Long-time readers will know that I am an unashamed fan of both speculative zoology, and of Dougal Dixon's hypothetical 'alternative' animals. Inspired by a comment made here in August by Jenny Islander, I have been having a re-think about the possible evolution of flightless pterosaurs: the fossil record gives no indication that such animals ever existed, and maybe they didn't, but that hasn't stopped people from speculating. The best known hypothetical pterosaurs are those from The New Dinosaurs (Dixon 1988), and among the most bizarre and memorable of them is the Lank Herbafagus longicollum, a long-necked, long-legged, long-faced flightless African pterosaur that eats grass and can swiftly out-run predators. Ridiculous, right? Well, the recent work on azhdarchid palaeobiology has led me to reconsider exactly how 'ridiculous' it was...
Like many of the animals from The New Dinosaurs, the Lank is suspiciously convergent with an extant animal, and in this case it's the giraffe [painting by Steve Holden © Dougal Dixon 1988, used with permission]. It's even patterned like a giraffe (and, specifically, like a Reticulated giraffe Giraffa reticulata [following the taxonomy of Brown et al. (2007)]). Yes, I will admit that, like others, I long found the Lank to be one of the daftest and most improbable of Dougal's 'alternative' animals.
In his review of The New Dinosaurs, Greg Paul (1990) dismissed the Lank as one of the stupidest creatures in the book, writing '[T]he "lank" is perhaps the worst beast in the book. That pterosaurs would beat their dinosaurian cousins into evolving giraffes, of all things, complete with hooves and reticulated orange-brown colour pattern, is unbelievable' (p. 311). David Unwin (1992) was similarly dismissive, writing 'Bipedal pterosaurs with wings free of the hind limbs, unlikely as they seem, cannot be ruled out, but I doubt whether evolution, at its most inventive, could produce 'lanks' and 'flarps', fully erect, four legged, grass-grazing, flightless pterosaurs!' (p. 63: note that this description actually applies only to the Lank as the Flarp Vexillala robusta is bipedal).
I would agree with Greg that the possibility of flightless pterosaurs successfully out-competing thriving and diverse herbivorous dinosaurs in adapting to plains-dwelling herbivory does not seem plausible, but as to whether the Lank is plausible in principle... well, I have, as so often happens, changed my mind. Now that the case for strong terrestrial adaptation in azhdarchids has been made (Witton & Naish 2008), we can reconsider the idea that, were the right conditions to arise, or were azhdarchids to hang on beyond the end of the Maastrichtian, they might well have reduced their already short wing-fingers and evolved flightlessness.
As you'll know from Witton & Naish (2008), or from the Tet Zoo or Wittoniana or Azh Pal articles that accompanied it, azhdarchids combined their proportionally short wing-fingers with elongate hindlimbs and small, narrow, well-padded feet. They walked with a narrow, parasagittal gait, and their tremendously robust, well-muscled arms indicate that they were the strongest launchers and, more importantly here, most powerful quadrupedal runners among the Pterosauria (the data supporting these assertions has yet to be published but is in the works and has been alluded to here on Tet Zoo before. For the time being it can be regarded as M. Habib, pers. comm.).
In view of the 'terrestrialization' of azhdarchids, I would say that, if any pterosaurs were to become flightless, then azhdarchids are at the top of the list. The fossil record gives us no indication that this happened. But if it did, it would (probably) only happen in environments devoid of predators like sebecosuchians or big theropods, as even a giant galloping flightless azhdarchid is probably under pressure to retain a flight ability so long as such animals are around. Note that, if the development of flightlessness happened at giant size, then flightless azhdarchids would probably not have to worry too much about small predators, like troodontids or small dromaeosaurs.
