural predators for these lizards. Wings are great for gliding, but they make you rather vulnerable to attack; a torn or broken wing will screw you up pretty bad, and gliding tends to prefer slender, skinnier anatomy, so the early protodragons wouldn't be flush with physical strength. It wouldn't take long for a predator to do serious close-in damage.
Thus, a defense mechanism which can keep predators at bay will be hugely useful. Certain cobras and even some vipers have the ability to spit venom, which would definitely come in handy. It's unlikely the protodragons would evolve the same type of venom that snakes have, though, since it would have to evolve independently. My chemistry-fu is not my strongest asset, but I'm fairly certain there are some biologically-constructible compounds which would serve as an excellent deterrent to attack. Perhaps something with a caustic peroxide component?
Peroxides, of course, can act as excellent oxidizers. The more caustic the venom becomes, though, the more dangerous it is to the protodragon itself. The lizards would likely evolve some sort of inhibiting compound in their saliva to prevent self-harm; once this was established, though, the causticity of the venom would skyrocket...and with it, the potential to act as a rapid oxidizer. I think the transition from caustic venom to oxidizing venom could happen pretty naturally. The faster the damage is done, the more of a deterrent it is to the predator. And an oxidizing venom replacing a caustic venom means the dragons can use it for hunting as well, because they wouldn't be swallowing anything very harmful.
Again, my chemistry is not the best, but I recalled reading about chlorine trifluoride:
In a comment to my post on putting out fires last week, one commenter mentioned the utility of the good old sand bucket, and wondered if there was anything that would go on to set the sand on fire. Thanks to a note from reader Robert L., I can report that there is indeed such a reagent: chlorine triflouride.
I have not encountered this fine substance myself, but reading up on its properties immediately gives it a spot on my “no way, no how” list. Let's put it this way: during World War II, the Germans were very interested in using it in self-igniting flamethrowers, but found it too nasty to work with. It is apparently about the most vigorous fluorinating agent known, and is much more difficult to handle than fluorine gas. That’s one of those statements you don’t get to hear very often, and it should be enough to make any sensible chemist turn around smartly and head down the hall in the other direction.
The compound also a stronger oxidizing agent than oxygen itself, which also puts it into rare territory. That means that it can potentially go on to “burn” things that you would normally consider already burnt to hell and gone, and a practical consequence of that is that it’ll start roaring reactions with things like bricks and asbestos tile. It’s been used in the semiconductor industry to clean oxides off of surfaces, at which activity it no doubt excels.
There’s a report from the early 1950s of a one-ton spill of the stuff. It burned its way through a foot of concrete floor and chewed up another meter of sand and gravel beneath, completing a day that I'm sure no one involved ever forgot. That process, I should add, would necessarily have been accompanied by copious amounts of horribly toxic and corrosive by-products: it’s bad enough when your reagent ignites wet sand, but the clouds of hot hydrofluoric acid are your special door prize if you’re foolhardy enough to hang around and watch the fireworks.
I’ll let the late John Clark describe the stuff, since he had first-hand experience in attempts to use it as rocket fuel. From his classic Ignition! we have:
"It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes."
Obviously, I doubt there is any peroxide compound nearly as powerful as chlorine trifluoride, and actually making chlorine trifluoride is out of the question, but I can imagine something more along those lines. Being able to spray a thin stream of venom that causes instant oxidization burns and can sometimes ignite what it hits...that's a HUGE weapon. And getting to an "always-ignites" status is the next logical step.
So there you have it. A six-limbed, fire-spitting, flying dragon.
Any objections? Any chemistry experts want to take this to task?