In our final post in this series my own PhD student Ross Elgin takes us through the work we did (with others I must hastily add) on the aerodynamics of pterosaur crests. This is one of those much discussed but little researched areas with (perhaps ironically) lots of hot air, but little moving air (like in a wind tunnel for example). So read on as we take to the skies with those crested pterosaurs.
Aside from the opportunity to congratulate Peter Wellnhofer, celebrate his excellent work on pterosaurs and enjoy a number of lectures on all manner of pterosaur goodies, the meeting in Munich highlighted another important thing…. just how many people we have now have with an interest in pterosaur research! And why not after all!? As I am sure anyone who has followed the posts on “the Musings” with any sort of continuity will be aware the pterosaurs are just one of those groups which despite over 200 odd years of constant scientific research we still don’t know all that much about them: where did they come from?; how did they grow and what did they eat?; were they efficient walkers on the ground or did they spend much of their life on the wing?; The list could get pretty extensive if I chose to include everything we still aren’t sure about, and I haven’t even mentioned taxonomy yet. Still the one thing I am certain of is that number of people looking to take up the challenge of “pterosaurology” is perhaps greater then ever and maybe just maybe, with the right degree of dedication and luck, we might be able to pry some more secrets from this ever more intriguing group. However I digress.
Ignoring all the intricate bits and bobs that we palaeontologists usually concern ourselves it wouldn’t be too far removed from the truth if I was to describe a pterosaur as little more than a flying head, and pretty spot on if we only consider the more derived pterodactyloids. Thus pick any derived pterosaur and you will be confronted with (hopefully) a large pair of wings and head often larger than the actual body itself, which is so reduced at times that I have often heard our engineers musing as to where they could possibly keep the fish after catching it. Of course in considering the size of the head we also must account for the large and extravagantly coloured, if Luis’s lovely renditions (on the banner above) are to be believed, crest that crowns the majority of pterosaur taxa. And here begins the small paradox that I found myself investigating many moons ago when I was but a lowly MSc. student back in Bristol. “Why would an animal go to such great lengths reducing its body size and hollowing out its bones, only to mount whopping sheet of bone/soft tissue on top of its head?”. My colleague at the time had a saying that “if anything in nature doesn’t make sense, blame it on the females,” talking, of course, about the function of sexual selection (before he or I are misinterpreted) and while he was certainly not the first person to have suggested this idea it has come to be more or less accepted by modern pterosaurologists.
This idea behind the sexual selection of the pterosaur crest is pretty simple, those males pterosaurs with the biggest, most brightly coloured or elaborate crests get all the girl pterosaurs and so pass their genes onto the next generation. Those with a less spectacular crest however are found wanting and more often than not simply have to do without, thus their genes disappear from the population. With such a strong positive selection acting on the crest (i.e. you aren’t going to mate without one) it is little wonder that as time marched on and old species split into newer species, larger and ever more elaborate crests were passed along as well. And thus we find the situation observed towards the end of the pterosaur lineage where the pressure to show off to the opposite sex was so strong that the crests themselves could easily end up dwarfing the original size of the skull, think Nyctosaurus and Tupandactylus navigans/imperator here. So, problem solved? Probably – after all many other types of animals have developed all manner of weird and wonderful ways to get noticed by the opposite sex (Bristol social life aside). Also several lines of evidence indeed suggest that the crest were sexually dimorphic and absent, or at least reduced, in female pterosaurs although the fossil record isn’t quite complete enough for this to be certain in many taxa. The obvious point being therefore that if it was performing some function that was not related to gender differences, we would see it in both genders.
However flying animals, and particularly large ones, have to be a bit more careful where they attach any extra bits and bobs and it is certainly no coincidence that all pterosaurian crests are extremely thin and streamlined from the front. Thus while sexual selection may, and probably does, explain the why of the crest it doesn’t consider the aerodynamic effects of attaching a whopping big sheet of bone/tissue on top of an already large head and this was what we wanted to test, and others in the past have suggested biomechanical benefits of the crest which we could look to support or refute with our analyses.
While such a feature might be imagined to impede a pterosaur during flight, for much of the last 100 years it has been an ongoing suggestion that the crest actually assisted it in one of several ways by a simple turn of the head and thus providing a possible alternative explanation to the origin of the crest. To test these predictions palaeontology (for a while anyway) regressed to an almost play school-like level and through the tireless effort of moulding play dough-like putty into banana shaped representations of pterosaurs and casting with the help of a rather large tub of latex rubber, several scaled down representations of Pteranodon were born. Things subsequently got a bit more scientific and the models strapped into a fairly sophisticated wind tunnel courtesy of City University, London and subjected to a number varying wind speeds to determine their aerodynamic profiles and test all the major aerodynamic possibilities that a crest might produce. So, ignoring the role of sexual selection in the development of the pterosaur crest what are these alternate theories and how exactly would a pterosaur benefit from having a crest mounted on the top of its head?
1) The crest as an airbrake.
Simply put if a large headed, large crested pterosaur turned its head against the air flow it would have been expected to produce a large amount of drag (the force that resists the movement of an object through a fluid) which, in turn, should have helped to slow the animal during landing or other manoeuvres. This however is one of these ideas that far sense in theory rather in practice. To produce the most drag possible the head would have to be turned broadside to the airflow so the animal wouldn’t be looking where it was flying and have big forces pushing on the fairly stiff neck. Also for maximum effect the crest should ideally have an aspect ratio (i.e. square) of 1 rather than the typical high aspect ratio crest (long and thin) associated with P. longiceps.
