Azhdarchid pterosaurs vs. the world

Azhdarchids: also available in flying. Depicted animal here is based on Quetzalcoatlus sp., but no taxon in particular.

In just a few weeks the world will stop for TetZooCon, a one day convention of all things we associate with the famous Tetrapod Zoology blog and podcast - tetrapods real and scientifically-speculative from wondrous, charismatic fossil reptiles to deceptively interesting small, brown herpetofauna. If you’re reading this, you’re slap bang in the middle of the TetZoo demographic and I guarantee* you’ll have a good time. Tickets are available until Friday 4th July, and it’s all going down one week later - Saturday 12th. Get your place while you can, or forever live with the shame.

*Guarantee not guaranteed.

In a surprise move, my contribution to TetZooCon features pterosaurs. Specifically, I’m looking at the way one group of pterosaurs has made major ripples in the palaeontology pond in recent years - and not just scientifically. The changing face of pterosaur science is certainly interesting, but an equally intriguing, rarely told story exists on the popular face of flying reptile research. To whet your appetite, here’s an 'extended abstract' of my TetZooCon talk, giving some insight into what I'll be covering in a couple of weeks.

PteroPop

Pterosaurs are not unfamiliar characters in popular culture. They have been mainstays of science fiction literature since at least 1874 (Jules Verne’s Journey to the Centre of the Earth), made the jump to the silver screen in 1925 (The Lost World), and since starred in uncountable stories of time-travel, lost worlds and Jurassic Park-inspired de-extinction fiction. Neither are they strangers to public education, from being part of Benjamin Waterhouse Hawkins’ 1854 Crystal Palace menagerie to modern appearances in £multi-million documentaries. These historic popular pterosaurs rarely showed much adherence to flying reptile science: their appearances, behaviours and lifestyles mostly reflect a shorebird-like 'pterosaur archetype' rather than the specific anatomy and habits of a once-living taxon.

One of the many curious things about early azhdarchid reconstructions is the head nubbin - a short, posteriorly-directed conical(?) crest on the back of the head. There is nothing like this known from any azhdarchid. Far from being an early mistake, reconstructions with head nubbins persist until at least 2000. Slide from my TetZooCon talk.

But all that is changing. One pterosaur lineage has overshadowed ‘generic’ popular pterosaurs to bring aspects of ‘real’ pterosaurology to the masses, and has even stolen some limelight from dinosaur celebrities in the process. Media as disparate as documentaries and comic books show these animals in (basically) anatomically correct forms, with accurate but atypical postures, and behaviours which are far removed from the idea of pterosaurs being ancient seabird analogues. The animals in question are, of course, the long-necked, toothless and gigantic flying reptiles, Azhdarchidae.

In some respects, the recent surge of azhdarchid pop-culture uptake is a bit strange. It is not, for instance, that azhdarchids are a newly discovered group. Far from it, their fossils were found by at least the latest 1930s or early 1940s; good remains were apparent the 1970s, and the concept of Azhdarchidae was formalised in the early 1980s. They’re not new to popular culture either, having hung around its periphery since the 1970s to be wheeled out as 'Largest Flying Animals Ever' on occasion. These early popular azhdarchids showed little uniformity in their reconstruction - maybe even less than other pterosaurs at that time. Most bore little resemblance to actual azhdarchid fossils, either anatomically of functionally (above). They were depicted with huge variation from the 1970s-1990s: hugely elongate and narrow wings, or broad, kite-like wings? Longirostrine skulls with snub-noses or tiny pin-heads? Toothed or toothless jaws? Short necks, long necks with swan-like flexibility, or long necks stiffer than broom handles? Art produced in the 1980s - 2D work by Greg Paul, Paul MacCready’s 1985 glider (below), and a (largely sculpted) azhdarchid skeleton mounted by the Texas Memorial Museum - were probably the first works to strike close to reality, but they’re still a bit short of the mark. John Sibbick’s better known and more influential 1991 snub-nosed Quetzalcoatlus was a step back from these more accurate works, accidentally making a chimeric azhdarchid from at least two Javelina Formation azhdarchids (this ‘snub nose’ almost certainly belongs to an unnamed, short-skulled azhdarchid from the same horizon as Quetzalcoatlus).