As Witton & Naish (2008) noted, the proportions of a quadrupedally walking azhdarchid are most similar to those of cursorial ungulates, specifically artiodactyls like wildebeest and giraffes. This is coincidental: cursorial ungulates and azhdarchids have not converged in proportions because of similar lifestyles. So what is a flightless azhdarchid - one with strongly reduced or absent wing-fingers - going to look like? By now you've already seen my attempt at imagineering one. This animal, Shemhazai ptychocheirus, might be from the Upper Cretaceous, or it might be from the Paleogene in a world where the end-Maastrichtian event never happened (like Spec). It has lost the uropatagia and most of the brachiopatagia and has a strongly reduced wing-finger. Manual digits I-III have hoof-like unguals, have rotated on the metacarpus to face forwards, and are more robust and better at weight-bearing. But that's about it, it really isn't that different from its volant relatives. Given that this animal cannot fly away when resources are dwindling, it could only have evolved in a place where there is a year-round supply of diverse foodstuffs, and I imagine it inhabiting semi-tropical mixed woodland where it forages for arthropods, molluscs, worms, small vertebrates, fruits, seeds, carrion and anything else edible.
Azhdarchids of the Miocene
If we now imagine another 20-30 million years of evolution on top of that, in which time members of this lineage have become even more terrestrial, better at running, and better at exploiting terrestrial resources, what might they look like? Well, kind of like the Lank actually. My cursorial, grassland-inhabiting Miocene azhdarchid - Qilin parungulatus* - is shown here (with Toni Naish for scale). What's happened now?
* Qilin was proposed by Alec T and parungulatus by Stevo Darkly. Thank you all again.
All traces of wings and wing-membranes are gone, and the even longer limbs are now more specialised for cursoriality and terrestriality. The body is far deeper and narrower than was the case in volant azhdarchids, and the forelimb and scapulocoracoid have become reoriented such that the humerus now projects posteroventrally. The distal end of metacarpal IV has become a heavily keratinised pseudo-hoof, and digits are entirely absent. Given that metacarpal IV is so large and robust in Cretaceous pterodactyloids, I thought it more likely that this - rather than digits I-III - would become the main organ of support in the forelimb. In the hindlimb, a need to become swift and cursorial has resulted in the development of digitigrady and of three hoofed digits on a super-long limb. The long neck is more flexible than that of its ancestors, and its serrated rhamphothecal tomia (= bill edges) make it an efficient browsing herbivore, capable of cropping and eating most plant material. It has voluminous guts, a big muscular gizzard, and is huge. And, yes, it looks like a giraffe, but that isn't because I really wanted it to.
A few discrepancies make the Lank 'inaccurate' if we compare it to Qilin parungulatus. For starters, the Lank walks on a strongly reduced, hoof-tipped wing-finger (Dixon 1988). As a result, its remaining 'free' fingers (there are only two) are now located up off the ground. The animal uses them in grooming but they are otherwise non-functional. However, given that azhdarchids (and other pterosaurs) walked with their wing fingers folded up and out of the way (only the base of the first phalanx could have played a role in weight support) and with their three 'free' fingers contacting the ground (indeed, these fingers were probably retained because of their role in quadrupedal walking), we should really expect the Lank to have a pretty different forelimb morphology. It might still have hoof-tipped digits, but these would correspond to digits I, II and/or III, not IV (the wing-finger). The wing-finger should either be absent altogether or, if retained, form some weird little display organ perhaps (as it does in Shemhazai).
Unlike any azhdarchid (or azhdarchoid), lanks possess teeth, and that might seem strange if lanks are meant to be derived azhdarchids. However, this isn't necessarily such a problem given that - as we've seen on Tet Zoo before - teeth can be, and have been, re-evolved in the jaws of clades that had otherwise lost them. Having said that, we might expect the Lank to have more a pterosaury-looking head: as a beaked herbivore it should perhaps look more like a long-billed goose. The Lank's head actually looked too mammalian. Qilin parungulatus has serrated tomia that function as pseudo-teeth, so teeth just aren't needed [in image below, giraffes from Longleat's giraffe page].
All in all however, the Lank does not fare that badly when put up against a more contemporary vision of what flightless azhdarchids might look like.
