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Wednesday, 6 July 2016

And drepanosaurs might fly... wait, really?

Minor update (06/07/16): Thanks to Andrea Cau, a few additional citations and points of discussion have been made below - the thrust and arguments of the post are the same, but the context is improved. Thanks Andrea!

Hypuronector limnaios restored as a glider. Have palaeontologists been smoking something of variable legality, or is there some basis to this?
Assuming you've reached level 5 of palaeontological geekdom you can't fail to know of the exceptionally weird Triassic clade Drepanosauromorpha. These generally small, long-bodied reptiles are largely, but not incontrovertibly, thought to nest at the base of Archosauromorpha (so between lizards and crocs in the landscape of modern animals) and are famous for their highly aberrant anatomy. Gracile, bird-like heads and necks sit atop long, robust and tubular bodies with deepened tails and stout limbs. The hands and feet are highly modified in each species, some bearing powerful claws, others having chameleon-like opposable digits. The end of their tails are modified into either grasping, prehensile organs or sharp hooks, these being interpreted as adaptions for anchoring the tail to vegetation or substrata. Exactly what drepanosaurs did for a living has long been a subject of discussion among academics, and they are nowadays generally considered arboreal or fossorial - or a blend of both. They're pretty awesome animals.

Because the Triassic was evolution's drug-fuelled, rebellious college days, it can't be considered shocking to learn that there's a drepanosaur species which is to drepanosaurs what they are to everything else. This distinctive, strange, and controversial species is Hypuronector limnaios (above). Reasonably good fossils of this small (c. 12 cm long) animal have been known for decades from upper Triassic deposits of New Jersey, but it received its name only relatively recently (Colbert and Olsen 2001). Hypuronector is often regarded as a swimming creature because of its dorsoventrally expanded, 'leaf-shaped' tail which lacks a hooked or prehensile termination (Colbert and Olsen 2001). Its tail is remarkable for the enormous chevrons (prongs of bone projecting downwards from the underside of tail vertebrae) which extend far below and behind their vertebra of origin to create the majority of the tail depth and its 'leaf-like' profile. Some authors have likened the outline of the tail skeleton to the body shapes of gymntoid or gymnarchid fish and suggested that it propelled Hypuronector through the deep, freshwater lakes its fossils were buried in, perhaps in a newt- or crocodile-like fashion (Colbert and Olsen 2001). Although possessing unusually long legs relative to other drepanosaurs and swimming animals, it's been argued that these were also related to an aquatic lifestyle. Specifically, it's suggested that they held the long, deep tail off the ground during terrestrial bouts, the tail apparently being incapable of elevation at its base (Colbert and Olsen 2001). This aquatic Hypuronector hypothesis has been around for some time. The animal was informally known as the 'deep tailed swimmer' in the 1980s (Fraser and Renesto 2005) and this moniker was transferred more or less entirely to its scientific name in 2001: loosely translated, Hypuronector means 'deep-tailed lake swimmer'.

Hypuronector limnaios skeletal reconstruction, from Renesto et al. 2010. Scale represents 10 mm.
At first glance at least, none of this sounds too outlandish: the tail of Hypuronector certainly has an oar-like shape, and we all know that lateral undulation of a tail is the commonest means of water propulsion for vertebrates. But there are other interpretations of Hypuronector which suggest it may not have been a swimmer at all. These alternative views suggest it was more like other drepanosaurs in being suited to climbing but, more remarkably, possibly a glider (Renesto et al. 2010). Sharing early versions of my gliding drepanosaur art (above) suggests that the latter hypothesis is not well known, even among experts. However, I want to stress from the outset that this is not All Yesterdays-style artistic speculation or the bizarre opinion of a 'fringe' worker. Challenges to the aquatic Hypuronector concept and suggestions that Hypurnoector was a more 'typical' arboreal form have been made by several authors (e.g. Senter 2004; Spielmann et al. 2006; Renesto et al. 2010; Castielloa et al. 2015), and the notion that it may have been a glider has been raised on reasonable (if perhaps not yet conclusive) evidence (Renesto et al. 2010). It follows older suggestions that some drepanosaurids - Megalancosaurus specifically - were gliders (see below; Ruben 1998; Renesto 2000) and, though this all might seem bizarre, there is some genuine scientific basis to it.

The aquatic Hypuronector hypothesis under scrutiny

Aquatic drepanosaurs are were first proposed in the early 90s (Berman and Reisz 1992) and quickly received criticism from drepanosaur workers (see Renesto 2010 for history). Hypuronector perhaps remains the best candidate for an aquatic, or at least amphibious species because of its unusual tail, but somewhat ironically, it's actually this paddle-shaped organ which seems to be the main problem for this hypothesis.

Holotype of Hypuronector limnaios, a partial skeleton with the 'paddle tail' (left), disarticulated torso and bits of limb and limb girdle. From Colbert and Olsen (2001).
One thing we should address straight out is that the resemblance of the Hypuronector tail to the body of certain fishes is not a the best endorsement for swimming habits. Fish do not swim using their whole bodies (the front end of any undulating swimmers needs to be stiff), and the gymntoid or gymnarchid fish likened to the Hypuronector tail don't really move their bodies at all when swimming. Rather, they propel themselves with oscillations of long, low fins along the top of bottom of their bodies. Thus, they may be a poor shape analogue for a sculling organ, and we're better off looking at the fins and paddles of swimming animals, not their entire bodies, for clues about the aquatic potential of the Hypuronector tail.

It stands to reason that Hypuronector would have swum like a crocodylian, newt or swimming lizard, where waves of lateral undulation in the tail generate forward thrust (Colbert and Olsen 2001). This requires tail anatomy which can accommodate a lot of lateral motion, and it's here that Renesto et al. (2010) suggest we hit a major issue. The caudal vertebrae of Hypuronector seem to permit some movement at the base and tip of the tail, but the mid-tail was pretty stiff. This is because the zygaopophyeses - processes of bone that overlap neighbouring vertebrae to guide their motion - are very long and have steep articular surfaces (below). In simple terms, they seem to have 'clamped' their adjacent vertebrae rather than - as expected for an undulatory tail swimmer - provided flat, horizontal surfaces for the vertebrae to slide over.

Further rigidity is provided by those amazing chevrons (Renesto et al. 2010). These rearward-projecting bones underlie the articulations of the adjacent 7-8 vertebrae, meaning any lateral motion at the vertebral joints had to overcome the stiffness of the 7-8 bony rods hanging beneath them. Although thin bones are somewhat compliant and the Hypuronector chevrons may have been flexible to a degree, it's difficult to see their arrangement as optimised for sculling habits: they may made the tail more paddle shaped, but to obvious detriment of tail flexibility and sculling potential. Indeed, we have to note that this configuration is very similar to biological structures adapted to resist bending. Tetrapod wings are a good example: the arrangement of bat fingers, pterosaur structural fibres and bird feather shafts with respect to the wing bones echoes the chevron distribution in Hypuronector. By contrast, deep-tailed swimmers, like crocodylians and newts, have chevrons which are short, robust, and do not significantly underlie neighbouring vertebrae. They are ideal structures for anchoring tail musculature, increasing tail depth and not interfering with tail motion. I have to agree with Renesto et al. (2010) that the potential of the Hypuronector tail as a swimming organ seems limited.

Hypuronector limnaios posterior trunk (left) and tail base (right) - note elevation of the latter with respect to the former, and the significant overlap of the zygapophyses. From Renesto et al. 2010, scale represents 10 mm.
Of further relevance here are the limbs of Hypuronector, which do not have obvious aquatic signatures. Aquatic, or even semi-aquatic animals tend to have proportionally short, squat limbs, often with expanded, paddle-like bones. But the limbs of Hypuronector are elongate, gracile and hollow (Renesto et al. 2010). Its hands and feet are not well known and variably interpreted, but the elements we have suggest that they were not paddle-like. Colbert and Olsen (2001) proposed that the limbs of Hypuronector were long to lift the tail from the ground when it left the water, their work suggesting that the vertebral column was too stiff to lift the tail on its own. But this can be seen as problematic for three reasons. Firstly, as pointed out by Renesto et al. (2010), articulated fossils of Hypuronector show the tail arcing upwards with respect to the trunk vertebrae (above): this is not thought to be taphonomic or diagenetic distortion. Secondly, the forelimbs of Hypuronector are somewhat longer than the hindlimbs, which is perhaps the opposite of what we would expect if dragging the tail was a concern - surely the body would tilt backwards with this arrangement? Thirdly, since when did reptiles, aquatic or otherwise, care about dragging tails? We need to be careful that we're not providing 'empty support' for hypotheses by inventing problems for our fossil animals to solve.

Maybe Hyperonector isn't 'the weirdo drepanosaur 'after all?

Taken collectively, these points about tail shape, tail arthrology and limb size must be viewed as problematic for the aquatic Hypuronector hypothesis, and maybe we should see if there are other interpretations of Hypuronector lifestyle which are more in tune with its anatomy. A good strategy for understanding strange fossil animals is putting the controversial, weird bits of anatomy to the side and first focusing on the more reliably interpreted components. With that said, let's ignore the controversial tail of Hypuronector for a moment and look at its limbs, limb girdles and trunk anatomy. As with all drepanosaurs, the shoulder and hip bones of Hypuronector are very tall and somewhat reminiscent of the limb girdles of chameleons (Renesto et al. 2010). It is thought both limb sets were highly mobile, although the drepanosauromorph fusion of the pectoral girdle into one solid structure, as opposed to having two separate halves like chameleons, would limit forelimb reach somewhat. The limbs were likely held in a sprawling pose and, because the femora and humeri are greatly elongated, Hypuronector likely had a wide, stable base to walk and stand on.

Bits and pieces of AMNH Hypuronctor specimens, including the only known cranial material (mandible, A-C) and the ventral view of a trunk and pectoral skeleton. Note the huge, curving ribs. From Renesto et al. 2010.
Hypuronector lacks the large, fused vertebrae over the pectoral region that we see in other drepanosauromorphs, but given that these likely reflect increased forelimb muscle mass and a reinforced pectoral region for digging and prey-capture (Castielloa et al. 2015), this may not impact locomotor mechanics too much. The trunk of Hypuronector was evidently powerfully muscled all the same, the tall neural spines of the dorsal vertebrae and the presence of large, curving ribs along the entire torso suggesting large muscles enveloped most of the body.

