Were the tail vanes of rhamphorhynchoids oriented vertically or horizontally on the living animal? Are they still believed to have served as a rudder when the pterosaur was in flight? Also, what supported the tail vane? And one more thing: what was the vane made of? Was it a simple skin membrane, or a more complex structure with layered tissues like the wings?

Oh boy, where to start. This is a great question Bryan (assuming you will not be confused or bored by the answer). For those who might be lost, some basal pterosaurs have a little flap of tissue on the end of the tail which would have helped them steer during flight, called a tail vane. The simple answer is that opinion is divided, most people favour the vertical orientation (i.e. half on top of the tail and half below) but there are others who prefer the lateral with the tail vane projecting left and right. Opinion is divided as there is quite good evidence for either.

For the vertical one, there are two good supporting reasons. The first is that some of the preserved vanes are assymetrical - one side is bigger than the other. If this was supposed to stick out laterally, this would not be a very effective aerodynamic structure! You wuld not want to fly in a plane where one wing was a different shape to the other, OK it would not be as serious, but it would be problemtic. It is possible that these are due to problems with the animal (i.e. a developmental deformity) as most *are* symmetrical, but it would be a surprise. Secondly, even where the pterosaurs are preserved flat on their backs, the tail is *still* twised to be shown in a lateral view (i.e. the bones are lateral, the vane is vertical) which implies that the vane indeed lies vetrically and has twisted the tail as it lay on its back (or front). Against this is the idea that a vertical tail vane would act as a rudder, helping to steer the pterosaur. At the end of a long and stiff (but very thin) tail, this kind of steering might well have been subjected to large forces which could have snapped it! Not much use as a rudder if it breaks.

In favour of the lateral position is the fact that aerodynamically an elevator would be of far more use to a primitive flying animal as controlling the height is more important when taking off and landing. Models suggest that this would indeed help a pterosaur maintain stable flight far better than a vertical vane. Against this is the fact that pterosaurs are not models - a model bird would be unstable in a wind tunnel - becuase of course in reality they would have been able to fine tune their wings, move their heads and feet etc. to help stay stable, a lateral vane might be useful, but hardly essential.

Overall then there are good reasons to be in favour of each interpreatation, and some decent reasons against, it really is pretty hard to say. Personally I favour the vertical orientation, but you cannot go too far worng with either.

As for support and structure, the vane appears to have been quite tough and was probably strengthened with some collgaen fibres which would have helped it stay stiff and bound it to the tail. Tail vanes are far less common than wings and none have been sectioned (to my knowledge) so the detail of their structure is unknown, but there is no evidence of the complex structures of the pterosaur wings (like actinofibrils, muscles tissue etc.) in them. They were probably just stong pieces of skin / collage tissue that were basically stuck on the end of the tail.

Hope that all helps!

I'm going to chip in with a couple of thoughts here, although I defer to Dave's far greater knowledge of pterosaur anatomy.

There are no extant flying animals with a vertically aligned tail (although grackles can fold their tails to approximate a vertical surface). This proves nothing, but it is interesting to consider that most flapping animals maintain a horizontal lift inducing tail for controlling their steering (in conjunction with their wing movement). Microbats are a little different because their tail membrane is also used in catching food.

A rudder is a high drag device with which to change direction - it is inefficient. By rolling an aerofoil about it's axis a combination of drag and lift allow direction change with lower levels of drag (and thus higher efficiency). This would suggest that the tail vane would be more efficient if was oriented horizontally.

Given that pterosaurs would have less scope for deforming their aerofoil (wings) than birds or bats,  it seems that forms requiring higher degrees of manouverability would benefit from an efficient steering mechanism. I would therefore predict that pterosaurs with tail vanes would be small, insectivorous forms.

Of course, my personal preference is for a vertical alignment. It looks better.

Dave - are there any trackways that might add some support here?

