Quadrupedal Launching In Bats And Pterosaurs

I can't recall when I first heard about 'quad-launching' as a serious suggestion for pterosaurs getting airborne, (I was under a rock, palaeontologically-speaking, between '06 and '12) though Mark Witton's excellent 'Pterosaurs - Natural History, Evolution, Anatomy' was the first time I remember anyone going to any effort to depict it pictorially. Indeed, all of his book's pterosaurs are shown mid-launch for their profile images, as if Mark is making a concerted effort to familiarise readers with the concept. Most of the other books on my shelves tend to hedge their bets, offering up a selection of methods, including (but not limited to) dropping from elevated perches, facing into the wind and spreading their wings, and taking a run up whilst flapping.

My biggest problem with quad-launching was that I found it hard to visualise. I've never seen anything get airborne like that. Given that birds are obligate bipeds and their legs are not connected to their wings by a continuous flight surface, they are free to either jump into the air, as with pigeons, or propel the animal along the ground with an energetic run-up, like swans and geese. Many palaeontologists agree that pterosaurs were obligate quadrupeds and that their fore-limbs and hind-limbs were, in life, connected by the wing membrane. Birds are, therefore, a poor analogue for launching pterosaurs, and it is for these, and other anatomical reasons, that palaeontologists believe that pterosaurs' primary launch method probably involved a highly-energetic 'push up'.

A recent post at Pterosaur Heresies again demonstrates its author's frustrations with the problems he sees with the forelimb launch mechanism. The article points out that vampire bats achieve a considerable height from an initial leap before they perform a single flap, and that pterosaurs would be unlikely to achieve such a feat. In a bid to attempt to understand bats taking off from the ground (only a few species can do this) I looked at video footage of a fringed myotis taking off. Adams et al, in their 2012 paper, looked at how bats use their uropatagium to facilitate launch, and made available the following video:

There are four video links in the online paper, showing launches from various angles. In order to get a better idea of what's going on, I rendered the bat as a very-basic stick figure, traced from screenshots of the first online video. The wings' tracings show the stroke, and the head shows the positions of the animal relative to the ground.

Sequence showing a bat (Myotis thysanodes) taking off from the ground, mapped from screen-shots of film footage. This section of the sequence totals around two-and-a-half seconds. (Sequence drawn by author, traced from footage available with Admas, Snode & Shaw 2012.)

In the next image, the seven stages are overlaid in order to get a slightly clearer view - though I think both diagrams are useful when taken in together. The bat accelerates quickly, with its wings in contact with the ground in stage 1-3 (in 1 and 2, they are still flush to the floor). In stage 4 it begins the upstroke, is preparing for its first proper downstroke at 5, and has achieved that downstroke by stage 7. It's already flying and is only a few inches off the ground. My understanding, at least for M. thysanodes, is that when it jumps its inertia carries it a little higher than it would appear when standing with its arms stretched out beneath it, but it's enough to get the first flap in, and by then it's already airborne.

The same bat's take-off sequence, overlaid in order to better show the small area required for a successful launch. Black numbers denote head positions during launch; red numbers denote left wingtip positions. (Sequence drawn by author, traced from footage available with Admas, Snode & Shaw 2012.)

About a year ago I began work on a graphic novel showing the birth, life and death of Nyctosaurus. I may have underestimated how long this would take to put together, so it's still filed under 'ongoing'. But in order to understand quad-launching, I put together a couple of graphics showing an adult Nyctosaurus getting airborne, both of which inspired the bat graphics:

Overlaid launch sequence for a male Nyctosaurus gracilis. (Copyright © 2014 Gareth Monger)

And the looong version:

Launch sequence for Nyctosaurus. Nicked from my deviantART profile, hence the whole lo-res thing. Copyright © 2014 Gareth Monger)

So there you go. Now that I've done the bat thing, I might refine the Nyctosaurus graphics. I might even put together a cel animation at some point. There's nothing overly scientific in all that, however it might prove useful for those of you out there who are into your leather-flappers and pterosaurs.


References:

Adams RA, Snode ER, Shaw JB (2012) Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight. PLoS ONE 7(2): e32074. doi:10.1371/journal.pone.0032074

Elgin, R.A., Hone, D.W.E., and Frey, E. 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica 56 (1): 99–111.

The Fossil Record Throws A Curveball: Yi qi

Those crazy, crazy theropods. If there's one thing palaeontology has showed us about dinosaurs, it's that you shouldn't get used to their popular reconstructions because, sooner or later, something will turn up that'll really screw with your mind. And it's not like these events are necessarily rare; Deinocheirus and Spinosaurus got make-overs in the last couple of years, and they're both pretty high-profile.

Less high-profile are the Scansoriopterygidae, small, feathered, theropodan dinosaurs with the long arms you'd expect of an arboreal, aerial-capable dinosaur, but with an immensely-long third digit. A popular notion is that this digit was an adaptation to an arboreal lifestyle, enabling the creature to wrap its arm around tree trunks and branches, like naturalist David Bellamy, just, y' know, sharing the love.