The famous 1985 'QN' pterosaur, a half-size gliding Quetzalcoatlus northropi and friends, including its designer, the late Paul MacCready (right of middle, in the tie). The model flew successfully multiple times and isn't a bad rendition of an azhdarchid, although many assumptions made in its construction conflict with modern pterosaurology. From MacCready (1985).

Why all the confusion over azhdarchid appearance? Most azhdarchid material known until the late 1990s was either too scrappy to inform artists about life appearance, while the more complete material (the small Quetzalcoatlus species) remained unpublished (and still isn't). The world at large was therefore not able to appreciate azhdarchid anatomy, so any artwork of them required more guesswork than usual. In some cases, entire 'reconstructions' were products of imagination. Not aiding the murky early phase of azhdarchid palaeoart was the transforming nature of pterosaur science which, in the 1980s and 90s, saw much of what we thought we knew about these animals turned on its head. Thus, artists who wanted answers to simple questions like standing postures, wing membrane attachment and so forth weren't always presented with straight answers. 1997 saw a potential change for the better when Unwin and Lü (1997) reclassified the Chinese Maastrichtian ‘nyctosaurid’ Zhejiangopterus linhaiensis as an azhdarchid, but few paid attention to this obscure species when reconstructing 90s azhdarchids, and artwork continued to remain of variable accuracy. The azhdarchid fossil record has not improved fantastically since 1997, only expanding via isolated, scrappy bits and pieces. Their sudden popularity and uniformity of reconstruction has nothing to do with a significantly improved azhdarchid fossil record, then.

Azhdarchids: over 40 Megafonzies of cool!

So, if azhdarchids aren’t new, and they’ve not sent a burst of insightful fossil material our way, why are they now so popular? Perhaps recent reappraisals of their appearance and behaviour have more influence here than anything else. Reconsideration of azhdarchid mass estimates (e.g. Paul 2002; Witton 2008; Henderson 2010; Sato et al. 2010), re-interpretations of lifestyles (Hwang et al. 2002; Witton and Naish 2008, 2013; Carroll et al. 2013) and flight characteristics (Habib 2008, 2013; Witton and Habib 2010) have recast azhdarchids from billboards of flighted animal size to muscular, terrestrially-competent predators and powerful fliers, which just also happened to be giants. This has seen azhdarchids landing ‘major roles’ in palaeo pop media. In the last five years, erect-limbed, terrestrially stalking and quad-launching azhdarchids featuring in the BBC documentary Planet Dinosaur, Atlantic Productions’ Flying Monsters 3D, the 20th Century Fox film Walking with Dinosaurs 3D, the 20,000AD comic series Flesh and recent comics of Teenage Mutant Ninja Turtles, been made into at least two figurines by CollectA, in the upcoming, Steam-released multiplayer game The Stomping Land, in Nathan Carroll's wearable pedagogic puppet form and even a rap. This uptake of the same pterosaur lineage is all the more surprising when you consider the diversity of influences and goals of these projects, as well as the near-infinite sea of fossil species which could take their place. More remarkably, these depictions of azhdarchids aren’t anatomically bad or variable, either: they have large, pointy heads with posteriorly placed crests, long necks, short wing fingers and long limbs. It seems azhdarchids have genuinely penetrated the pop-palaeo zeitgeist.

Azhdarchids in recent comic books, including Teenage Mutant Ninja Turtles and the fabulous 20,000AD series Flesh. Note the far right panel - terrestrially stalked TO DEATH!!! Awesome stuff - should really cover Flesh in more detail here some time. Slide from my TetZooCon talk.

The times, they are becoming very different

I think there's several points of interest here. Firstly, we seem to be witnessing a relative rarity within palaeo pop culture: the indoctrination of a new lineage into the mainstream palaeontological canon. What takes a fossil species from an occasional extra and bit-part player to relative superstardom in the space of a few years? There must be aspects of ‘new’ azhdarchids which have made them more marketable and appealing in short time. And before anyone mentions it, I can vouch for azhdarchid uptake not being overtly pushed by the scientists involved in reinventing them. I've acted as a consultant for three of the projects listed above because ‘new’ azhdarchids were sought after by the media producers, and not as a generic pterosaur expert who pushed his own ideas. The other media, as far as I'm aware, just moved forward with these ideas on their own. 