It can be seen that Hypuronector trunk and limb anatomy matches pretty well with what we see in other drepanosaurs: powerful torsos and mobile limbs that seem well suited to walking and climbing. We might view its limb elongation as an adaptation to climbing, the increased length of the upper limb segments simultaneously increasing stability and enhancing reach while also keeping the centre of mass close to the substrate. Perhaps more surprisingly, Hypuronector is also similar to other drepanosaurs in certain aspects of tail anatomy. Although its tail has a different overall shape and lacks the derived tail-tips of true drepanosaurids, it shares the specifics of drepanosaur tail motion - flexible base and tip, rigid mid-length - with the rest of the group (Renesto et al. 2010). So perhaps the tail of Hypuronector was just a simpler, oddly-shaped variant on the drepanosauromorph tail and used for similar purposes: stability when climbing (a simple prop can aid traction, balance and recovery from accident), a brace when rearing to dig and feed, or simply for showing off (Renesto et al. 2010).

Putting these lines of evidence together, several authors have started to interpret Hypuronector as a more 'typical' drepanosaur, albeit a less-specialised species that lived like a modern arboreal lizard rather than a reptilian tree pangolin or pygmy anteater (Spielmann et al. 2006; Renesto et al. 2010). If this is true, we might view the shape of its tail as a mechanical red-herring, something which seems more important to Hypuronector behaviour than it actually was. Perhaps it had no more significance to locomotion and behaviour than do the cranial ornaments of dinosaurs and pterosaurs, structures which most now agree were more to do with communication and display than the mechanics of day-to-day life.

Yes yes yes, but we're here for the gliding stuff

Taking this idea of a climbing, generalist Hypuronector a step further, Renesto et al. (2010) note that there are several features of Hypuronector which might indicate it was a patagial glider - that is, an animal with membranes extending between its limbs to facilitate slower falls from elevated positions or glide between perches. The chief features of interest here are the the elongate limbs and, in particular, the forelimbs being as long, if not slightly longer, than the hindlimbs. This configuration is uncommon among reptiles. Well known reptiles with disproportionately long arms include canopy-browsing herbivorous dinosaurs, completely aquatic lineages like ichthyosaurs, derived sauropterygians and turtles, and flying animals like pterosaurs. It's clear that the former animals are playing an entirely different game to drepanosaurs, but the basic similarity between pterosaurs - small, gracile boned creatures which probably had climbing and gliding ancestors - and Hypuronector might be a little more intriguing. Forelimb elongation occurs again and again in patagially gliding tetrapods - pterosaurs, cologus, scaly tailed gliders etc. - and it's not unreasonable to wonder if the same phenomenon in Hypuronector betrays the presence of gliding membranes. The limb proportions of this species are not so extreme as to think it was an exemplar glider and able to travel long distances from vertical starts, but they may have housed membranes of sufficient size to cushion the fall of these small animals if they jumped or fell from high places. The deep, rounded shape of the tail becomes something to pay attention to here as well, it perhaps being well-shaped to help 'correct' a tumbling Hypuronector into the right posture for a steady glide.

Which might have been handy if the initial glide trajectory was what glider pilots call 'less than ideal'
As noted above, at least Megalancosaurus has been also posited as a potential glider in the past (Ruben 1998; Renesto 2000). These conversations were inspired (at least in part) by long-defunct (if you could ever really consider them credible!) ideas that birds may have had shared, close ancestry drepanosaurs or drepanosaur-like animals - let's quickly duck aqay further discussion of that. But why has the idea of gliding Megalancosaurus not caught on? Although not ruled out entirely (Renesto 2000), gliding doesn't seem to have stuck with this species because it its spiked tail, highly mobile wrists and ankles, and grasping appendages suggest it was quite highly adapted to climbing. While climbing and gliding are not incompatible, it also lacks features like the long, gracile limbs we would expect from flighted animals. The anatomy of Hypuronector, by contrast, is a little more generalised and ticks enough boxes in the glider column to think it could be possible.

Of course, it's worth stressing that any gliding drepanosaur is hypothetical at this stage, but we should not take this as reason to dismiss the idea out of hand. In addition to the evidence mentioned above, consider that many, perhaps all drepanosauromorphs seem to have been climbers of one kind or another, and we know from extant faunas that the step from climbing to gliding is often a short one (Renesto 2000). It's really not crazy to think extinct lineages were any less able to develop gliding forms than our modern ones, and drepanosaurs were exapted for gliding flight in many ways. Their skulls had large brains and overlapping visual fields (Renesto and Dalla Vecchia 2005) (ideal for judging distance and processing flight data); they were generally small animals with hollow limb bones (lightweight); their torsos were stiffened and reinforced (aids stability); their limbs were powerfully muscled and highly mobile (control of aerofoils) and their deep, strong tails might be ideal rudders and stabilisers. And as bizarre as it may seem to be discussing the possibility of gliding in an animal only known from bones, recall that pterosaurs were identified as flying animals in the early 1800s long before we discovered fossil remains of their wing membranes: we can identify flying animals if we look carefully enough at their bones. The challenge now is to see if we can test these ideas, perhaps carefully comparing the limb anatomy and myological signatures of Hypuronector with other drepanosaurs, modelling the effects that crazy tail has on a falling animal and so on. We can also look for Renesto et al.'s membranes on Hypuronector fossils, examining them with UV light and being extra-careful when preparing future Hypuronector specimens: experience with other delicate reptile specimens shows that it helps to know where to expect soft tissue when removing matrix.

So there we go, then: the Triassic, and drepanosaurs, might have just got even weirder/cooler/complicateder/more frustratinger than we all knew. I'm thinking we need to hang out in the Triassic even more in future blog posts - check out this label for previous conversations on Triassic topics. And note that my new art book, Recreating an Age of Reptiles, has several pages dedicated to Triassic animals - including Drepanosaurus.

This blog glides on the gentle, supportive updrafts of Patreon

The paintings and words featured here are sponsored by the organisms almost as awesome as Hypuronector: my Patreon backers. Supporting my blog from $1 a month helps me produce researched and detailed articles with paintings to accompany them, and in return you get access to bonus blog content: additional commentary, in-progress sneak-previews of paintings, high-resolution artwork, and even free prints. For this post, we'll be taking a look at a (currently unpublished) painting of a more familiar drepanosaurid.

References

  • Berman, D. S., & Reisz, R. R. (1992). Dolabrosaurus aquatilis, a small lepidosauromorph reptile from the Upper Triassic Chinle Formation of north-central New Mexico. Journal of Paleontology, 66(06), 1001-1009.
  • Castiello, M., Renesto, S., & Bennett, S. C. (2015). The role of the forelimb in prey capture in the Late Triassic reptile Megalancosaurus (Diapsida, Drepanosauromorpha). Historical Biology, 1-11.
  • Colbert, E. H., & Olsen, P. E. (2001). A new and unusual aquatic reptile from the Lockatong Formation of New Jersey (Late Triassic, Newark Supergroup). American Museum Novitates, 1-24.
  • Fraser, Nicholas C., and Silvio Renesto. Additional drepanosaur elements from the Triassic fissure infills of Cromhall Quarry, England. Virginia Museum of Natural History, 2005.
  • Renesto, S. (2000). Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia (Research In Paleontology and Stratigraphy), 106(2).
  • Renesto, S., & Dalla Vecchia, F. M. (2005). The skull and lower jaw of the holotype of Megalancosaurus preonensis (Diapsida, Drepanosauridae) from the Upper Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia (Research In Paleontology and Stratigraphy), 111(2).
  • Renesto, S., Spielmann, J. A., Lucas, S. G., & Spagnoli, G. T. (2010). The taxonomy and paleobiology of the Late Triassic (Carnian-Norian: Adamanian-Apachean) drepnosaurs (Diapsida: Archosauromorpha: Drepanosauromorpha): Bulletin 46 (Vol. 46). New Mexico Museum of Natural History and Science.
  • Ruben, R. R. (1998). Gliding adaptations in the Triassic archosaur Megalancosaurus. Journal of Vertebrate Paleontology, 18 (3), 73A.
  • Senter, P. (2004). Phylogeny of Drepanosauridae (Reptilia: Diapsida). Journal of Systematic Palaeontology, 2(3), 257-268.
  • Spielmann J. A., Renesto S. and Lucas S. G. (2006). The utility of claw curvature in assessing the arboreality of fossil reptiles.Bulletin of the New Mexico Museum of Natural History and Science 37: 365-368.

Monday, 4 July 2016

Recreating an Age of Reptiles: the motion picture (and other promotional material)

Last week I put my new palaeoart book Recreating an Age of Reptiles on sale: you can see previews and buy copies here, and check out this post for some basic details.

If you'd like a more in-depth introduction to the project and enjoy the experience of disembodied voices narrating over slides, why not make a cup of tea and watch this 24 minute book launch video? It features some of the new art, explains why the book has such an old-timey title, and outlines some of the palaeoart philosophising that takes place therein.


To whet your appetite further, here's the full set of adverts that I put out for the book on Twitter. They set the tone for the book pretty well.





Wednesday, 29 June 2016

New palaeoart book, Recreating an Age of Reptiles, out now!


Finally, my long promised palaeoart book Recreating an Age of Reptiles is available from online retailers! Conceived as a short, 'how long can it take to publish a print-on-demand book where I have full control?' sort of project, today marks the end of the year of design, illustration and writing work it actually took to take this broad, palaeoart-led look at various parts of the Mesozoic. The result is a Letter page-sized (that's 21.59 x 27.94 cm) full-colour paperback with 108 pages of text and imagery, and over 90 bits of artwork. About 20% of the artwork has not been published anywhere before, at least not in entirety, and virtually none of the pictures have been featured in other publications. So if you're after some new entries on your palaeoart bookshelf, or hard copies of images of mine that you've seen around the internet, this might be the book for you. You can access a preview of the book interior via its page at Lulu.com.