Noting in the track and footprints record. Only rhamphorhyncoid pterosaurs had tail vanes and they left no tracks, all pterosaur footprints and traces are from pterodactyloids. One thing I would argue with though, I think pterosaurs had as much, if not *more* ability to deform their wings that other vertebrate fliers. Their wing membranes are incredibly complex structures and we have good evidence that they were under close control and could fly as well, if not better than birds and bats.

Obviously we consider the more basal rhamphorhyncoids to be less adept in the air and perhaps this belies the need for the tail vane, though at least one family of rhamphorhyncoids (the anurognathids) lost the tail vane so it was not critical for all of them, and of course vanished in the pterodactyloids.

Which of course brings us to why you propose that pterosaurs had a greater ability to deform the wing membrane than bats and birds.

Here's my take on wing structure.

Birds: Wing composed of multiple aerofoils (feathers) that can be used to form either a continuous surface or can be "feathered" to alter the flow of air and production of lift and drag forces. The flight surface can be altered by combinations of movements from the shoulder, elbow and wrist. The alula is a digit that forms an aditional small wing for use in low air speed manouvers to prevent stalling due to flow seperation over the steeply angled main wing. Secondary flight feathers also act to control stalling by flow seperation from the rear of the wing, lifting under low pressure progression from the rear wing margin to contain the seperation vortex. The vast diversity of wing shape in birds reflects the flexibility and efficiency of the feathered wing. Most importantly perhaps is the ability of the wing to alter shape by allowing the feathers to slide past each other. This provides a remarkably stable lifting surface, regardless of wing deformation.

Bats: Wing composed of an elastic and muscular membrane supported by the arm and hand and attached to the body and ankle. Flight surface can be deformed by combinations of movements from the shoulder, elbow, wrist, digits and the legs. Bats have a propatagium that deflects flow over the wing at high angles of attack, helping to prevent stalling at low air speed - the ridges of the digits would also assist in maintaining suitable airflow at low speed. The lifting surface of bats can become compromised at low extension (the surface can bunch, reducing efficiency of airflow and shape of aerofoil). The diversity of wing morphology is quite conservative in bats when compared to birds.

Pterosaurs: Wing composed of an elastic membrane supported by the arm and fourth finger and attached to the body and ankle - it also had actinofibrils that had a structural role in maintaining wing camber and planform. Flight surface could be deformed by combinations of movements from the shoulder, elbow, wrist, fourth digit and the maybe the legs. Pterosaurs had a propatagium, supported bythe pteroid, that deflects flow over the wing at high angles of attack, helping to prevent stalling at low air speeds. The lifting surface of pterosaurs is likely to have become compromised at low extension (due to surface bunching - reducing efficiency of airflow and the shape of aerofoil). The diversity of wing morphology is quite conservative in pterosaurs when compared to birds.

Overall I would agree that pterosaurs would be truly excellent fliers, however, they do not have quite as many features available to them for efficient deformation of the wing as birds or bats (although I happily admit that they are not far off). When talking about lift generation pterosaurs are way up there. The problems arise with manouvering. The large pterodactyloids are not likely to require great manoeuvrability, but the smaller rhamphs probably did. Their tails would provide stability in forward flight, but the added mass and aerodynamic qualities of the vane would provide a mechanism for introducing the controlled instability needed for manoeuvrable flight.

A horizontal vane could contribute to the lift required to keep this relatively heavy (for its position on a flying animal) device supported, but a vertical vane would only contribute to drag. I find myself favouring a horizontal orientation based on energetic reasons, although I would still rather think of the vane being vertical!

Perhaps pterodactyloids used crests on their heads to perform a similar role as the rhamphorhynchoid tail for improving manoeuvrability? These may have subsequently been taken over by runaway sexual selection forces originating in display flights demonstarting manoeuvrability and resulting in visual signals about fitness? Interesting speculation...

Last edited by Paolo Viscardi (4th Apr 2008 12:01:37)

Actually the wing of the pterosaur is rather more complex that you suggest (for an in depth essay on this by me, see here: http://dinobase.gly.bris.ac.uk/forum/vi … hp?id=531) and in fact they could probably alter the subtle shape of even very minor areas of the main wing membrane (to increase or descrease tension and thus alter the camber and lift of parts of the wing). This is probably more effective than the 'sectioned' wing of bats (slit into segements by the wing fingers) thouhg of course, perhaps not as sood as the slotted wings of at least some birds.