In case you're wondering, this does not mean that the other Dixonian flightless pterosaurs are ok. The world of The New Dinosaurs was populated by a diverse pterosaur assemblage that, besides lanks, included the savannah-dwelling Flarp, the wading sifts and parasos, the raptor-like Harridan Harpyia latala, shorerunners, the immense, long-winged oceanic Soar Cicollum angustalum, penguin-like plungers, and the flightless, muppet-like wandles and kloons of New Zealand. Paul (1990) noted that, given their low diversity prior to the end-Maastrichtian event, the survival of pterosaurs into the Cenozoic might be doubtful even if a mass-extinction event failed to occur. Unwin (1992) noted that many of the lineages portrayed in The New Dinosaurs seemed unrealistically diverse and speciose in view of the fortunes of their Maastrichtian ancestors. This was particularly true of pterosaurs: so far as we know at the moment, azhdarchids were the only pterosaurs present in the Maastrichtian (thanks to Mark, I've had to revise my previously-held idea (Martill & Naish 2006) that tupuxuarids also made it into the Maastrichtian), so they'd be the only ones able to make it into the Cenozoic.
Cursorial Miocene lank-like azhdarchids never did exist, of course. But I now think it's possible that something like Shemhazai could have existed, in which case its fossils might await discovery in some yet-to-be-described Upper Cretaceous island-endemic assemblage. We can but dream.
Refs - -
Brown, D. M., Brenneman, R. A., Koepfli, K.-P., Pollinger, J. P., Milá, B., Georgiadis, N. J., Louis, E. E., Grether, G. F., Jacobs, D. K. & Wayne, R. K. 2007. Extensive population genetic structure in the giraffe. BMC Biology 2007, 5: 57 doi:10.1186/1741-7007-5-57
Dixon, D. 1988. The New Dinosaurs: An Alternative Evolution. Salem House Publishers, Topsfield, MA.
Martill, D. M. & Naish, D. 2006. Cranial crest development in the azhdarchoid pterosaur Tupuxuara, with a review of the genus and tapejarid monophyly. Palaeontology 49, 925-941.
Paul, G. S. 1990. An improbable view of Tertiary dinosaurs. Evolutionary Theory 9, 309-315.
Unwin, D. M. 1992. The New Dinosaurs: An Alternative Evolution (review). Historical Biology 6, 61-71.
Witton, M. P. & Naish, D. 2008. A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLoS ONE 3 (5): e2271. doi:10.1371/journal.pone.0002271
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Quite extraordinary Darren. The only element, (disregarding the switchblade fingerbones), would be ear tufts in flying ancestors, developing with flightlessness into full fledged antler-tufts, but then, that might be a bit "out there"...
Hey Darren,
This chain of posts has gotten more and more compelling, particularly for us working on SPEC. We've actually seriously talked about the need for a flightless pterosaur in the final book, especially when Quetzalcoatlus and Zeighiangopterus have fared so well under scientific scrutiny.
Currently, we have been talking about an island endemic flightless azhdarchid. And what we were planning on would look almost identical to your first flightless pterosaur, if not evolved and tweaked into context, and turbo-charged with some of whatever is in Spec's air (psycho penguins anyone?)
I again commend you on your ability to enchant, entertain, and be Scientific.
Thanks Tim. I await a giant kaiju-azhdarchid of doom!!!1!
A longer time ago (I don´t know anymore if it was here or at "Ask Dr. Vector" or "The Lord Geekington" I wrote about the probable evolution of animals on isolated islands prior 65 million years ago. To find fossils of those animals seems to be extremely unlikely, but there surely were extremely extraordinairy animals on islands. Even today we seen all those strange island endemics, not to speak about those which were exterminated during the last millenia.
I had to think about the possibility that some pterosaurs possibly evolved to become more terrestrial and perhaps even completely flightless on remote islands where they had no predators to fear. Especially such animals which already found their prey mainly on the ground (like Pterodaustro). I have to say that my flightless island pterosaur look very different to Shemhazai ptychocheirus because I played with the idea that it could undergo a similar evolution like dodo and other big flightless island birds (although I had to rethink my ideas about flightless pterosaurs after you wrote at TetZoo I about Quetzalcoatlus and imagined a huge flightless and stork like pterosaur on a big island also as possible).
I made some sketches of a pterosaur with comparably robust wings and a body much more robust than those of real pterosaurs, and with a proportionally smaller head but stronger jaws. I speculated that if those animals would life on an island without any predators, they could also possibly evolve striking features like many island birds, for example a very colourfull coat or strange crests on the head (which should be of different shape than those of flying pterosaurs because they should not hamper a walking animal during its way on the ground. All in all a really bizarre creature, but how knows, perhaps there were really some flightless pterosaurs on isolated islands.