The opening spread of the azhdarchid pterosaur section. This is one of three sections featuring flying reptiles.
The book is divided into a number of thematic sections based around animal clades, specifics of behaviour, or types of habitat. In selecting the art and generating new pieces for this project I tried to keep things varied and interesting. This is not a book dominated by any one particular type of animal, nor a tome where every picture shows prehistoric animals ripping each other's throats out (if, indeed, that can be said to feature at all). Dinosaur groups account for 50% of the book's content, with the rest taken up by mammal-like creatures, Crocodyliformes, Triassic archosauromorphs, pterosaurs and others. Many of the pictures show atypical behaviours such as burrowing, swimming, sleeping, falling over, shyness and nocturnality, and weather - rather than just variably coloured skies - plays an active role in a good number of illustrations. I'm not going to boast that "you've never seen the likes of this before!" but, presented as a collective, I hope it presents a nuanced take on Mesozoic palaeoartworks.

Brontosmash! needs a double page spread.
Although primarily an art book, I've tried to make this something worth reading too. Each picture is accompanied with details about the research, artistic decisions and researcher collaborations that informed their production. The book is bracketed by essays musing on aspects of the palaeoartistic process: how many ways we can reconstruct extinct animals without leaving the realm of scientific credibility; the role of artistic personality and biases in palaeoart; whether we constrain our art by adhering too tightly to familiar parts of science, and whether we should view the inevitable outdating of our work as positive or negative. While (hopefully) avoiding naval gazing, I've tried to outline some of my own inspirations and philosophy concerning palaeoart production throughout this text. We don't really discuss our individuality as palaeoartists very much - why we prefer certain colours or animal behaviour, why we choose certain compositions - but it's something I'm curious to hear more about from the palaeoart community, so I've shared some of my views in this book. It seems that discussing palaeoart as 'art' rather than a strictly illustrative or scientific endeavour seems like an important step to improving its standing and perceived value among its patrons.

So, where can you buy it from, and how much is it?

The cover price for Recreating an Age of Reptiles is £26, and it's available now, direct from Lulu.com (below). You can also buy it at all major online book stores (e.g. Amazon, Barnes and Noble etc.). But before you click the Amazon link, note that Lulu.com is, and will always be, the cheapest place to buy Recreating an Age of Reptiles. I've set a 5% discount at their store which means it's retailing for £24.70, not £26. I'll be honest about why I've set this incentive: major retailers take 50% of sale profits before the rest can be divided up among printers, publishers and authors, which means book authors are not left with much from their sales. Lulu offers the same shop service as anywhere else online (and you can pay with Paypal, too) and their service, in my experience, is swift and efficient - you should have the book within a week from ordering.

There are other ways you can get a copy. One way is to support me on Patreon, a copy of the book being a reward for the highest support tier. This copy will be signed and doodled on if requested. If you have any requests for a small sketch in the front pages, please let me know when you place your order!

The final way is to buy a signed and doodled copy through my website store. These are a bit more expensive than the unsigned copy, because there's two sets of shipping to factor (once to me, and then again to you) but hopefully not too steep at £30. These will be on sale any day now. As above, if you have any requests for a small sketch along with my signature please let me know when you place your order!

I'll have more info and promotional material for the book here in a few days - in the meantime, if you have any comments or questions, be sure to ask them in the comment field below, on Facebook or Twitter (#RecARep is the Recreating an Age of Reptiles hashtag). And for those who buy the book, I hope you enjoy it!

Thursday, 23 June 2016

Why the giant azhdarchid Arambourgiania philadelphiae needs a fanclub

Two giant azhdarchids, Arambourgiania philadelphiae, attempt to portion a troodontid. The troodontid objects.
When people talk about giant azhdarchid pterosaurs (odds are most readers of this blog don't need an introduction to azhdarchids, but if you do, click here) they typically mention two taxa. The first is Quetzalcoatlus northropi, a giant Texan pterosaur discovered in the 1970s and now one of the most famous pterosaurs of all (Lawson 1975, Langston 1981). The second is Hatzegopteryx thambema, a relatively robust giant discovered in the 1990s and initially - because of its size and reinforced bone construction - thought to be a giant predatory dinosaur (see Buffetaut et al. 2003). From internet forums to TV show producers, if you want to chat about giant pterosaurs, these species are your most likely subjects.

Many readers will be aware that these aren't the only giant azhdarchids, however. The record of these animals cannot be described as extensive, but it is sufficient to indicate that they were present across most of the world and probably not particularly rare in Late Cretaceous ecosystems. But most fossils of giant azhdarchids are unnameable on account of being too fragmentary, being represented by parts of undiagnostic anatomy, or being too poorly preserved. This makes it all the more surprising that the third named giant azhdarchid doesn't get much attention: the Maastrichtian species Arambourgiania philadelphiae, known from several bones from phosphate mines in Jordan.

I'm not sure why we generally overlook this giant. Perhaps it's because Arambourgiania - 'Arambourg's giant' - is one of those old-fashioned names which works better in translation than the original Greek. It certainly doesn't sound as evocative or exotic as Quetzalcoatlus or Hatzegopteryx. Moreover, it's the least known of the three named giants, being primarily represented by a long - 620 mm - cylindrical neck vertebra, and not much else. The other named giants are not well represented either, but we have more than a handful of bones for them, and they're represented by intuitively intriguing anatomies: giant wing skeletons, bits of skull and jaw and so on. But whatever the cause, there are reasons to consider our relative neglect of Arambourgiania as unwarranted. It may not be as well-known as Quetzalcoatlus, or as immediately intriguing as Hatzegopteryx, but if you're interested in giant azhdarchids (and, hey, who isn't?) you this animal deserves your attention just as much as the other species. Here are just three reasons why.


History has been unkind to Arambourgiania

We typically start the story of giant azhdarchid studies in the early 1970s and the discovery of Quetzalcoatlus, but Arambourgiania was found and described long before then. Indeed, it's among the first accounts of an azhdarchid in scientific literature. When exactly the first Arambourgiania material was unearthed remains mysterious - it was likely the late 1930s or early 1940s - but the holotype cervical vertebra emerged in a scientific paper in 1954 thanks to French palaeontologist Camille Arambourg. Five years later, he would name this bone Titanopteryx philadelphiae (Arambourg 1959), a title which would be modified to Arambourgiania in the 1980s once the preoccupation of Titanopteryx by a black fly became apparent.

Aramboug misidentified this vertebra as being wing metacarpal of a large pterosaur (below). This might seem surprising - how do you confuse a vertebra for a wing bone? - but this tubular bone must have been a bizarre object to him. Consider that no-one in the 1950s had a clue what an azhdarchid was; that no-one imagined pterosaurs could have the incredibly long necks now known for azhdarchids; and that there weren't any pterosaur specialists at this time (pterosaur researchers collectively took a breather in the early-mid part of the 20th century, only really returning to work from the 1970s onwards). The vertebra itself is near-devoid of features we would expect from an axial element, with only the lightest development of typical vertebral processes, and it has a near circular cross section, a condition at odds with a typical pterosaur vertebra but pretty typical of limb bones. In the context of the time, wing metacarpal was not a silly suggestion.

Despite his misidentification, Arambourg made one thing very clear in his reports: his animal was big. In both his 1954 and 1959 works he wrote that this bone, fragmentary as it was, clearly indicated an animal vastly superior in size to the 7 m wingspan Pteranodon, then considered the largest flying animal of all time. This is important: as early as the 1950s Arambourgiania was being interpreted as evidence that pterosaurs with wingspans rivalling small planes once existed.
Arambourg's (1954) illustration of the Arambourgiania vertebra as a wing metacarpal.
What Arambourg didn't do was elaborate on this point further: he made no fanfare about 'largest flying animal of all time' or whatever, though he might have been justified in doing so. I quite admire Arambourg's restraint in not running too far with the size of his giant: sometimes it's good to admit we don't have enough data to provide a full answer to certain questions, and given how bizarre this bone must have seemed he probably made the right call in being conservative. But his lack of excitement about his gigantic animal might explain why little fuss was made over Arambourgiania after the 1950s. The discovery of Quetzalcoatlus in the 1970s made the vertebral identification of the Arambourgiania holotype clear (Lawson 1975; Wellnhofer 1978), but no mention was made of its significant size compared to the then newly discovered Quetzalcoatlus vertebrae, nor its implication that giant azhdarchids were not only gigantic in wingspan, but must be enormous in neck proportions too.

Other authors missed the significance of Arambourgiania too. For instance, when writing about giant pterosaur flight in 1974, Cherrie Bramwell and G.R. Whitfield stated that Pteranodon was the largest flier ever. Ross Stein's (1975) work on a similar topic provided the same fact, and Wellnhofer's (1978) review of Pterosauria made no mention of the size of Arambourgiania. It wasn't until the 1980s and 1990s that Arambourg's interpretations finally penetrated the pterosaur research zeitgeist, but by this time a flurry of media and scientific attention had made Quetzalcoatlus 'the' giant pterosaur. Arambourgiania would eventually get more dedicated scientific treatment - including wingspan estimates - in the mid 1990s (Frey and Martill 1996; Steel 1997; Martill et al. 1998), but this did little to elevate the status of Arambourg's work and his giant in the story of giant azhdarchid research.

I have to admit that I'm as guilty as anyone in not been kind to Arambourgiania. In Witton (2010), a paper on the history of giant pterosaur discoveries, I didn't even feature it in this figure of 'world record' claims of pterosaur wingspans and equivalent standing heights. A, a 3 m span Andean condor (Vultur gryphus); B, 3 m span wandering albatross (Diomedea exulans); C, Marsh’s 1876 7.6 m span Pteranodon longiceps; D, Stoyanow’s 1936 (apocryphal, and never published in a peer reviewed journal) 10 m span Jurassic pterosaur; E, Harksen’s 1966 9.1 m span Pteranodon sternbergi (now considered too big - 6-7 m max is likely for Pteranodon); F, Lawson’s 1975 11 m span Quetzalcoatlus northropi; G, Buffetaut et al. (2002) 12 m span Hatzegopteryx thambema (probably a smidgen too large); H, another apocryphal giant, a 20 m wingspan form announced at the BA Festival of Science. I want to stress that this animal really, really doesn't exist. Humans used for scale are 1.75 m tall.
Of course, it's easy to see why the 1970s discovery ofQuetzalcoatlus had the impact it did: the fossil material was better, it was announced in Science, and the Texan team did a lot of work to promote their discovery (indeed, there might be more information about Quetzalcoatlus in popular articles than in scientific papers...). By contrast, Arambourg presented Arambourgiania in a couple of very dry articles, published all his work on this animal in French*, and without fanfare. Needless to say, history is more likely to record the bigger splashes than the ripples on the pond, and Quetzalcoatlus made a big splash. But with hindsight, I think we can say that the sidelining of Arambourg's work in historic accounts and our frequent omission of Arambourgiania in discussions of these animals is something we should address. Arambourg was saying decades before anyone else that Arambourgiania was significantly bigger than Pteranodon, and we have to recognise the concept of 'truly' giant pterosaurs as his creation. We might have put numbers to his animals with our 10 m wingspan estimates and 200-250 kg mass predictions, but he put the concept on paper first. The fact he did this from such scant material, and at a time when our knowledge of pterosaur palaeontology was rusty, is impressive, and it really doesn't matter that he got a few things wrong. So yeah, from now on I'm saying that Arambourgiania - not Quetzalcoatlus - was, and always has been, the original giant azhdarchid, and that Arambourg knew this decades before anyone else.

