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.

Standing Tall: Stegosaurus

This blog was never conceived with the intention of filling it with speculative palaeoart, but it's as good a place as any to put it. Stegosaurus has had a fair bit of coverage in recent months, with the NHM's mount being used to estimate the animal's mass, and Saitta 2015 looking at apparent differences in individuals' plates to determine the animals' genders. Padian and Carpenter disagreed, and Theropoda looked at the health implications of stegosaurs dragging their tails (such as constipation).

Superb illustrations by John Conway and Mark Witton got me thinking about those plates. Palaeontologists have put forward various ideas regarding their purpose, the most popular of which being thermoregulatory aids, display structures and defensive structures. In nature, structures often have multiple functions, with secondary functions being unrelated to their primary function. Feathers, for example, probably developed initially for insulation, but could have been easly modified for use in display, either through behavioral means or by changes in pigmentation. Structural modification of the feather - and other key anatomical features - then endowed the owner with an aerodynamic advantage.

That's a long-winded way of suggesting that Stegosaurus's plates probably did not perform one single function. Some of that's already been touched on in this earlier post, but I'm keen on the idea that part of Stegosaurus's display is concerned with how tall an individual looks, i.e., how much vertical space it occupies, especially in the eyes of potential mates, conspecific rivals and would-be predators. With fuzziness now known to be present in (some) ornithischians, I'm happy to speculate that some stegosaurs may have used stiff fur or 'fuzz' as it's often called, to extend the margins of the dorsal plates. Many palaeoartists, palaeoillustrators and palaeontographers already restore those dorsal plates with a sizable soft-tissue (see comments) keratinous extension. An additional growth of stiff hairs as a light-weight projection could, in theory, increase the size of the plates' appearance. Compared to a bone-and-flesh plate, the hair component would be less demanding on the animal, given that once the hair as at the surface, it's a dead structure, and no longer requires a blood supply in order to maintain it.  Of course, if it's concerned with sexual display, it may be renewed seasonally, and shed after mating. This would get around the problem of it getting trashed through day-to-day activities, and filling up with dirt, mould and parasites - which nobody wants.

Stegosaurus stenops, displaying some serious fuzz. Not unlike a filthy old coconut husk. (Copyright © 2015 Gareth Monger)

Anyway, it's just a thought. And this post is supposed to be short and sweet, like the Holocene.

Next up: Yi qi (again).

My Home, The Sauropod - Part I: Getting Filthy

The idea of an organism playing host to a multitude of other organisms is well known, and is something which most of us are familiar with - even if it's only through catching headlice at school or pushing a worming tablet down a cat's throat. On a smaller scale, there are gut bacteria, some of which perform an important role in keeping the digestive tract healthy. Others are less helpful, as those who frequent dodgy kebab shops will testify - if they're still alive. And it's not all restricted to Animalia, with trees being well-studied examples of a complex ecosystem centred around an individual.

It's not just about parasitism. Sharks, large fish, turtles and whales are often seen with a posse of remora, unusual fish with a sucker-like organ on top of the head, allowing them to hitch a ride. They take advantage of a steady food supply, in the form of particles of food dropped by the host animal, sloughed skin and faeces, but they also help keep the host healthy by consuming parasites which may attempt to adhere to its surface. In return, the host doesn't (always) eat them. On land, large mammals may be parasitised by blood-sucking invertebrates, but these, in turn, may be preyed upon by birds, which the larger mammals are big enough to bear the weight of - and the additional irritation. So, there's a constant battle being enacted on and in many animals, by other organisms, and often this affects the way these animals look and behave. A consideration of how parasites, symbiotes and associated organisms interacted in ancient settings may steer palaeoart in the direction of increased accuracy. It may also result in depictions which look rather different to what we're used to. After all, nature isn't always a tidy place.

Now, portraying these kinds of interactions probably drops this type of speculative palaeoart somewhere in the All Yesterdays camp. Of course, it's only 'speculative' with regard to how one might decide to show those interactions; they undoubtedly happened, but fossilised remains of those interactions are exceedingly rare. And not all of those interactions will be targeted encounters between a parasite or symbiote, and host, and this is what I have attempted to show in the following sketches.

Sauropods represent an interesting branch of Dinosauria, and include the largest terrestrial animals ever to walk the earth. The idea of these keystone species forming some sort of walking ecosystem is an attractive one, as they no doubt carried a contingent of parasites which may have attracted animals which predate those parasites. In addition, they may have provided a perch for flying reptiles and insects, and a surface for non-parasitic organisms to grow on. It's probably also not unreasonable to imagine some sauropods accumulating leaf litter and a twigs, especially if one restores certain species with a row of spines and other ornamentation, as some palaeontologists and artists do. It's hard to imagine sauropods successfully ridding themselves of all the material which rains down on them from the forest canopies under which they must have spent some time. That's not to say an apatosaur would never have dislodged accumulated forest junk from its back, but it would probably have found cleaning itself with any kind of precision difficult.

An apatosaurine sauropod, sketched up quickly and based on Scott Hartman's 'Apatosaurus' excelsus skeletal illustration. The dorsal spines form a trap for falling leaf and branch material, whilst the mosaic of scales and osteoderms provides a surface on which lichen fragments and diaspores adhere.  (Copyright © 2015 Gareth Monger)

That probably looks like a lot of twigs and branches, but these are long-lived animals which may have spent a long time in wooded environments. A smooth-backed sauropod probably won't accumulate twigs in any appreciable quantity, but you might expect to see a bit of lichen growth, especially on those surfaces which see little abrasion from rubbing against trees and other surfaces, or areas of skin which experience minimal flexing. A sauropod might look rather colourful - and different - if adorned with a collection of brightly-coloured lichens, dried on leaves, and small branches. And different populations of a single species might look different, depending on the differences in the vegetation of their respective home-ranges.

An apatosaurine sauropod, as viewed from above, wandering through a pine forest. Dead twigs and branches constantly rain down onto the forest floor, with some inevitably landing on passing sauropods. Reference photo: SV-POW! (Copyright © 2015 Gareth Monger)

For the fans of wild speculation, it might be fun to imagine that certain sauropods used a pile of vegetation in sexual displays, with those male sauropods carrying the largest pile of woody compost most likely to attract a female. Behaviour is one aspect of palaeoart which is wide open to ideas. You've only to look at extant animals' courtship displays to realise that from the future skeletal remains of, say, the bird of paradise, you'd never come close to guessing how they go about impressing a potential mate. There's every reason to think that some non-avian dinosaurs could have been at least as weird. And for palaeoartists, that's where a little imagination can prove useful.

Next up: some more thoughts on Yi qi...   ...and maybe something quick on Stegosaurus.

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.