BTW, in the book "The World of Kong: A Natural History of Skull Island" there is also an extremely strange flightless pterosaur which walks bipedally on its hindlegs and has very atavistic paddle-like wings, and which lives by hunting fish in small rivers. I have to say that this is one of the animals I don´t like that much in the book, but most of the rest is really great. There are lots of really good pictures, and even if most of those animals were designed to evolve to monsters, it is still highly interesting. They created even a whole bunch of parasites and dung-eaters for some of the more special creatures, and they had really a lot of cool ideas. If you like speculative evolution you should really take a look at this book.
"powerful runners"
Were their plantigrade hindlimbs able to produce long thrust? Possibly real azhdarchids were like storks in this aspect - good at slow walking but unable to run, taking off into flight for fast locomotion.
(On pictures of these hypothetic flightless forms, their hind legs are changed into digitigrade feet similar to ungulates).
BTW - Modern birds in defense bend neck, extend and peck forward. Did azhdarchids have enough neck mobility for it?
In dorsal view it looks exactly hadrosaurian...
The fossil record of the Campanian and Maastrichtian is actually too bad to tell. Worst of all, anurognathids are only known from outright Konservatlagerstätten, and there aren't any known from the entire Late Cretaceous...
Isn't there supposed to be a toothed jaw fragment of a pterosaur from the Maastrichtian of India?
Why, the earth-shattering stench of the Mitey Poogod, of course. :-)
Having read your paper and seen Mark Whitton's amazing illustrations, I was already wondering if, as they attained adulthood, these animals underwent a shift in lifestyle. The juveniles flew more than the adults, which settled down in a territory and retained flight only as a self-defence measure. I presume there's no way of ever testing that idea, however...
> a long-necked, long-legged, long-faced flightless African
> pterosaur that eats grass
If it is meant to be the ecological equivalent of a Giraffe,
shouldn't it be a high browser rather than a grazer?
Another crazy speculation:
Considering that the forelimbs are stronger than the hindlimbs, would it be possible that a flightless azhdarchid evolves to use a three-legged gait similar to someone walking on crutches, i.e. swinging the body and hindlimb in between the forelimbs and then swinging both forelimbs forward simultaneously? Is such gait biomechanically feasible?
Considering Specworld: What exactly do you mean by "in the final book"?
Is there a book in the making? Really? That would be incredibly cool.
Maybe the cursoriality is easier to achieve than the ability to efficiently process grass or leaves, although it has been done before of course.
Would it be totally hopeless to specifically look for terrestrial pterosaurs in known former island habitats, rather than wait and see?
Tilsim writes...
Get me a grant and I'll be right on it. Several thousand dollars/euros/pounds will do it. Surely super-rich, generous anonymous benefactors aren't entirely fictional? Oh wait, they are.
They are not `entirley` fictional, Darren, read the first chapter of Tim Severins `The Sinbad Voyage`
(Even though there seems to be a lot of reasearch funding going on at the moment in the Islamic world, I doubt its for life sciences though.)
The Dougal Dixon animal I hated the most was the parashrew.
Alright, let me check my lottery ticket. Someone won that $15M prize and it might have been me. If so, you've got your grant.
BTW, I ran across this quote from historian Hugh Trevor-Roper that seems apropos this discussion: History is not merely what happened: it is what happened in the context of what might have happened.
Problem is that today's isolated islands without land mammals had land dinosaur fauna during Mesosoic. Maybe some paleogeology freak can point then - isolated islands. They probably had to be isolated since Permian extinction. And what happened to them? Collided with one or another continent?
Reading about these speculative flightless pterosaurs has me wondering about the ancestry of pterosaurs. What do we know about the lineage of pterosaurs? (From what little I've read, we don't know much, but I'm hoping you have more recent info.) There must have been flightlessness in their past as well as in any hypothetical future.
"Surely super-rich, generous anonymous benefactors aren't entirely fictional? "
Contact this teen who got published a book about ropens. And this student who got a grant for looking for orang-pendek. Just say you wanna find bones of post-cretaceous pterosaurs.