Predicted size and neckage of Arambourgiania next to a Masai giraffe and a human wife. C. Arambourg predicted this 20 years before anyone else, yet we rarely give him any credit for his insight.

Arambourgiania is more than just a neck bone

It's rarely mentioned that Arambourgiania is known from material other than just a gigantic neck bone: a smattering of other bones from the same Phosphate mines might - probably- pertain to the same species. These were re-discovered and outlined by Frey and Martill (1996), and comprise the proximal and distal end of first wing phalanx (below), and a heavily eroded bone interpreted as a second cervical vertebra. Given the uncertainty about their association with the holotype - remember that the circumstance of its collection are lost to history - Frey and Martill classified these as cf. Arambourgiania.

Line drawing and reconstruction of the lesser seen cf. Arambourgiania first wing phalanx fragment (a, c-d). That's the wing phalanx of Quetzalcoatlus sp. in panel b. Scale bars equal 20 mm, which shows the cf. Arambourgiania bone as pretty darned big. From Frey and Martill (1996).
There isn't that much which can be said about the additional cervical - it has some identifiable features, but it's a few flecks of broken bone and bumps of internal mould away from being a featureless tube. It's a little smaller in diameter than the big holotype vertebra, and much shorter. I'm not sure it should be considered as belonging to an animal of the same size as the holotype individual.

The wing phalanx elements however, are more interesting. For one, they're enormous, and look proportionate to the holotype vertebra when juxtaposed in a skeletal reconstruction (below). If they're not from the same individual, they must be from a very similarly sized one. Frustratingly, the wing phalanx ends are broken in a way that hints at the bone shaft bone surviving to the modern day as well, but being lost in recent times.
Arambourgiania (known elements in white, restored, hypothetical neck length of 2.6 m indicated by grey vertebrae) compared to Quetzalcoatlus sp. Note the chunky wing finger bones.

It might be difficult to understand why these scraps of a wing bone are exciting, but they inform us of some fundamental aspects of giant azhdarchid anatomy and wing structure. There aren't many giant pterosaurs where we have recognisable wing and neck material from the same species so, however scrappy it might be, this is already useful material for building a picture of their proportions and appearance. From a functional perspective, they are interesting in showing that wing finger of Arambourgiania articulated with the metacarpal in exactly the same way as it did in smaller pterosaurs. This is good to know, as it confirms the notion that understanding the smaller azhdarchid species is our best route to fathoming the bigger ones. And of further mechanical note is that these elements show the wing finger as proportionally robust, with a big articular surface for the metacarpal/phalanx joint and a wide space for insertion of ligaments pulling the wing open in flight. Increased robustness is a sign of greater resistance to stresses and strains, and a good indication that Arambourgiania had scaled its wing bones to be flightworthy. This is an important counterpoint to proposals from some researchers that the extreme size of giant azhdarchids rendered them flightless. Of course, these scraps of wing bone don't tell us much about flight performance or style, but they are a good indication that flight of some kind was happening in these forms.

The neck of Arambourgiania was a high point of tetrapod evolution, and we need to learn more about it

Of course, we can't talk about Arambourgiania without mentioning its long, tubular neck skeleton. To appreciate it fully, we should outline some generalities of azhdarchid neck anatomy. Proportionally speaking, azhdarchids have some of the longest necks of any tetrapod, a feat all the more remarkable given several aspects of their head and neck skeleton. While the idea of their necks being made of nothing more than simple, near-featureless tubes is overstated, we can't escape the fact that the majority of the azhdarchid neck skeleton had highly reduced features: no big processes, no elongate cervical ribs, no complicated corporeal geometry. This means they had atypically reduced opportunities for muscle attachment and soft-tissue neck support, and they must have been doing something clever to keep their necks aloft - exactly what that was remains a mystery. Like all pterosaurs, azhdarchids also only had seven 'true' cervicals (cervicals eight and nine are 'dorsalised') so that their neck length largely had to stem from just a few bones. This can be seen as peculiar as other long necked reptiles tend to increase their cervical counts to aid elongating their necks, but azhdarchids made do with their ancestral condition. The job of the azhdarchid neck was a significant one: most long necked animals have proportionally small heads, but azhdarchid heads were enormous (see Quetzalcoatlus skeletal restoration, above) and, even allowing for pneumaticity, they probably represented a good chunk of their body mass. Indeed, azhdarchid skulls are big for any tetrapod, their jaws being about about three times longer than their bodies, and those of the giants are predicted as being among the longest of any terrestrial animals, ever. The fact these huge heads were atop these long, skinny neck skeletons is pretty remarkable. In my view we should consider the azhdarchid neck as a real marvel of evolution: these animals did some pretty amazing things with an outwardly simple approach, and achieved some pretty extreme anatomy using a seemingly maladapted to enlarging neck tissues.

The 620 mm long holotype of Arambourgiania philadelphiae as illustrated by Martill et al. 1998. Top is ventral view, bottom is left lateral. Anterior is to the left of the image, scale bar is 100 mm. This bone is predicted to reach 770 mm when complete.
Taking all these points and multiplying them across the Arambourgiania holotype cervical suggests this tubular bone is a pretty fantastic piece of anatomy. We can reconstruct the length of the holotype cervical (presumed to be a fifth, the longest bone in the neck) as 770 mm, and this translates to a neck length estimates of 3 m using scaling based on Quetzalcoatlus (Frey and Martill 1996), or 2.6 m using a range of azhdarchid necks (specifically lengths of cervicals III-VII - this from my an unpublished dataset). However you want to cut it, it's clear this was a very long-necked animal, perhaps up there with the longest necked of all non-sauropodan terrestrial animals (below). On top of this we have to put a big azhdarchid skull, which is going to be about 2-3 m long for a giant. If these estimates are correct, Arambourgiania would be loaded with 5 m of neck and head, and was supporting the whole lot with a small number of bones resembling packing tubes. It has to be regarded as one of the most 'extreme' tetrapod bodyplans known.

Mike Taylor and Matt Wedel's (2013) take on the non-sauropod contest for longest tetrapod neck. It's a close call in my mind as to who wins out of Arambourgiania and a large Tanystropheus, but the important point is that Arambourgiania has an extremely long neck.
So how did the neck of Arambourgiania work? How did a series of bony tubes support a 2-3 m long head? Where did the muscles attach to on the simple structure of cervical V? Full answers to these questions remain part of a broader mystery about the functionality of azhdarchid necks, and this is something that researchers are only just starting to address. But what we know of Arambourgiania is sufficient to give some provisional, partial insight here. The basic construction of the Arambourgiania cervical is basically similar to what we see in smaller azhdarchids, where large, stiffened joints between the neck bones helped support and reinforce the neck (artists: please stop drawing azhdarchids with S-shaped necks in flight!). But subtle modifications to its vertebrae likely enabled each element to grow to much greater lengths without failing. Most azhdarchid cervicals are dorsoventrally flattened, which makes them weakest against vertical loads. Most of the time, vertical loading is created by the weight of the neck and head, but it will also include any food being picked up. The Arambourgiania cervicals are expanded dorsoventrally to the extent that they are slightly taller than wide (Frey and Martill 1996), reinforcing them against vertical bending, and thus potentially able to support greater weights than their smaller cousins. Furthermore, in expanding the bone dimensions to a near circular cross section, and all the while retaining a characteristically thin pterosaurian bone wall, Arambourgiania likely had vertebrae more resistant to torsion and bending than those of the smaller forms.

So counter-intuitive as it seems, making a neck out of tubes is a good way to produce a strong, long, lightweight skeleton, especially if it has to support heavy loads like a huge head. Pterosaurs used the same tactic to enhance their wings, and it seems azhdarchids - especially Arambourgiania - transferred some of these mechanical properties to their vertebral column. We can only guess at the exact proportions of the Arambourgiania head, but adaptations of its neck bones indicate it might have been just as large as those of its smaller cousins. While assessments like this are very basic and clearly only the tip of the iceberg as goes azhdarchid neck mechanics, they demonstrate that Arambourgiania is, and will continue to be, a critical species for understanding the neck proportions, mechanics and scaling of giant azhdarchids.

So, what I'm saying is...

These are just three reasons why we shouldn't be overlooking Arambourgiania when considering the largest pterosaurs. It might not have the sexiest name, and it might not be known from as many elements as the other named giants, but it has historic and anatomical significance that cannot, or should not, be eclipsed from other species. It's clearly an animal that needs to be brought back into the fold of popular science so, the next time giant azhdarchid pterosaurs come up in conversation, remember that there are three named giant species, not just those other two, and that forgotten, old-timer Arambourgiania still has plenty of things to tell us about giant azhdarchid palaeobiology.

Coming really, really soon: you guys like pterosaurs, right?