There is doubtless a myriad of factors making azhdarchids popular - good publicity, a sudden glut of TV and movie interest in prehistoric animals etc. - but I suspect the most important factor is that science accidentally gave azhdarchids a more appealing ‘character’. When we restore fossil animals in art and science we cannot help but impose certain ‘character traits’ into them, and, as with fictitious characters, those with traits we consider desirable are more likely to be popular. 'New' azhdarchids embody everything which is classically cool: they’re original; imagined as assertive, confident animals of great skill and energy; visually interesting and striking; instantly recognisable, as well as being gigantic: metaphorically and physically bigger than us and our problems. Plus, they have that edge of danger: big, predatory species which harvest smaller ones for their own use - let's face it, bad guys and anti-heroes are always cooler than the good guys. In short, it’s not surprising that ‘new’ azhdarchids are popular because they embody the same characteristics as most iconic literary monsters. The traits outlined above could easily apply to H. G. Wells’ Martian tripods (below) or the Star Wars mechanical walkers. Prior to their reinvention, azhdarchids didn’t - and couldn’t - have this appeal, as their appearance was ill-defined, their lifestyles too poorly constrained (skim-feeding? sediment probing? aerial hawking? aquatic pursuit predation? wading?), and much of our science pointed to rather ineffective, flimsy animals. This not only prevented crystallisation of a consistent palaeo pop ‘character’, but also didn't give them much popular appeal.

The uptake of azhdarchids in pop-culture may reflect their recent recasting as stylish, dominating predators of small, defenceless animals, a formula known to strike a nerve with the public - ask H. G. Wells. Do our palaeo pop-culture icons attain iconic status because, like some literary ones, they simply evoke cool characteristics and styles? 

Perhaps more importantly, azhdarchids have - for what seems like the first time - persuaded popular culture to widely depict actual pterosaurs rather than an anonymous set of wing membranes and toothy jaws. That's pretty neat, as it means we're starting to break the notion that different pterosaurs are just minor variants on the same basic animal. I wonder if this applies to pterosaur science too, as research into azhdarchid lifestyles and habits is providing compelling evidence of palaeoecological variation within the group: the concept of shorebird, fish-eating habits applied almost universally to Pterosauria just doesn't work for these guys. With it being increasingly obvious that azhdarchids were doing their own thing, it's easier to start seeing other pterosaurs as potentially having distinctive lifestyles as well. Azhdarchids may be the thin end of the wedge in this respect for both popular and scientific circles.

Of course, no-one can predict how long our current interest in azhdarchids will last, nor what will happen to hypotheses concerning terrestrial stalking, quad-launch and so on. My gut feeling is that these ideas will stand up to scrutiny, but we can never predict what the fossil record or new studies will tell us. Whatever happens, these ideas and the animals they concern have gone some way to superseding generic ‘pterodactyls’ in palaeontological culture, replacing them with a more accurate and detailed appreciation of pterosaur diversity. But what next for azhdarchids? What advances in azhdarchid science are on the horizon? How might these impact their portrayal in popular culture? For that, you’ll have to attend TetZooCon and my talk. Tickets!

References

• Carroll, N. R., Poust, A. W. & Varricchio, D. J. (2013). A third azhdarchid pterosaur from the Two Medicine Formation (Campanian) of Montana. In: Sayão, J. M., Costa, F. R., Bantim, R. A. M. And Kellner, A. W. A. International Symposium on Pterosaurs, Rio Ptero 2013, Short Communications. Universidad Federal do Rio de Janeiro: pp 40-42. 
• Habib, M. B. (2008). Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana, 159-166.
• Habib, M. (2013). Constraining the air giants: limits on size in flying animals as an example of constraint-based biomechanical theories of form. Biological Theory, 8(3), 245-252.
• Henderson, D. M. (2010). Pterosaur body mass estimates from three-dimensional mathematical slicing. Journal of Vertebrate Paleontology, 30(3), 768-785.
• Hwang, K. G., Huh, M., Lockley, M. G., Unwin, D. M., & Wright, J. L. (2002). New pterosaur tracks (Pteraichnidae) from the Late Cretaceous Uhangri Formation, southwestern Korea. Geological Magazine, 139(04), 421-435.
• MacCready Jr, P. B. (1985). The great pterodactyl project. Engineering and Science, 49(2), 18-24.
• Paul, G. S. (2002). Dinosaurs of the air: the evolution and loss of flight in dinosaurs and birds. JHU Press.
• Sato, K., Sakamoto, K. Q., Watanuki, Y., Takahashi, A., Katsumata, N., Bost, C. A., & Weimerskirch, H. (2009). Scaling of soaring seabirds and implications for flight abilities of giant pterosaurs. PloS one, 4(4), e5400.
• Unwin, D. M., & Lü, J. C. (1997). On Zhejiangopterus and the relationships of pterodactyloid pterosaurs. Historical Biology, 12(3-4), 199-210.
• Witton, M. P. (2008). A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana, 143-158.
• Witton, M. P., & Habib, M. B. (2010). On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PloS one, 5(11), e13982.
• Witton, M. P., & Naish, D. (2008). A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLoS One, 3(5), e2271.
• Witton, M. P., & Naish, D. (2013). Azhdarchid pterosaurs: water-trawling pelican mimics or "terrestrial stalkers"?. Acta Palaeontologica Polonica doi: http://dx. doi. org/10.4202/app, 5.