Sorry, sorry, sorry, you understand I have terribly boring programming to do...
Re: Spec's flightless pterosaurs, they will be Australian, and probably look from the neck down fairly similar to what Darren has created. Their niche will be totally different however, as they go terrestrial after all large carnivores die off in Australia during the Miocene. Omnivorous ornithipods become the dominant ambush predators, and azhdarchids become the large cursorial predators. I'm assuming they'll have more raptorial heads, but I'm going to allow Mark (Mr. Witton has agreed to illustrate them once he finishes his thesis) to use some creative license.
As to a Speculative Dinosaur Project book, it's been discussed, but I think for now we're trying to get the newer website fully updated. Considering it's been an ongoing venture for at least 7 years AFIAK, and many contributors have moved on, I think it would be hard getting permission from everyone to publish.
Fun stuff all around!
With regards to running azhdarchids: plantigrade posture is not actually as much of a hinderance to running as it might seem. Bears, for example, can run quite quickly. While a running azhdarchid wouldn't win any speed records, they could probably gallop at a reasonable clip. Interestingly enough, we might expect them to be obligate pacers while moving below the gallop transition, based on the ratio of limb length to torso length (D. Hone, pers comm).
Soon as I finish the art show and some spec-Permio-Triassic stuff, Karl, I'm all in!
Great post and delicious illustrations, but I think there are just too many air sacs, etc. in the wing to make it completely "naked," even as a walking limb.
The front limbs of such an animal would look like a chador-wearing muslim "woman" in crutches, if you know what I mean.
Furthermore, I think such animals, if they lived on islands, would be smaller than their flying, giant counterparts - no bigger than a small gazelle. They would be "large," but not that "large".
I think there may be something about the giant size that was supported through the abundance of food that was only accessible as a flyer.
I have to admit that "The New Dinosaurs" was a great disappointment to me after Dougal Dixon's earlier book, "AfterMan",simply because his New Dinosaurs simply weren't as interesting as the "old" ones.
And "Man after Man" was simply depressing.
"Man After Man" felt like a boring, somewhat troubling sci-fi story.
Both Man after Man and the New Dinosaurs are made even worse by the damning presence of a certain "brain tumor artist" (I won't give his name,) who draws everything and anything as if they were composed purely of the aforementioned tissue. Not only that, but he has fingers emerging straight from the wrist, "broken" legs, inverted feet, shadows going towards the light, and other visual atrocities. What a shame.
I just want to say that I am very proud to have half-won this naming contest on this, my favorite science blog of all time. (Or possibly tied for Laelaps' blog for favorite. If I have to share the glory, you can too.)
I just want to say that I am very proud to have half-won this naming contest on this, my favorite science blog of all time. (Or possibly tied for Laelaps' blog for favorite. If I have to share the glory, you can too.)
Stevo; Thanks for the compliment! I think Tet Zoo has got me beat, but it's good to know that you enjoy my writing.
I haven't looked at Dixon's books since I stumbled across them in the library of my old high school, but some of the illustrations struck me as a bit teleological. I don't recall all the names (the "Gimp" is the only one I really remember), but the Lank is a good example of another kind of animal being designed to fit a particular mold. If giraffes never evolved, then something would have evolved that would have looked like a giraffe, right? Not necessarily, particularly since historical contingency opens some potential evolutionary pathways and closes others.
Given that this is speculative zoology I try not to get too wound up, but it is interesting that even when we try to imagine new creatures we often push different creatures into familiar archetypes. This tendency reminds me of the "Dinosauroid," as well as Carl Sagan's thoughts on why so many purported aliens have big heads, large eyes, and small bodies;
"...the UFO abduction syndrome portrays, it seems to me, a banal Universe. The form of the supposed aliens is marked by a failure of the imagination and a preoccupation with human concerns. Not a single being presented in all these accounts is as astonishing as a cockatoo would be if you had never before beheld a bird. Any protozoology or bacteriology or mycology textbook is filled with wonders that far outshine the most exotic descriptions of the alien abductionists. The believers take the common elements in their stories as tokens of verisimilitude, rather than as evidence that they have contrived their stories out of shared culture and biology."
I feel extremely honored! Thanks Darren!