This bout of championing an old, somewhat forgotten dead reptile was sponsored by Patreon

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References

  • Arambourg, C. (1954). Sur la presence dun pterosaurien gigantesque dans les phosphates de Joradanie. Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, 238(1), 133-134.
  • Arambourg, C. (1959). Titanopteryx philadelphiae nov. gen., nov. sp., ptérosaurien géant. Notes et Mémoires sur le Moyen-Orient, 7, 229-234.
  • Bramwell, C. D., & Whitfield, G. R. (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society B: Biological Sciences, 267(890), 503-581.
  • Buffetaut, E., Grigorescu, D., & Csiki, Z. (2002). A new giant pterosaur with a robust skull from the latest Cretaceous of Romania. Naturwissenschaften, 89(4), 180-184.
  • Buffetaut, E., Grigorescu, D., & Csiki, Z. (2003). Giant azhdarchid pterosaurs from the terminal Cretaceous of Transylvania (western Romania). Geological Society, London, Special Publications, 217(1), 91-104.
  • Frey, E., & Martill, D. M. (1996). A reappraisal of Arambourgiania (Pterosauria, Pterodactyloidea): One of the world's largest flying animals. Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 199(2), 221-248.
  • Langston, W. (1981). Pterosaurs. Scientific American, 244, 122-136.
  • Lawson, D. A. (1975). Pterosaur from the Latest Cretaceous of West Texas. Discovery of the Largest Flying Creature. Science, 187: 947-948.
  • Martill, D. M., Frey, E., Sadaqah, R. M., & Khoury, H. N. (1998). Discovery of the holotype of the giant pterosaur Titanopteryx philadelphiae ARAUBOURG 1959, and the status of Arambourgiania and Quetzalcoatlus. Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 207(1), 57-76.
  • Steel, L., Martill, D.M., Kirk, J., Anders, A., Loveridge, R.F., Frey, E. J.G. Martin (1997). Arambourgiania philadelphiae: giant wings in small halls. The Geological Curator, 6(8): 305-313
  • Stein, R. S. (1975). Dynamic analysis of Pteranodon ingens: a reptilian adaptation to flight. Journal of Paleontology, 534-548.
  • Taylor, M. P., & Wedel, M. J. (2013). Why sauropods had long necks; and why giraffes have short necks. PeerJ, 1, e36.
  • Wellnhofer, P. 1978. Handbuch der Paläoherpetologie. Teil 19: Pterosauria. Gustav Fischer Verlag, Stuttgart. 82 pp.
  • Witton, M. P. (2010). Pteranodon and beyond: the history of giant pterosaurs from 1870 onwards. Geological Society, London, Special Publications, 343(1), 313-323.

Friday, 10 June 2016

The dinosaur resting pose debate: some thoughts for artists

Three Sinornithoides youngi, one standing, one sitting, one sunbathing. But would these animals really have adopted resting poses like those of modern birds, as shown here, or would they have relaxed in completely different postures?
A recurrent topic of conversation among palaeoartists concerns how non-avialan dinosaurs rested. Specifically, were they constrained to crouching down and lying on their bellies like modern birds (above), or could they lounge on their sides, rest with limbs beneath their bodies, and generally adopt more varied resting poses? I've always thought that there's no reason to confine depictions of reposed dinosaurs to avian-like squatting poses and, based mostly on personal experiences of modern animals, it has never seemed outlandish to depict a theropod resting on its side, or a horned dinosaur sitting on its legs or whatever. This is evidenced by some of the art I've posted here in the last few years, some of which is reproduced below. But reasonably frequent mention of this topic on social media suggests that not everyone has this attitude, so I thought it might be of interest to discuss this in more depth. What do Mesozoic dinosaur skeletons, trace fossils, and modern animals tell us about Mesozoic dinosaur resting poses, and how might we approach this topic as artists?

A necessary caveat

Asking general questions about how extinct dinosaurs did anything is increasingly difficult to answer in a succinct, concise manner. Dinosauria is an enormous group of animals with huge diversity in body size, gait, proportions, soft-tissue anatomies and so on. These are all things which impact the way an animal might sit or lie down, and in all likelihood there is no one answer to this inquiry. What works for a troodontid may not work for an ankylosaurid, and what works for these may not apply to a sauropod. While there's some merit to taking a general approach to this discussion (and that's what we'll be doing here), this point is something to bear in mind as we go through. The actual answer to this question will be multifaceted, and found through dedicated study of specific dinosaur groups.

What the fossil record tells us

Crouching dinosaur traces attributed to bipedal theropods and ornithopods are known from the track record (e.g. Lockley et al. 2003; Milner et al. 2009 and references therein), and these are often used as evidence for dinosaurs generally adopting bird-like, crouched resting postures. Such impressions cannot be regarded as common, but are easily identified by the elongate footprint impressions where the long metatarsal bones and ankle are impressed into the ground behind the toe prints. That at least some of these crouching traces show stationary behaviours, and not crawling or stooping, is evidenced by the symmetrical position of the footprints and impressions of an ischial callosity (the soft-tissue covering the posterior prong of the dinosaur pelvis). These pelvic traces show that the body was in contact with the ground when these traces were made, and that it was not being dragged forwards. Clearly, these animals were, at least partly, letting the ground take some of their weight.

We don't just have to look to trace fossils for evidence of crouching behaviour. On rare occasions, remains of dinosaurs are found that were more-or-less entombed alive in ash or sediment, revealing details of their postures at time of death. Famous examples of such occurrences include several troodontids (Russell and Dong 1993; Xu and Norell 2004; Gao et al. 2012) and protoceratopsids (Fastovsky et al. 1997). These also consistently show dinosaurs resting on their bellies in crouched postures, legs folded up either side of their bodies in a very avian manner.

Protoceratops (P. hellenikorhinus shown here) is among the dinosaurs known from skeletons that are thought to reflect near-life position at death. The pose depicted in this painting, with the animal leaning on its left leg, is at odds with the poses of such skeletons.
This is starting to seem like we've already solved the debate, but we might want to think about what these data are actually telling us. Our footprint data, and much of our crouching-death-pose skeletons, pertains to smaller bipedal dinosaurs, and we don't have any comparable data for the other bauplans. I don't know about you, but my chief interest in this discussion isn't really the small bipeds: it's the stegosaurs, the sauropods, the ceratopsids etc: those big animals that are different enough from modern species that their day-to-day behaviours are not obvious. And there's certainly enough strange stuff going on in their anatomy to caution against simply applying what we see in small bipedal dinosaurs across the entire group.

Secondly, it's interesting that we only sometimes see hand prints associated with crouching traces (Milner et al. 2009). When we do, we tend to see evidence of the palms and digits, not of whole forearms, as we might expect from a fully resting, lying animal. So are these animals actually lying down, or just sitting? Have they crouched down to truly rest, or are they performing some other behaviour (e.g. foraging, preening etc.)? I find it interesting that crouching traces can occur multiple times within trackways (Lockley et al. 2003), and that others show evidence of animals shifting weight and changing direction. Such instances seem to record sitting, but still 'active' individuals. Studies of modern animal behaviour are relevant here. When researching bird sleeping postures, I found Almaner and Ball (1983) had similar misgivings about the idea of immobile birds being 'inactive' or merely 'resting'. Based on their observations, they divided avian 'loafing' behaviour (that is, behaviour adopted when the bird is generally immobile) into multiple types of activity, of which only one is resting. Looking through their categories of loafing behaviour (below), none seem outlandish when applied to dinosaurs and I wonder what those prints made by stationary, crouched dinosaurs really represent: resting is really only one option. It may be that further examination of 'resting' traces can turn up more information. That said, I'm aware of slightly frustrating experiments with modern emus where even optimal substrates for track formation do not record additional trace evidence of activities like feeding from crouched positions (Milàn 2006). But, hey, we can still be optimistic that more data and insights will come in time.

What it means to be a stationary bird. Clearly, being crouched and immobile does not always mean 'resting'. From Almaner and Ball (1983).
Those skeletons preserved in life position are also worthy of further comment. Generally speaking, these represent animals caught in catastrophic events - volcanic eruptions, sandstorms and so forth - and we probably should not assume that these animals were just 'resting' when they were entombed in sediment. Indeed, the orientation of Protoceratops skeletons in Mongolian bone beds is non-random, and sedimentological data indicates they were facing into strong sandstorms when they died (Fastovsky et al. 1997). They were certainly crouched, but in all likelihood they were not relaxed and taking it easy: quite contrarily, it seems reasonable to assume they died during attempts to weather a storm, doing their best to hunker down against flying sand, collapsing dunes and all manner of other terrible events. An apt analogy here might be reading the Pompeii ash mummies as representing stereotypical human resting postures: some were found lying down, but that doesn't mean they were relaxing when they died.

On a related note, I also wonder how we would identify a dinosaur that died deliberately resting on its side rather than being moved into that pose by taphonomic processes. Most animals require effort to remain vertical, be it crouched or otherwise, and it's obvious when we find crouching dinosaurs that their pose reflects something of their final behaviours. But how do we distinguish a dinosaur preserved having a nap on its side from one that simply died and fell over, or was washed up on a riverbank or whatever?

All this considered, my point here is not that these data are meaningless when it comes to discussions of dinosaur resting postures. They clearly show that many Mesozoic dinosaurs did naturally crouch in an avian-like manner, and there's no problem with assuming this has some bearing on resting poses. But I do not think this data is without complications, nor that it is complete enough to tell us the whole story here. We probably need to look elsewhere for additional information.

The search for modern analogues

Torvosaurus tanneri in controversial 'reclining on a recline' pose.
Another approach we can take to this problem is to look at how modern animals sit and rest. Birds are often hailed as the best insight here, and for obvious reasons. But are birds good models for Mesozoic dinosaurs? Modern birds represent an extremely derived group of dinosaurs, and their anatomy has been heavily influenced by the development of flight adaptations. That means that many anatomical aspects we should consider here - body shape, flexibility of vertebrae and limb joints, muscle mass and so on - are quite far removed from their Mesozoic cousins. Their torsos, for instance, are proportionally short and broad, and rendered inflexible by osteological fusions and large flight muscle masses. Their hips are similarly broad, thanks to reconfiguration of their internal organs and their hindlimb musculature is immense - a consequence of their launch strategy as well as reconfiguration of the leg to primarily flex at the knee, rather than the hip, during terrestrial locomotion. Their necks and heads are also extremely lightweight, and capable of being withdrawn over the body to rest on the chest. All these things considered, it's not surprising that birds almost always (see below) rest in crouching postures. Some parts of their anatomy are well suited to it, and others almost dictate it. Despite these derivations, some dinosaurs - theropods and small bipedal ornithischians - are certainly closer to this morphology than any other living groups, and birds probably are their best analogue for resting behaviours.