Azhdarchid pterosaurs - invading TetZooCon this July!

Mountains of respect to whoever can name the sources and years of each of these silhouettes. If your surname is 'Naish', you're not allowed to play. Title slide from my TetZooCon talk.

What are you doing on the 12th of July this year? If you're in London and you fancy a day of tetrapod-themed fun - and who doesn't? - you could do a lot worse than attending TetZooCon, a whole day inspired by the famous Tetrapod Zoology blog and its podcast. TetZoo really needs no introduction to anyone reading this, being renowned for mixing semi-technical zoological discussions of Recent and fossil tetrapods with speculative biology, fictitious creatures, cryptozoolgy and, well, whatever else takes the fancy of hosts Dazza Nash and Johnny Conners.

The full TetZooCon timetable of events is appropriately broad and covers dinosaurs, cryptozoology, conservation, speculative biology, primatology, wildlife watching and two regular stalwarts topics of this blog - pterosaurs and palaeoart. I'm taking reigns for the pterosaur aspect of the morning (you can get a sense of what I'll be talking about above - more details on this soon) as well as taking part in an interactive palaeoart event with two real artists - Bob Nicholls and John Conway. This promises to be great fun, and allows audience members an unusually good insight into palaeoart processes. I'll be selling prints of my work alongside other artists and merchandisers - the Palaeoplushies will be in town! - so bring your pennies for products you won't find anywhere else. And if that doesn't already convince you that there's something for everyone, the venue is none other than the London Wetland Centre, and delegates are free to wander around it all day.

In all, it sounds like it's going to be a terrific day, and I'm pretty stoked to be invited along. If this post tickles your fancy, and especially you'd like events like this to become regular fixtures in your calendar, support the event by spreading the word and grabbing some tickets. Looking forward to seeing you there!

Darwinopterus vs. Bat Out of Hell

Darwinopterus robustodens and his pal, a European robin (Erithacus rubecula). When not posing for artwork, they drive around in a van solving mysteries.

Wukongopterid pterosaurs have been on my mind this week thanks to a near-complete manuscript about them. This required* rendering everyone's favourite wukongopterid Darwinopterus - results above (note that this is D. robustodens, not the more familiar modularis). In the final version he's joined by something else, but you'll have to wait to just what that is. Wukongopteridae is the group of recently discovered Chinese pterosaurs which bridge early pterosaur and pterodactyloid-grades of pterosaur morphology, famously combining head and neck characteristics of the latter with the bodies of the former. They're best known for Darwinopterus modularis, but there's actually now nine(!) taxa known from the same horizon in north-east China, all morphologically very similar and almost certainly oversplit.

*By 'required', what I really mean is that I'm compelled to construct papers with extraneous artwork in them somewhere, because I'm a sucker for punishment.

Illustrations of Darwinopterus and I go way back. I was asked to do press images for two early publications on it: the initial description and assessment of its unusually 'modular' evolution (Lü et al., 2010), and the equally fantastic discovery of its sexual dimorphism and an egg-mother association (Lü et al., 2011a). Below is the first of these images, published in 2009.

Ah, 2009. Shrink wrapping, relatively light integuments and very low soft-tissue crests were still in fashion. Fun fact: Darwinopterus was referred to as 'Frank' before it was given a binomial, a nod to it being a Frankenstein's Creature-like mash of body parts.