I agree with Brian, as well. I think what Dixon does should be called "anthropozoocentrism." He tries to fit hypothetical animals into familiar archetypes, and that just smacks of unoriginality. He should know that nature would be much more creative than evolving just another giraffe.
Hmm, I'm not buying the flexi-neck.
If a stiff neck was good for their predecessors, it should be good enough for them, too. More to the point, all the musculature and support that was "given up" in exchange for a light, stiff neck would need to be evolved anew. Giving up its stiff-necked attitude is about as likely as it taking up wearing a dashiki. (Speaking of which: Mark, can we see how one of these things would look in a dashiki and mod haircut?)
As a strictly armchair enthusiast, I'm flattered! Thank you!
About habitat for big flightless pterosaurs: Perhaps a land mass comparable to New Zealand or Caledonia, if any existed at the time--? Of course, even if they did exist, any fossils might be long gone or not accessible in our lifetimes. ISTR an account of the islands of the Subantarctic that included a photo of a guano-encrusted rock about 200 feet high with a remark about its origin: the last remnant above sea level of a fragment of Gondwana that rode a terrane (microplate?) out into the Southern Ocean and quietly eroded away . . .
*New Caledonia. I can proofread my own posts, really I can.
I really like this idea! I can imagine them evolving this way after ending up on an island somewhere, like moa birds.
Speaking of flightless pterosaurs, one thing I've wondered about for a while - if a group of bats were to evolve flightlessness, what would become of their wing-hands? Is it plausible to imagine that they could evolve into bipedal creatures with usable hands? And given that bats are fairly intelligent to begin with, might they have the potential to evolve into an intelligent species, given tens of millions of years of evolution?
Oh, and as an addendum...Damn you Darren, for making me open up my copy of the New Dinosaurs! I spent around half an hour trying to figure out, given what is now known of dinosaur phylogeny, if a halfway decent cladistic tree could be erected for the species within.
Some things seem doable. The Cribrium could be a basal ornithomimosaur. The Gormand is a silly tyrannosaur, but somewhat more plausible as a carnotaurine. But the wyrms...I'm just left shaking my head. While I enjoyed it as a small child, It's really hard for me to believe Dougal Dixon was actually a paleontologist at all (has he ever published anything scholarly?), and not just a science fiction writer with a dim grasp of paleontology.
Picture a small lizard like animal, about a foot in length. It's posture is semi erect, and it's hind limbs can be rotated so that it can climb down a tree trunk face forwards; much like a gray squirrel or ocelot. Between the front and back limbs is a sheet of skin that imparts to the animal the ability to glide for some distance. The skin also boasts a fuzz of sorts, sort of like a mammal's hair. As the generations pass the forelimbs will grow longer, the fingers especially so. The primitive gliding wings will become more sophisticated true wings, and the first flying vertebrates will take to the skies. That's my speculation regarding the ancestor of pterosaurs.
And unlike any known pterosaur.
Now if the Azdarchids really do show that adaption to ground life was feasible for pterosaurs I feel pretty sure that flightless pterosaurs did evolve, probably repeatedly. However as the avian record shows, flightlessness almost exclusively evolves on oceanic islands where there are no tetrapod predators. Unfortunately terrestrial deposits from old oceanic islands are exceedingly rare (usually they disappear into an oceanic trench), so we are unlikely to ever know for sure.
And for those that might point to emus, rheas, ostriches etc as counterexamples, all these continental flightless birds seem to have originally evolved in the early Paleocene, when there were practically no land predators. It seems that large (and fast-running) flightless birds can survive in the presence of predators, but they can't evolve there (they have to start out as smallish birds).
Bats on the other hand don't seem to be able to evolve into flightless forms at all, not even on oceranic islands. New Zealand bats admittedly forage more on the ground than other bats, but they are anything but flightless. Hoewever bats do adapt to predator-free island environments. The Azores Bat for example forages extensively in broad daylight (no hawks!).
"...so far as we know at the moment, azhdarchids were the only pterosaurs present in the Maastrichtian (thanks to Mark, I've had to revise my previously-held idea (Martill & Naish 2006) that tupuxuarids also made it into the Maastrichtian)"
Actually, some Nyctosaurids (including possibly Nyctosaurus itself) did survived until the Maastrichian; however, being specialized fish eaters, I do doubt they would pass behond the Neogene.