But what about other species? We might look to other reptiles for further insight here. Lizards, turtles and crocodylians are like birds in that they rest on their bellies, although they tend to be less fussy about the placement of their limbs (I often find my own pet reptiles looking like they just flopped down mid-step, legs and arms at all sorts of angles. It doesn't look comfortable, but I guess it must be). But do these animals really have an alternative? Their bodies are very broad but shallow, and their limbs project laterally from the torso. It's hard to imagine them achieving a stable resting posture by doing anything other than lying on their bellies.

The body shapes of living reptiles are pretty distinct from those of sauropods or many ornithischians, and I don't think these animals provide much assistance with our inquiry. For these groups, a case can be made for their basic form being more akin to those of modern mammals than any living reptile. Like mammals, they tend to have deep, narrow chests, (see illustrations in Paul 2010 and Goldfinger 2005), and many lack the rigid structural bracing and expansive chest muscles that we see in birds. For some dinosaur groups, the limbs of large land mammals are better models than the light, flexible limbs of birds, and the fact many mammals are quadrupedal is also of utility here. The relative weight of mammalian heads, and flexibility of their necks, may be more comparable to some dinosaurian bauplans than avian ones too, and mammals are also our best (and only) modern analogue of larger Mesozoic dinosaur body masses. The latter is important to this discussion as shifting weight around between standing and reclining, as well as considering weight bearing during the rest phase, are factors here.

All these points considered, maybe the body shapes and masses of mammals offer some of the most useful analogues for non-bipedal dinosaur resting poses and related mechanics? Mammals are, of course, far more flexible in their approach to resting than reptiles. Even large mammals like elephants, hippos, rhinos and large bovids are capable of crouching and lounging on their sides, and even modest-sized species will sometimes rest on their backs. I imagine this is because deep-chested large animals are top-heavy when crouched, so flopping over to one side is likely to be far more relaxing and stable. The fact that our largest land animals can spend hours on their sides without dying of asphyxiation is a good indication that this may not have been a concern for large dinosaurs, either. A big elephant is going to weigh as much as many big dinosaurs and, while the biggest hadrosaurs and sauropods were likely heavier, it's useful to have confirmation that 5-6 tonne creatures can lie down for extended periods without problem. I often wonder if the idea of animals crushing their lungs and other organs when lying down is a bit of a myth, or at least overstated. Even large stranded whales, weighing many times more than our largest elephants, can survive for days on land before dying. The fact is most beached whales die of complications related to the injuries and diseases that led to their stranding in the first place, and this generally happens long before their lungs or other organs are crushed.

To cheer everyone up after all that talk of dead whales, here's a male Asian elephant napping. D'aw. Photo by Wikimedia user Fruggo.
So maybe our best models are birds for bipeds, and mammals for everything else? Perhaps, but for all this talk of typical resting postures and so on, we should mention that some animals lie down in ways that would not be expected from their anatomy or taxonomic associations. This includes modern dinosaurs. For example, resting ratites (particularly young individuals) will sometimes sit with their legs completely stretched out behind them (Amlaner and Ball 1983). Sleeping ratites, and some other birds, do not tuck their beaks under their wings, or rest them on their chests, but rest their entire neck on the ground (Amlaner and Ball 1983). Sunbathing gamebirds (including poultry) are known to roll on their sides and back, both feet clearly visible on one side of the body, and will fall asleep in that posture if undisturbed (example). As is usual in biology, there are enough complications in our nice, neat rules to make us question whether we can ever predict anything with more than shaky confidence.
Resting postures of the Greater Rhea, depicted by Amlaner and Ball 1983. I recall seeing a rhea using the upper posture at Edinburgh zoo. It's... weird seeing a bird sitting like this in real life.

Functional studies of dinosaur anatomy

We've looked at direct evidence of reposed dinosaurs and their modern analogues, which leaves functional considerations of dinosaur skeletons as our last main area of consideration for this topic: is there anything about their anatomy to suggest resting on their sides or using other postures might be prohibited? The fusion of some dinosaur vertebrae is often mentioned as a problem here, particularly the ossified tendons common to many ornithischian dinosaur groups. These are suggested to have limited the motion of the vertebral column and limited dinosaurian abilities to shift their mass/wiggle out of lounging postures. Such suggestions are probably overstating the stiffening effect of ossified tendons. As a general point, it should be mentioned that ossified tendons are common across animals of all kinds, and occur in many places in their bodies. For example, they occur in the bodies of fish, in bird and human legs, along bird backs, in sauropod necks and in pterosaur forearms (e.g. Bennett 2003; Organ 2006; Organ and Adams 2010; Klein et al. 2012). Moreover, they are not necessarily anything to do with restricting skeletal motion. Sometimes the opposite seems true: they may be something to do with storing and releasing energy to increase arthrological efficiency, or simply reduce strain on musculature. Their functional roles are still being worked out, but it seems well grounded that their role varies with their position in the skeleton and their associated musculature. We also know that their histological composition varies, and this likely affects their mechanical properties too (Organ and Adams 2010).

In dinosaurs, ossified tendon distribution along the vertebral column is quite varied. As a general rule, ossified tendons occur around the hip and tail base, but they can cover many of the torso vertebrae in things like hadrosaurs. Studies suggest that their effect is to reduce vertebral motion in some planes, but they do not eliminate movement altogether (Organ 2006). The vertebrae can still move in all directions, and even relatively freely in some axes of motion, and that's likely all that was needed to enable animals to lift themselves from a non-crouching resting posture. We only need a few degrees of motion here and there to liberate a limb, or to gain better purchase on the ground, before the limb skeleton can take over in levering the body into a standing pose. For the sake of completeness, it's worth mentioning that the trunks of other dinosaurs - those without ossified tendons - were probably mobile enough for this job, too (e.g. Mallison 2010a, 2010b).

Finally, a practical consideration

From the new ITV show "Abelisaurs do the Darnedest Things".
One final point to make on this issue is a relatively pragmatic one. The idea that dinosaurs only adopted crouched resting poses implies a certain rigidity to their form, and one that would compromise their ability get into, or out of, anything other than deliberately chosen, specific poses. Against this I cite the general clumsiness of animals everywhere. Much as we like to glamourise and romanticise nature - beautiful in its savagery, red in tooth and claw, survival of the fittest and all that - the truth is that animals are as clumsy as we are. You don't have to be a dedicated wildlife observer to see animals slip, trip or fall. These things happen routinely, and they almost certainly did in the Mesozoic, too. Fighting and jostling animals would also almost certainly find themselves forced in compromised, awkward positions from time to time, too. We can be confident that Mesozoic dinosaurs fell on their sides, rolled onto their backs and got into other mischief by accident even if not by intent, and it seems unrealistic to assume they would not have recovered from these accidents as effectively as modern animals. If we assume they could escape such poses in emergencies, why should they not be able to rise from them at other times as well?

So, in summary...

Putting all these lines of evidence together - the limited direct fossil data, our ability to interpret that fossil data, the anatomy and behaviour of modern animals, and what we know of dinosaur anatomy - I still don't see any reason to think Mesozoic dinosaurs were constrained to crouched resting poses. I stress my use of the word 'constrained' there: as mentioned above, there is good reason to think crouching was utilised by dinosaurs for a variety of reasons, and I'm sure many of them rested in this way. Moreover, we can probably assume that most dinosaurs entering or rising from repose would have assumed a crouched position during that process. This seems fairly true of modern animals, after all. But there seems no reason to think they were incapable of other resting in other attitudes as well, such as reclining in classically 'mammalian' poses, using some of those strange ratite or galliform postures mentioned above, or doing something else entirely. It seems almost certain that different dinosaurs were suited to different poses, and different ranges of poses, when resting: maybe this is something to explore in future art.

Coming next: this:

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References

  • Amlaner, C. J., & Ball, N. J. (1983). A synthesis of sleep in wild birds. Behaviour, 87(1), 85-119. 
  • Bennett, S. C. (2003). New crested specimens of the Late Cretaceous pterosaur Nyctosaurus. Paläontologische Zeitschrift, 77(1), 61-75. 
  • Fastovsky, D. E., Badamgarav, D., Ishimoto, H., Watabe, M., & Weishampel, D. B. (1997). The paleoenvironments of Tugrikin-Shireh (Gobi Desert, Mongolia) and aspects of the taphonomy and paleoecology of Protoceratops (Dinosauria: Ornithishichia). Palaios, 59-70. 
  • Gao, C., Morschhauser, E. M., Varricchio, D. J., Liu, J., & Zhao, B. (2012). A second soundly sleeping dragon: new anatomical details of the Chinese troodontid Mei long with implications for phylogeny and taphonomy. PloS one, 7(9), e45203. 
  • Goldfinger, E. (2004). Animal Anatomy for Artists: The Elements of Form: The Elements of Form. Oxford University Press.
  • Klein, N., Christian, A., & Sander, P. M. (2012). Histology shows that elongated neck ribs in sauropod dinosaurs are ossified tendons. Biology letters, rsbl20120778. 
  • Lockley, M., Matsukawa, M., & Jianjun, L. (2003). Crouching theropods in taxonomic jungles: ichnological and ichnotaxonomic investigations of footprints with metatarsal and ischial impressions. Ichnos, 10(2-4), 169-177. 
  • Mallison, H. (2010a). The digital Plateosaurus II: an assessment of the range of motion of the limbs and vertebral column and of previous reconstructions using a digital skeletal mount. Acta Palaeontologica Polonica, 55(3), 433-458. 
  • Mallison, H. (2010). CAD assessment of the posture and range of motion of Kentrosaurus aethiopicus Hennig 1915. Swiss Journal of Geosciences, 103(2), 211-233. 
  • Milàn, J. (2006). Variations in the morphology of emu (Dromaius novaehollandiae) tracks reflecting differences in walking pattern and substrate consistency: ichnotaxonomic implications. Palaeontology, 49(2), 405-420.
  • Organ, C. L. (2006). Biomechanics of ossified tendons in ornithopod dinosaurs. Paleobiology, 32(04), 652-665. 
  • Organ, C. L., & Adams, J. (2005). The histology of ossified tendon in dinosaurs. Journal of Vertebrate Paleontology, 25(3), 602-613.
  • Paul, G. S. (2010). The Princeton field guide to dinosaurs. Princeton University Press. 
  • Russell, D. A., & Dong, Z. M. (1993). A nearly complete skeleton of a new troodontid dinosaur from the Early Cretaceous of the Ordos Basin, Inner Mongolia, People's Republic of China. Canadian Journal of Earth Sciences, 30(10), 2163-2173. 
  • Xu, X., & Norell, M. A. (2004). A new troodontid dinosaur from China with avian-like sleeping posture. Nature, 431(7010), 838-841.