B-b-but - what's this? A new paper by Xu, Zheng, et al, announces the discovery of a new scansoriopterygid, Yi qi, preserving not only the long fingers and feathers, but also a new, hitherto unseen structure. A long bony, or cartilaginous, rod projects backwards from each wrist, and patches of membrane suggest a set-up not totally unlike that of bats or pterosaurs. Or dragons, but I didn't say that. There's still some debate as to how the proximal margins of the wing chord may articulate, i.e., does it merge with the thoraic region or something else. And what is the true arrangement of the manual elements, in particular, the rear-pointing 'prong', referred to in the paper as the styliform element? They offer up a couple of possible arrangements, such as something superficially bat-like, and a set-up where the styliform elements are directed inwards, towards the body, helping to maintain a narrower chord. If this animal did indeed undertake powered flight, it's not too difficult to imagine it 'scooping' the air with its membranous hands, as bats do. Bats' hands' 'palms' form a sort of concave shape as they fly, which looks like a sort of arial butterfly stroke. Their fingers are fully jointed, enabling them to alter the shapes of their manus as required, resulting in a rather effective wing. The paper offers up three potential arrangements for Yi qi's 'wings', the two more plausible (to me) of which are shown here:

Two of three different arrangements proposed in Xu, Zhen, et al (2015), showing a proximally-pointing styliform element running parallel to the forearm (left), and the same feature, free of the forearm, pointing posteriorly and supporting a much-deeper membrane. (Illustrated by Gareth Monger; modified from Xu, Zhen, et al 2015.)

With regards the styliform element, I wonder if, rather than being curved in a horizontal plane (as restored, left) it instead curved ventrally (right), helping to maintain the aerofoil section - and a bat-like scoop. Some time after death, and prior to fossilisation, it has tipped over, rotating approximately 90 degrees, and settling in an unnatural position (left). Compression of the bones and associated remains during preservation could be masking the true shape of this apparently-unique element, but some lateral compression in life would make structural sense in terms of giving it strength during a downstroke. But that's all speculation.

In the paper, the wing reconstructions (shown in dorsal view) show the hind limbs of the animal trailing behind it. Although the main point of the graphic is to demonstrate the possible extent of the membrane, a trailing position for the hind limbs is unlikely; it pushes the centre of gravity back, and increases turbulence. For a volant theropod, it would seem unlikely that it would extend its legs behind it if they're not supporting part of a flight surface, and it also seems unlikely that a volant animal would rely on a narrow wing as suggested in the left-hand diagram. The right-hand diagram shows a deep chord, within which the (estimated) centre of gravity comfortably sits, when the legs are brought up, underneath the body, and out of the airflow.

Speculative illustration showing possible extent of contour feathers on Yi qi, and a possible centre of gravity. Note that the animal brings its legs in under itself, out of the airflow and therefore reduces turbulence. This also maintains a more-central centre of gravity. (Copyright © 2015 Gareth Monger)

Where the trailing edge of the membrane attaches (e.g., the body, or the hind limb) is not clear. Flying dinosaurs which use feathered wings benefit from legs which are independent of the wings. They can run into the airflow to achieve lift-off, or they can jump into the air, with the wings already committed to the flight strokes and not involved in the jump (compare pterosaur quad-launching). Having a skin membrane attached to the leg might be problematic since the legs (if not held out behind) would need to be elevated in order to maintain a level flight surface, and not one which partially faces into the airflow. However, that brings the leg and the membrane attached to it forward, reducing the tautness of membranous wing. Bearing that in mind, one might expect the membrane to attach on the body, somewhere in front of the hip, and not to the leg. The styliform element could work as a means by which the animal adjusts the tautness of the membrane, in a similar way to how a pterosaur is thought to do so with its apparent ankle attachment. Without that extra strut, the animal might enjoy less control and increased flutter in the membranes.

Yi qi in flight. (Copyright © 2015 Gareth Monger)

One of the key questions raised by this is why would a theropod go the route of developing a membranous flight surface when so much experimentation with flight (and there seems to be a lot of it!) is concerned with forming a continuous flight surface from elongated feathers? A major difference between scansoriopterygids and other, flighted, theropods is their elongated third digit. As suggested earlier on, it could be that this is an adaptation towards an arboreal lifestyle, enabling the animal to climb trees and other steep surfaces more easily. And it could be that selective pressures favoured the extension of the postpatagium instead of the feathers present on the arms. Whatever the case, feathers for flight persisted, and the theropodan flight membrane proved an evolutionary dead-end. Hopefully, additional specimens will come to light, adding to our understanding of this weirdo dinosaur.

Many thanks go to Mike Boyd for enabling me to write this particular article.

Speculative Palaeoart: Pterosaur Embryos

This micro-project was born out of one of those Facebook art challenges which I tried (and failed) to ignore, and for which I was nominated by Bob Art Models's Bob Follen (remember him from this post?) and Palaeoplushies's Rebecca Groom. The 'rules' of this particular Facebook challenge were pretty simple: produce three pieces of art for five days. That was it. It didn't specify whether they were to be new pieces, or whether they had to be posted over five consecutive days.