Looking back on Darwinopterus 2009, I'm not enormously happy with it. This in itself isn't that unusual. Artists often look back with dissatisfaction with older works, but this has an additional reason for dissatisfaction: I never really agreed with the notion of Darwinopterus as an aerial-hawker of flying tetrapods, and I think this comes across in its execution. I outlined my basic concerns with this idea in Pterosaurs (Witton 2013):

Given that wukongopterids have provided an insight into macroevolutionary processes, filled a gap in pterosaur phylogeny, and present a very unique pterosaur bauplan, expectations may be high that their proposed foraging strategies will also be rather amazing. Fittingly, some have proposed that wukongopterids were pterosaur top guns, their newly evolved long necks and oversize heads being used to prey upon dinosaurs, other pterosaurs, and even gliding mammals in midair (Lü et al. 2010). Such acts would be rather remarkable because, with even a generous mass estimate, the biggest Tiaojishan wukongopterids would not weigh much over 300 g (extrapolating data from Witton 2008), which is about the same as a modern feral pigeon.

At that size, tackling squirrel- sized mammals or crow-sized dinosaurs on the wing would be a feat earning praise from even the hardiest modern raptors, and wukongopterid skeletons would have to be brimming with offensive weaponry for this purpose. Vertebrate-hawking birds are renowned for their talons, incredibly strong feet, robust skulls, and powerful beaks (e.g., Hertel 1995; Fowler et al. 2009), while bats that subdue large vertebrates in flight are also armed with formidable teeth and powerful jaws (Ibáñez et al. 2001). These adaptations provide the means to immobilize their prey quickly and efficiently, and are obvious advantages for animals grappling with large prey while in “flight. Vertebrate hawkers are also powerful fliers that can chase down their quarry and, once immobilized, carry the prey to a safe spot to eat. Pterosaurian equivalents would therefore require equally powerful “flight musculature to permit the same tasks. Unfortunately for the Darwinopterus raptor hypothesis, wukongopterids do not possess any of these requirements. None of their appendages bear the chunky digits and talons ideal for subduing large aerial prey items, and their long, comparatively delicate skulls and unimpressive teeth are ill suited to this task. Nor, for that matter, do they have the expanded shoulder regions indicative of the powerful “flight muscles needed to chase and eventually carry their prey. With this in mind, raptorial pursuits look doubtful for wukongopterids.

Witton (2013), p. 141-142.

There's a lot more that could have been said about this, but you get the idea: Darwinopterus and chums were small (jackdaw-sized - see image at the top of the post), delicately built animals for which aerial predation seems counter-intuitive and unlikely. I'm not the only person saying this, either: Lü et al. (2011b) and Sullivan et al. (2014) also raise points against the aerial hawking idea. We could go so far as to to label wukongopterid aerial hawking as another example of an 'extreme' palaeoecology based on cherry picked characteristics rather than considering a full suite of functional data.

So, on reflection, we probably started on the wrong foot but, hey, what can you do? Working as an artist is quite different to being involved in research: ultimately, you're a guy with a paintbrush being told to illustrate someone else's idea, even if you don't necessarily agree with it (also see Csotonyi and White 2014). With the notion of aerial predation being raised in the paper, specifically as a possible explanation for the development of pterodactyloid head and neck features Lü et al. (2010), it was an obvious choice for the image. But we didn't help matters by walking into some of the cheesiest, silliest things stereotypes of palaeoart, primarily because we were trying to make a small, fairly inoffensive animal look like a skydiving, dinosaur-eating-badass. We tried several different takes on this, varying aerial and terrestrial prey, and compositions which upped the voracity, such as this:

You'll need to make your own whooshing jet fighter noises and 'pew pew' laser sounds.

Eventually death-from-above was decided over death from sideways. But a key issue to tackle was that that the proposed prey for Darwinopterus was the same size as the predator itself (Sullivan et al. 2014 go into this more), and it's obvious that Darwinopterus is not a fighting, wrestling creature. To make the image work at all we had to play liberally with animal sizes: the maniraptoran (loosely based on Anchiornis in the final version) is tiny, and the Darwinopterus is a huge, ferocious juggernaut. I'm not the only artist who used this trick: the disappointing pterosaur documentary Flying Monsters 3D also had their Darwinopterus plundering undersized theropods. After we tinkered with reality, we then started piling on the cheese: the maniraptoran was made more reptilian-looking, it's head twisted to stare into the Maw of Destiny and, of course, it's mouth open to 'NOOOOOO!!!' it's impending fate. The result is 'Meat Loaf palaeoart', the sort which resembles the worst kind of rock album covers more than nature.