Wednesday, 18 May 2016

Quetzalcoatlus: the media concept vs. the science

Background: Javelina Formation forest. Mid-ground: the 4.6 m wingspan, super-famous azhdarchid pterosaur Quetzalcoatlus sp. Foreground: lunch.

As a consultant, the pterosaur I get asked about more than any other is the azhdarchid Quetzalcoatlus. In recent weeks I've been speaking to two completely independent media producers about this animal, and I think just about all my prior TV and film work has involved it somehow, even if not prominently. I suppose Quetzalcoatlus is so popular because it's not just the most famous azhdarchid pterosaur - which are now more popular than ever - but the first animal most people think of as the biggest ever flying creature.

All the pictures, museum exhibitions, sculptures and animations of Quetzalcoatlus suggest it must be a well-understood animal, but it's actually very difficult to provide consultancy on, for several related issues. The first is that the science on the animal itself is unfinished, largely unpublished, and the existing body of work is decades old. The second is that our general understanding of azhdarchid pterosaurs has moved on considerably in the last two decades, and largely without a good grounding of what Quetzalcoatlus actually is. The third, and final, issue is that most parties think Quetzalcoatlus is better understood than it really is, to the point where whole media projects are locked in around it, and consultants are asked to make calls that have little scientific backing. That third point is an important one: 'Quetzalcoatlus the media concept' is a very different beast to 'Quetzalcoatlus the scientific entity'. This can create difficulties when creating programmes, games or artwork of this animal, as the expectations for what can be achieved with Quetzalcoatlus are often beyond what scientists can provide.

In the interests of helping out those who want to use Quetzalcoatlus in their projects, I thought it might be useful to show how 'common knowledge' about Quetzalcoatlus differs from its actual, objective status within 21st century pterosaur science. An important caveat before we go further is that scientific work on Quetzalcoatus is ongoing. A full description and functional assessment is rumoured to be in the works (as, er, it has been for 40 years...) and this will change the way we view this animal tremendously. It can't not do this: the data available on this animal are minimal, so publication of any new details will augment the current situation. In all likelihood this post will be moot, at least in part, when this document is finally published. This piece is being written in May 2016, so please bear in mind that things might have changed if you're reading this at a later date.

The concept. Quetzalcoatlus is a giant azhdarchid pterosaur from Texas, known from substantial remains.


The science. The popular view of Quetzalcoatlus is really a conflation of two taxonomic entities, Quetzalcoatlus northropi and Quetzalcoatlus sp. Both occur in the Upper Cretaceous (Maastrichtian) Javelina Formation of southern Texas*. Q. northropi is currently the only 'officially' named species of Quetzalcoatlus, and is the reason we think this animal was so huge. It's also only known from bits and pieces of a gigantic left wing and cannot be regarded as a well known animal. Do not, if you're a TV producer or whatever, expect to film a room full of real fossils for this thing - any skeleton you see of a giant Quetzalcoatlus is almost entirely reconstruction - still impressive, but nearly all calculation and extrapolation.

*It's worth mentioning that Quetzalcoatlus has been identified outside of Texas, scraps of azhdarchid bones from both North America and Europe being allied to this genus. I suspect that most of these should not be referred to Quetzalcoatlus proper, as they're fairly 'generic' azhdarchid bones and, moreover, we have no criteria for what constitutes Quetzalcoatlus within Azhdarchidae itself (see below). I don't want to discuss these now, however, and only mention them for the sake of completeness. 

Q. sp., by contrast, is much better represented. Several incomplete skeletons are known (an example can be seen below) that collectively give a near complete picture of Q. sp. osteology. Its this material that gives us our familiar image of Quetzalcoatlus: the long neck, the oversize pointy face, the long limbs and so on. Almost all discussions about the detailed anatomy of Quetzalcoatlus pertain to this material, not the giant wing. The Q. sp. fossils are also a key source of the proportional data used in calculating the 10 m wingspan for the giant Q. northropi animal, even though Q. sp. is much smaller - the complete wing metrics of one specimen give a wingspan of 4.6 m (Unwin et al. 2000).

Q. sp. partial skeleton figured by Kellner and Langston (1996).

How the two Quetzalcoatlus species are related to each other, and other azhdarchids, remains critically unexplored and uncertain. It is no exaggeration to say that taxonomic considerations of this important, well-known genus comprise no more than a few sentences in the entirety of technical pterosaur literature. The name Q. northropi was not even erected in a descriptive paper, but as an aside in a brief comment on the likely wingspan of the giant wing specimen (Lawson 1975a). This gave us the northropi name and designated a type specimen (the big wing), and suggested that the smaller specimens were just diminutive, presumably immature versions of the giant species. However, other elements key to species creation - diagnoses, specimen inventories, supporting description or illustrations were not provided, the best alternative being a very short 1975 science paper (Lawson 1975b). Nessov (1991) provided an attempt to diagnose the genus (along with other azhdarchids) but his characters are not useful. Later, Kellner and Langston (1996) suggested that the smaller Quetzalcoatlus skeletons were not juveniles of the giant species, but a distinct species that would be named at a later date. It was this paper that created the 'Q. sp.' moniker, a place-holder for a name we'll perhaps see published in time. Alas, Q. sp. was also created using drive-by taxonomy without justification for separating sp. from northropi, or providing characters to unite these species in a distinct genus among other azhdarchids.

This might all sound like tedious detail irrelevant to reconstructions, media portrayals etc., but the upshot is that the pterosaur community is still largely in the dark about Quetzalcoatlus. We can't really comment on what makes it unique, whether all these bones from Texas should be considered one or two species, how accurate it is to scale up the smaller Quetzalcoatlus to the size of the big wing and so on: that information has been made public yet. Some specific folks might be able to provide those details, but they are not peer-reviewed 'common knowledge'. Therefore, most researchers (including myself) can only talk about Quetzalcoatlus in terms of the few details that have been published, and fill the rest in with 'generic' details of azhdarchid pterosaur palaeobiology.

The concept. Quetzalcoatlus was like all azhdarchids: a long-necked, long-skulled creature with long limbs.


The science. As alluded to above, we have a basic idea of Quetzalcoatlus sp. appearance, even if the vast majority of it remains unpublished. The core aspects of this animal can be built from data titbits gleaned from different papers: a good description of the skulls and mandibles was provided by Kellner and Langston (1996); Witton and Naish (2008) and Steel et al. (1997) gave some details of the cervical vertebrae, and Unwin et al. (2000) provided measurements for nearly all major limb bones. From this, we can be confident that Quetzalcoatlus sp. is the long-necked, long-faced, toothless, short-winged and gracile-limbed creature we have traditionally associated with the Quetzalcoatlus name. I've attempted a skeletal reconstruction of this 4.6 m wingspan species below using these data: some of the bone anatomy is 'generic azhdarchid', but the basic proportions, skull and anterior neck vertebrae should be OK. This skeletal is the basis for the painting at the top of the post.

Quetzalcoatlus sp. skeletal, using data from Kellner and Langston (1996); Witton and Naish (2008), several other sources of azhdarchid neck data, and Unwin et al. (2000). Yes, it was that long-necked and long-legged.

A question I have much more trouble answering is what Q. northropi looked like. When we see a giant long-necked Quetzalcoatlus in TV shows or comics, we're seeing a hybrid of the two Quetzalcoatlus animals - the anatomy of sp. crossed with the size of northropi. This approach is not without merit: it's consistent with the existing taxonomy of this animal (such that it is), and other pterosaur fossils confirm that some giant azhdarchids were long-necked, perhaps Q. sp-like creatures.

On the other hand, we only have an incomplete northropi wing skeleton to work from. It's widely recognised that pterosaur wings are among the more conservative aspects of their anatomies - great for identifying pterosaurs to specific groups (we have characterised azhdarchid wings, for instance) but above a basic level of taxonomy they don't tell us much about life appearance. It wasn't always this way. When Quetzalcoatlus was found in the 1970s the smaller Q. sp. skeletons provided our only comprehensive insight into azhdarchid anatomy, and thereafter we assumed that Q. sp. typifies the group. However, azhdarchid pterosaur science has progressed considerably in the last two decades and the group can no longer be considered anatomically uniform. Their skulls can be short and broad, long and narrow, and have deep or slender lateral profiles (Witton 2013). They can have cranial crests (Kellner and Langston 1996), but they might not (Cai and Wei 1994). Their necks can be extremely long, or of more typical pterodactyloid lengths (Vremir et al. 2015). Some had stilt-like limb bones, but others had short forelimb anatomy (McGowan et al. 2001). Such variation seems present in the giants as much as their smaller cousins. I don't think we know what an 'average' azhdarchid looked like yet, but Q. sp. should be viewed as representing only one, relatively exaggerated take on azhdarchid anatomy. It historically seemed safe to make Q. sp. a giant version of this form, but that cannot be regarded as the only option today. For all we know, Q. northropi could be a short-armed, short-necked species with a truncated, deep jaw - quite the opposite of Q. sp..