It started well enough as I paved the way to its inevitable non-completion with the usual good intentions. In fact, it's not abandoned, just hard to complete, what with everything else I've got going on. Given that I've got a fairly sizeable pterosaur project in the works, I figured it would make sense to stay close to this subject, and I was inspired to try my hand at portraying pterosaur embryos as close as possible to how extant animal embryos are.

It started with a quick mooch around Google, looking at photos of embryos of familiar animals, to get an idea of general bauplans. Most people are used to the proportions of young animals, with their disproportionately large eyes and heads, and comparatively short bodies and small limbs, but embryonic animals, depending upon their stage of development, can look altogether different to how they will appear at the time of their birth. Indeed, in the early stages, many disparate lineages' embryos may look broadly similar to one another. The diagram below shows representatives of the major vertebrate groups.

A selection of early-stage embryos, representing the major vertebrate groups. (Copyright © 2015 Gareth Monger)

You'll no doubt have seen versions of this line-up. I've opted to cut out most of the internal detailing so common to this style of diagram since I'm only dealing with external features in the pterosaur embryos I'm illustrating. It's easy to appreciate the broad similarities in this selection. The embryos, although perhaps not quite the same age, are at a very similar developmental stage, which shows how vertebrates follow a similar pattern of development in their earliest stages before they begin to specialise. As you can probably appreciate, there's not much point in producing a speculative, days-old pterosaur embryo to add into the above line-up as it won't look significantly different enough to add anything to the subject. The extant maniraptor in that diagram doesn't exactly scream 'DINOSAUR!', or at least not to this non-embryologist.

Today, the only flying vertebrates are birds and bats, and since only bats and pterosaurs share a membranous wing, I chose to look at bat embryos. Of course, bats and pterosaurs are not particularly closely related, having both developed flight apparatus independently, separated by an enormous chunk of time. (View this illustrated lineage at Phylopic to see just how distantly-related they are.) The framework on which they support their wing membranes is different, too, with pterosaurs using an enormously-elongated fourth finger and an internal network of aktinofibrils to stiffen the wing, while bats support a comparatively loose membrane on an enormous, five-fingered hand. Presumably this is the trade-off: pterosaurs have a skeletal scaffold reduced to a single spar, but there's more going on in the membrane and the skeletal mechanism by which they fold the wing away may not be as effective as that of at least some bats.

Embryos of the black mastiff bat (Molossus rufus), showing various stages of forelimb development. Compare 1's short manual digits with those of 2 and 3. (Copyright: Dorit Hockman. Used with permission)

This photo of three black mastiff bat embryos, courtesy of award-winning Dorit Hockman, a junior research fellow at Oxford, shows clearly the embryo pups' 'hands' and dactylopatagia (interdigital wing membranes) which are relatively small compared to those of newborn pups. Roll back several days in these animals' development, and at some point the animals' limbs will appear somewhat unspecialised, without any suggestion of the flighted animal to come. It was this intermediacy which I wanted to explore in my pterosaur embryo illustrations.

There's probably not too much that bat embryology tells us about pterosaurs; after all, bats are altricial and, although relatively well-developed at birth, are unable to fly until they are several weeks old, so there is a period of development and growth during this period. Some exceptionally well-preserved pterosaur fossils suggest that they are precocial, to the point that they can probably even fly within a very short time of hatching. It's reasonable to expect this to be reflected in the anatomy of their embryos and that their embryos go through a stage of rapid development before hatching, at which point they are essentially miniature adults. Unfortunately, early-stage embryo skeletons comprise a lot of cartilage which doesn't often fossilise well anyway, hence the speculative nature of illustrating pterosaur embryos.

A relatively-quick digital 'sketch' of a pterodactyloid embryo, with large head, closed eyes and stubby wings. 10a scalpel blade for scale. (Copyright © 2015 Gareth Monger)

It's difficult to make any scientific claims as to the accuracy of these illustrations, given the lack of direct evidence, which is why I tend towards filing these under 'speculative palaeoillustration'. That's also why I don't feel comfortable tying these down to too specific a taxon, leaving it as loose as 'pterodactyloid'. Other than full-term pterosaur foetuses illustrated to support fossil finds, there doesn't appear to be much in the way of palaeoart for prenatal pterosaurs. It's probable that, owing to the lack of any physical evidence, and the fact that the embryonic/foetal stage is comparatively brief, it's simply not important enough to necessitate producing illustrations of an animal at a point in its development where nothing would see it anyway, particularly when the adult reconstructions are being reviewed and refreshed as frequently as they are.

Another generalised pterodactyloid pterosaur embryo, with wing-digit and brachiopatagium beginning to develop. (Copyright © 2015 Gareth Monger)

So, in short, I don't think this is necessarily a critical aspect of palaeoart, but as science and scientific art, including photography, creeps further and further into our lives, whether that be online or through television or in print, the previously-hidden lives of vertebrates become more familiar to us. It makes some sense for palaeoartists to let themselves be influenced by this.

Biggest of thanks go to Dorit Hockman for letting me use her photograph of the bat embryos, which greatly influenced the final look of my pterosaur embryos. View her professional profile here.