As a whole, I do agree that pterosaurs, even if the K-T event hadn't occured, would never reach the diversity they reached in The New Dinosaurs, but I don't think they would die out completly either. Azdarchids, being generalists, probably would have as many chances as troodontids to survive at least until the Miocene., and maybe they could survive isolated in New Zealand or Madagascar.
As a whole though, I think Dixon's big mistake was how he depicted his flightless [terrestrial] pterosaurs; most if not all of them have their heads designed in a very reptilian fashion, even without beaks, a feature present in all known pterosaurs (with the exception of Anurognathids).
If pterosaurs develop flightless forms, though, it will have to be in a different way than in birds; avians usually become flightless via neoteny, as most birds only develop flight when full grown. Pterosaurs though are known to had been capable of flying as hacthlings. Bats also develop wing membranes when very young, so that may be the reason why they remain capable of flying even in islands as well.
What about the ergilornithid cranes?
Good point. There are cases where birds lost flight through other means. That said, AFAIK all of those examples involve gigantism (the only certain one is the du, Sylviornis, a huge galliform of New Caledonia that was extirpated a few thousand years ago or so). I shudder to imagine an azhdarchid so huge it couldn't fly!!!
Becoming flightless by becoming simply too big to fly certainly isn't the best option, as a huge animal would require large quantities of food. Although the massive Hatzegopteryx from Cretaceous Romania could had been flightless simply for being too huge; since it was an island habitat, only the native abeliosaurs could be a threat (and even so they were smaller than those in the mainland, so even those dinosaurs could hardly be a big threat to a massive azdarchid).
Most likely pterosaurs would become flightless just in behaviour, or somehow loose their wing membranes at some point in their lives (I'm assuming babies would still need to fly to avoid predators like lizards or even adults of their own specie [canibalism is known in several reptiles])
I was thinking perhaps flightlessness first, huge size second. Picture, say, tropical islands that happen to have a niche open for a raccoon-analogue, but no land predators. A flock of pterosaurs arrives one day. Pterosaurs that spend most or all of their time on the ground, closely examining everything around them for food value, have more young than the ones that fly about spending their energy searching for a particular type of food. Also, the ones that have the behavior "when in doubt, fly away" tend to be swept further out to sea. (This assumes a species that practices at least brief parental care of the young instead of abandoning eggs in a nest mound or something, so that they would not immediately need to escape adults.) So there are two forces selecting against wings: selection for nimble paws and long legs regardless of what that does to wing shape and no disadvantage to being a poor flier.
Some time sfter the development of the flightless pterosaur, sea level drops and more land becomes available. New niches open as well. Perhaps the original islands had a rainforest that dropped tasty seeds and fruit here and there, but the new landmass also has a more dry, open forest with lower trees that reward animals able to reach up into them with nestlings, tender leaf buds, etc. So another flightless pterosaur develops: a high-browsing omnivore. Meanwhile, the island group is riding a plate that is colliding with another one and more and more critters are landing on the islands. Perhaps the new high-browsign species finds itself contending with a new species of giant lizard, forcing it to develop some speed becuase it's too big to hide. So by the time the island group ends up joined to the land mass on the other plate, you've got a thing rather like a lank that can hold its own on the mainland, plus something like a beaked raccoon that may also persist.
Just armchair speculation.
I always thought that flightless pterosaurs would best develop from small, likely arboreal or parasitic pterosaurs, but considering how well they were faring in the Cretaceous (a.k.a. they weren't doing good), I suppose azhdarchids may just be the best bet for terrestrial pterosaurs.
My issue, which really isn't that big of a deal, is that this Miocene pterosaur is a herbivore when I'd see them being opportunistic omnivore/carnivores and stork-like scavengers.
http://spec-evolution.sytes.net/index/
Same here; I actually see Quilin paraungulatus more as a terror bird analogue.
If birds never evolved, I guess azdarchids wouldn't be the only pterosaurs to develop flightless forms, but I think flying reptiles would produce as many flightless species as birds did.
Sorry, I meant "would never produce as many flightless species as birds did"