A selection of azhdarchid skull and mandibles. A and B, posterior skull bits of Hatzegopteryx; C-D, Q. sp., E. Zhejiangopterus; F-G, Bakonydraco; H, TMM 42489-2, the Javelina Formation azhdarchid which isn't Quetzalcoatlus. Q. sp. must be regarded as a long-snouted, gracile form: does this also apply to Q. northropi? Image from Witton 2013.
I would be more comfortable with reconstructions of Q. northropi as a giant, scaled-up Q. sp. if we had good reason to believe the two were closely related**. As mentioned above however, necessary work on this has yet to be presented and, as evidence-led scientists, it is not unreasonable to call the situation ambiguous until more data is forthcoming. I suppose one reason we might think northropi and sp. are congeneric is because they're from the same formation. However, the remains of northropi and sp. were not associated, being found 10s of kilometres apart (Lawson 1975b), and we have increasing evidence of multiple azhdarchid taxa occurring in the same geological units (e.g. McGowen et al. 2000; Godfrey and Curry 2005; Vremir et al. 2013). Where azhdarchids do coexist, they differ markedly in anatomy and overall form (Vremir et al. 2013). An additional complication is TMM 42498-2, a large, deep-jawed Javelina Formation azhdarchid which is clearly not Q. sp. (bottom panel in the image above). If this represents cranial remains of Q. northropi and not an additional Javelina species (we currently have no way of telling), northropi would look would look very different to our usual depictions.

**I'm expecting people to wonder if Quetzalcoatlus taxonomy has been looked at in detail via cladistic methods. The most complete published assessments I'm aware of are those by Brian Andres (e.g. Andres and Myers 2013) which use seven azhdarchid OTUs, including the two Quetzalcoatlus taxa. The results of such studies remain ambiguous about the affinities of Quetzalcoatlus (Q. sp. and northropi form a polytomy with Arambourgiania). I'd like to see an analysis with more azhdarchid taxa, including some of the more unusual types such as Hatzegopteryx and Montanazhdarcho.

I don't have any real answers or insight on these points. Rather, I'm getting at the fact that our 21st century understanding of azhdarchids and other flying reptiles is becoming complicated, and with our current, superficial insights into what Quetzalcoatlus is and how it's related to other azhdarchids, there may not be one 'right' way to restore northropi. We might be correct to represent it as a giant Q. sp., as per tradition, but we might not.

The concept. Quetzalcoatlus was the biggest animal to have ever flown, even larger than other giant azhdarchids


The science. As with modelling the size of most giant extinct animals, it's difficult to say what giant azhdarchid was the biggest. Q. northropi was certainly up there, recent estimates of its wingspan being in the region of 10 m and predicted body masses of 200-260 kg. But other giant pterosaurs are predicted to be about the same size (see Witton and Habib 2010 for a recent discussion)... and that's about all we can really say. Every giant azhdarchid is represented by scrappy material, so the error bars on any size estimate are large and the calculations themselves are highly sensitive. We would be foolish to use them as anything other than ballpark figures. We can say that Q. northropi was one of the biggest flying animals of all time and, along with other giant azhdarchids, it dwarfed other flying species including most pterosaurs, and all birds and bats. I appreciate some folks will find this lack of clarity frustrating, but that's just how it is: we can't say any more until we understand all the giants - not only Quetzalcoatlus - in more detail. 

The concept. The outstanding flight capabilities of giant azhdarchids allowed Quetzalcoatlus to be a continent-hopping animal that occurred in Europe as well as the USA, although the European bones were given a different name: Hatzegopteryx. Some chap called 'Witton' suggested this.


The science. Thanks, I think, to Wikipedia, I've been confronted several times about suggesting the Romanian giant azhdarchid Hatzegopteryx thambema should be synonymised with Quetzalcoatlus. I feel a bit misquoted on this. Yes, I (and colleagues) mentioned this as a qualified possibility in a 2010 abstract and conference talk (Witton et al. 2010), but as part of a broader, detailed discussion about the need to tighten up giant azhdarchid taxonomy. Specifically, we discussed the three named giant azhdarchids - Arambourgiania philadelphae, Quetzalcoatlus northropi, and Hatzegopteryx thambema - and stressed issues with their current taxonomy. These issues - hitting some points already tackled here - included the lack of a description and diagnosis for Quetzalcoatlus; the uncertainty over the relationships between Q. northropi and Q. sp.; the scrappy nature and general incomparability of giant azhdarchid fossil remains; their representation by anatomies generally considered unnameable by pterosaur workers, and the identification of several alleged autapomorphies of giant species in other azhdarchid remains.

Holotypes of giant azhdarchids. A, Arambourgiania philadelphiae, B, Q. northropi (humerus only, the other holotype wing elements have never been published) C, the damaged proximal humerus of Hatzegopteryx thambema (humeral head and cranial remains not figured here). From Witton (2013).
Concerning Quetzalcoatlus and Hatzegopteryx, we pointed out that overlapping bits of Q. northropi and Hatzegopteryx (proximal humeri) are similar enough that they could be considered synonymous, but this says more about the use of wing bones for the Q. northropi type material than anything else. As mentioned above, wing bones aren't always that useful in detailed taxonomy. The overlapping bits of Hatzegopteryx and Q. sp. (jaw joints), however, are different enough to demonstrate they are separate taxa. We went on to say that the significance of all this isn't clear because the relationships between Q. sp and Q. northropi are not evaluable at this stage. Our take-home message, then, wasn't that Hatzegopteryx and Quetzalcoatlus are the same thing, but that the diagnoses, validity and relationships of named giant azhdarchids warrant detailed assessment in future. Since writing this abstract six years ago, ongoing work on Hatzegopteryx seems to be supporting its separation from other azhdarchids - more on that in time.

So... how can Quetzalcoatlus be used in art, film, games etc.?

It's clear by now that the way we imagine and depict Quetzalcoatlus as a media construct is very different to its status in science. My take on this animal is about as cautious and conservative as you'll find, and I suppose that's because my experience with azhdarchids in both a research and artistic capacity has frequently found Quetzalcoatlus as a tricky animal to work with. While we can't point to anything being 'wrong' with those older interpretations of Quetzalcoatlus, shifts in our understanding of azhdarchid and other pterosaur science means we can't accept those 40- or 20-year old interpretations of this animal with the same confidence as we used to. Quetzalcoatlus, as a scientific concept, needs modernisation.

Still, my overall goal here is not to be defeatist, rather to simply say what we might and might not be confident about when depicting Quetzalcoatlus. For instance, while Q. northropi is a can of palaeobiological worms, Q. sp. (or whatever it turns out to be) does offer a lot of scope for use in reconstructions and media projects. I frequently feel that these sorts of animals (i.e. the better known mid-sized or small species, not the specrapularly known giants) make interesting enough subjects for artwork and media projects. There's certainly a lot more to work with and say about them, and we can be far more confident in what is being conveyed to the public. Not everything with palaeontological content has to focus on the biggest animals!

Of course, I also appreciate that some projects just can't do without giant azhdarchids. For these, I stress that there are alternatives to Q. northropi which boast the same approximate wingspan, are known from anatomies that provide more insight into life appearance, and have a better grounding in contemporary science. These include the long-necked Jordanian form Ararmbourgiania and its Romanian contemporary, the wide-skulled, robustly-built Hatzegopteryx. Both have interesting stories of discovery and scientific development (for instance, Arambourgiania was actually the first giant azhdarchid on record, not Quetzalcoatlus; and Hatzegopteryx is so chunky that it was initially interpreted as a giant predatory dinosaur) and are palaeobiologically interesting. If these won't do - perhaps for reasons of geography - then consider that unnamed bones of giant azhdarchids are widespread, being known from North America, Europe, Africa and Asia. Although these offer fewer anatomical details than the named taxa, they can also be handled more generally because they don't need to look like a (theoretically) well understood, diagnosed and named species. This means data can be pooled from other finds into a more 'generic azhdarchid' melting pot, and that gives more wiggle room when considering appearance and form.

The take-home here, then, is that Quetzalcoatlus might be the 'best known' giant azhdarchid and the one that everyone wants to feature in their TV shows, films and games, but be forewarned that the scientific status of this animal is rather different to what popular depictions suggest. Moreover, there are alternatives which might be (at time of writing) better understood and just as interesting. If you're planning a TV show, video game or artwork of a giant azhdarchid, remember that there are choices other than the obvious.

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References

  • Andres, B., & Myers, T. S. (2012). Lone star pterosaurs. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 103(3-4), 383-398.
  • Cai, Z., & Wei, F. (1994). On a new pterosaur (Zhejiangopterus linhaiensis gen. et sp. nov.) from Upper Cretaceous in Linhai, Zhejiang, China. Vertebrata Pal Asiatica, 32(3), 181-194.
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  • Kellner, A. W., & Langston Jr, W. (1996). Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from Late Cretaceous sediments of Big Bend National Park, Texas. Journal of Vertebrate Paleontology, 16(2), 222-231.
  • Lawson, D. A. (1975a). Could pterosaurs fly?, Science, 188: 676-678
  • Lawson, D. A. (1975b). Pterosaur from the Latest Cretaceous of West Texas. Discovery of the Largest Flying Creature. Science, 187: 947-948.
  • McGowen, M.R.; Padian, K.; de Sosa, M.A.; Harmon, R.J. (2002). "Description of Montanazhdarcho minor, an azhdarchid pterosaur from the Two Medicine Formation (Campanian) of Montana". PaleoBios 22 (1): 1–9.
  • Nessov, L. A. "Giant flying reptiles of the family Azhdarchidae. I. Morphology, systematics." Vestnik Leningradskogo Universiteta, Seriya 7, no. 2 (1991): 14-23.
  • Unwin, D. M., Lü, J., & Bakhurina, N. N. (2000). On the systematic and stratigraphic significance of pterosaurs from the Lower Cretaceous Yixian Formation (Jehol Group) of Liaoning, China. Fossil Record, 3(1), 181-206.
  • Vremir, M., Kellner, A. W., Naish, D., & Dyke, G. J. (2013). A new azhdarchid pterosaur from the Late Cretaceous of the Transylvanian Basin, Romania: implications for azhdarchid diversity and distribution. PLoS One, 8(1), e54268.
  • Vremir, M., Witton, M., Naish, D., Dyke, G., Brusatte, S. L., Norell, M., & Totoianu, R. (2015). A Medium-Sized Robust-Necked Azhdarchid Pterosaur (Pterodactyloidea: Azhdarchidae) from the Maastrichtian of Pui (Hateg Basin, Transylvania, Romania). American Museum Novitates, (3827), 1-16.
  • Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.
  • 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., Martill, D. M. and Loveridge, R. F. 2010. Clipping the wings of giant pterosaurs: comments on wingspan estimations and diversity. Acta Geoscientica Sinica, 31 (1), 79-81.