Think I'm being daft comparing palaeoart to cheesy hard rock album covers? How many depictions of extinct animals have the exact same pose as the titular bat here? See links below for more examples. From Wikipedia.

The result isn't awful, but I think I've had more successful collaborations with the same authors, just because this doesn't feel realistic at all. From the science to the composition, the whole thing just seems 'forced'. Don't get me wrong: I'm sure Darwinopterus could be terrifying instruments of mortality to the right prey (probably terrestrial invertebrates according to Lü et al. 2011b and Witton 2013), and I'm not belittling the drama or viciousness which can occur in the lives of small animals. But life - thank God - doesn't look like the cover of a Meat Loaf album. There are some fossil animals that can - just about - pull off Meat Loaf palaeoart, but it's just not possible to turn small, fluffy, unthreatening animals into monsters. When we do, they're more liable to look goofy than impressive. Despite this, we do it all the time. I understand why we do - it's exciting and marketable, and feeds into expectation that Deep Time as a monster-filled fantasy realm - but it also reinforces the perception that palaeoart is unsophisticated and aimed at children. But there may be another way.

Palaeontologists: next time you have a new, diminutive animal you want illustrated, forget the monster: play the cute card. The chubby little Darwinopterus at the top of this post intuitively seems closer to the reality of this animal than Darwinopterus 2009: AwesomeoSuperKiller. It's proportions, fluffiness and posture are all accurate, and it only looks cute because, well, it probably was: it's a small fuzzball with an oversize head and mischievous grin. If the Internet's obsession with cats and baby sloths has taught us anything, it's that cute sells better than violence. Indeed, this image is one of the most popular things I've ever put on Facebook, and (at time of writing) it was posted less than a day ago. The great thing about the cute card is that everyone wins: the artwork should promote research just as well or better than it's Meat Loaf variant, because it appeals to wider demographics. The artist gets to render an animal in a more realistic light without jumping through hoops to monsterise it, and the world gets a new picture to 'coo' at. A few folks might even start to appreciate extinct animals in ways they never could when they're always shown screaming and fighting.

And if nothing else, it will mean we'll never have to discuss Meat Loaf album covers here again.

May God have mercy on us all. (Wikipedia)

References

• Csotonyi, J. & White, S. (2014). The Paleoart of Julius Csotonyi: Dinosaurs, Sabre Tooths and Beyond. Titan Books, London.
• Fowler, D. W., Freedman, E. A., & Scannella, J. B. (2009). Predatory functional morphology in raptors: interdigital variation in talon size is related to prey restraint and immobilisation technique. PloS one, 4(11), e7999.
• Hertel, F. (1995). Ecomorphological indicators of feeding behavior in recent and fossil raptors. Auk, 112(4), 890-903.
• Ibáñez, C., Juste, J., García-Mudarra, J. L., & Agirre-Mendi, P. T. (2001). Bat predation on nocturnally migrating birds. Proceedings of the National Academy of Sciences, 98(17), 9700-9702.
• Lü, J., Unwin, D. M., Jin, X., Liu, Y., & Ji, Q. (2010). Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B: Biological Sciences, 277(1680), 383-389.
• Lü, J., Unwin, D. M., Deeming, D. C., Jin, X., Liu, Y., & Ji, Q. (2011a). An egg-adult association, gender, and reproduction in pterosaurs. Science, 331(6015), 321-324.
• Lü, J., Xu, L., Chang, H., & Zhang, X. (2011b). A new darwinopterid pterosaur from the Middle Jurassic of western Liaoning, northeastern China and its ecological implications. Acta Geologica Sinica‐English Edition, 85(3), 507-514.
• Sullivan, C., Wang, Y., Hone, D. W., Wang, Y., Xu, X., & Zhang, F. (2014). The vertebrates of the Jurassic Daohugou Biota of northeastern China. Journal of Vertebrate Paleontology, 34(2), 243-280.
• Witton, M. P. (2008). A new approach to determining pterosaur body mass and its implications for pterosaur flight. Zitteliana, 143-158.
• Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.