The other question that must be asked is could the crest compete with the drag production of the wings……probably not! Cambering the main wing by depressing the propatagium (fore wing) or lowering the hind limbs, which in turn cambers both the wing and brings the uropatagium (hind wing) into the air flow would have been far superior in this respect. Though the results of the wind tunnel experiments unsurprisingly confirm that a crested Pteranodon produces around 25-50% more drag than an uncrested form (confirming that crested models produce larger forces) they also unusually indicates that if the animals pitched their heads downwards (as suggested for some derived pterosaurs based on other research into the orientation of the ear) the profiles would collapse onto one another and all the models ended up producing similar coefficients of drag. (The coefficient of a force just being a convenient way of comparing the force production of models by removing any differences in size).
While a crest must have undoubtedly helped increase drag during a head turn, providing the head itself was not depressed, it is often of the wrong shape to maximise drag and the resulting force is actually very small, only a few newtons (N) – made even worse by the assumption that a pterosaur must have been travelling slower than 10m/s when such a tactic might have been employed to any use. As noted above, if the feature was also genuinely effective and useful one must wonder why only putative males had large crests and not the females too (a point that applies to the other situations below). Use of the crest as an airbrake, slim to none.
2) The crest as a forward rudder.
Put an object into a flow, turn a feature acting as a rudder and the object will naturally develop a torque (force producing a torsion or rotation) yawing (turning) it in the desired direction thus, as the theory goes, why could a large crest not be used in a similar manner to assist the animal during sharp turns? Bats and birds however it seems have not evolved such a feature for assisting in turns, relying on their wings instead (and note that large headed birds like hornbills and toucans don’t seem to use their heads in this way, though of course structurally these are very different features), and while it is a valid argument to state that there is no such need for a crest to play a role in such aerodynamic manoeuvres it is useful to see exactly how much more efficient the wings are. Bypassing the actual maths a pterosaur (or flying animal/vehicle of your choice) of weight 4.3kg that banks at an angle of 45° will produce a turning force of 30 N. That is more than three times the maximum generated by the head and crest at an angle of yaw of 40° and six times that generated at 15° yaw. Thus we can confidently state that the crest would have been of little additional use in helping the animal to turn. More likely the crest would have been a nuisance to the animal being simply one extra torque that would have had to have been balanced by the wings, resulting in an increase in drag (and again an unwanted pull on the neck).
The effect the crest, however, both for better or worse, it seems was ultimately limited by a fairly low flight speed. Falcons have been noted to have some problems with developing undesired torques as they usually approach their prey with the head turned out to the side, partially explaining why they choose to approach in a circling pattern rather than diving directly along the shortest possible path. With a flight speed of around 10-12m/s however even large pterosaurs were unlikely to have generated any seriously detrimental torques with a turn of the head. Use of the crest as a forward rudder, unnecessary and in fact probably slightly detrimental to stability.
3) The crest as a counter balance.
When not busy developing extra weight in the form of crests pterosaur took a number of weight saving measures and there was obviously a strong positive selection for reducing weight as much as possible. As muscle is a good deal heavier than bone perhaps pterosaurs developed the crest to balance the head and protect the neck against unwanted torques, thus becoming of evolutionary value by allowing for a reduction in neck muscles. So was this the case? Surprisingly enough, no. While all manners of variations in crest shape and form have been observed, most located forward of the neck joint, even the caudally directed crest of P. longiceps proved pretty lousy at counterbalancing the head. Other tests with a large variety of Brazilian pterosaurs confirm that pterosaur crests across the lineage just can’t counterbalance the head. So was the crest useful as a counterbalance, not in any of the studied taxa (probably including Tupandactylus imperator).
Thus in a nutshell while the cranial crest of Pteranodon succeeds in lowering the yawing moment of the head and limiting the movement of the centre of pressure, the overall aerodynamic effect is rather modest. Were the animal to try to use its head to assist in steering, braking, or manoeuvring, it would have instead created aerodynamic instabilities (which would have to be countered by the wings) and imposed additional twisting loads on the neck attachment (which is also pretty bad). So in the end while sexual selection seems to have won out in being the primary cause of the pterosaur crest it also appears that aerodynamic influences on the development of the crest were very few indeed. Other than having to maintain the compulsory streamlined front profile for drag reduction, the slow flight speeds of the pterosaurs allowed the crest almost unlimited freedom to develop in whatever way was possible from lateral view. It should probably therefore come as no surprise if more pterosaurs turn up in the future with ever larger, stranger and more elaborate crests, the latest batch of fossils out of Brazil unveiling the true extent of the crests of Ludodactylus and T. imperator for example as well as others like Raeticodactylus or Dsungaripterus.
With the aerodynamics of the head neatly tied up it is hopefully on to the much more taxing and interesting matter of determining the flight characteristics of the wings, bodies and pro/uropatagia. Hopefully these might eventually warrant another post in the not to distant future and we may well on our way to understanding the aerodynamics of a fully integrated pterosaur.
Elgin, R.A., Grau, C., Palmer, C., Hone, D.W.E., Greenwell, D., & Benton, M.J. 2008. Aerodynmic characters of the cranial crest in Pteranodon. In: Hone, D.W.E & Buffetaut, E (eds.) Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana B, 28, 169-176.