The pterosaur wing could have been altered by moving the legs as well as various parts of the arm. The propatagium could be moved a little in the anterior-posterior plane, and could also have been lowered to change the wing shape. Both the fingers and toes were webbed and preliminary studies (of a research group I am part of) suggest they were in a position to influence the airflow. Oddly enough the heard crests had no real aerodynamic effect at at all apart form a minor increase in drag (a paper of mine now in press ;-). Finally, studies of the flocculus (the part of the brain that controls movement and the body's sense of 'self' in terms of position) have shown that in pterosaurs (both rhamphorhyncoids and pterodactyloids) it is as big as, if not bigger than, that of birds, and probably bigger than in bats.

The implications are thefore that they had excellent close control over their flight surfaces, with vary fine tuning availbale throuhg a multitude of mechanisms and a large part of the brain devoted to the feedback and control of these flight surfaces. OK, so we can't say 'they were better than birds' with any great degree of certainity, but we can say that they were good fliers and in key areas, despite the different wing constructions, were probably on a par with most birds and bats and probably better than at least some of each.

The way I see it, pterosaurs and bats have remarkably similar wings in their composition (elastic skin with a network of blood vessels that supply muscles in the membrane). The main difference is that pterosaurs need an additional structure to offer support across the membrane (actinofibrils) that bats don't need because they have their digits incorporated in the membrane. These two wings sound very similar from a comparative perspective - except for the ability to *easily* make alterations of wingplan. If anything bats seem to have an advantage because the digits are articulated and their movements are easily controlled by existing neural pathways. Perhaps pterosaurs needed a large flocculus precisely *because* their wing morphology was difficult to maintain close control over if it was to be used for complex flight.

The need for complexity in a control system may reflect deficiencies in the system that is being controlled. I'm sure we're all familiar with the "querty" problem in evolution - i.e. the inherited solutions to a problem may no longer be the best solutions, but they are too heavily relied upon for a new solution to be practicable, so they are maintained - usually with a whole bunch of secondary adaptations that "fix" the problems of the inherited system (a bit like Microsoft operating systems). This is always something that should be considered.

Again, I am not "dissing" the pterosaurs - I am sure they were very good in the air, but I am still not convinced that their flight structures are comparatively more specialised than those of bats or birds - they just used a different mechanism that had to be adapted in different ways to do the same thing. Who knows, early on the tail vane may have been another such adaptation that was later replaced by a lighter weight mechanism?

I would love to see a copy of the head crest paper when it comes out - could you send me a reprint please? I find it hard to imagine how turning the head with a crest on it would not deflect air in such a way as to cause yaw.

Last edited by Paolo Viscardi (5th Apr 2008 10:05:16)

I still disagree. While bats can collapse the wing through bringing the fingers toghther, they are still stuck with the nacelles of the fingers which would disrupt the airflow, and while we can hardly compare them directly, pterosaur wings were clearly very hughly elastic, and based on my experience with bats, far more so that a bat wing. Bats might not need actinofibrils, but then pterosaurs don't need several other 'heavy' fingers wedged into their wings! :-)

Thus they (potentially) had more more elasticity and control over their wing shape. Pterosaurs could lower and raise, and extend and retract the legs to change wingshape as well as having the fine muscular control and additional flight surfaces. I think it's a bit harsh to suggest a big flocuulus is a likely to be linked to poor flight (even as a possibility) - it is not something we see in gliding animals for example.

I'm not saying that the large floccus is linked to poor flight - I am suggesting that good flight in pterosaurs required greater processing power than it does in bats, because the pterosaur flight system was unable to utilise preadapted deformation mechanisms. Therefore, new solutions had to be adapted, increasing the required "computational" power of the control system.

I find it hard to see how elasticity in a pterosaur wing can directly be compared to that of a bat, since there are no in vivo pterosaur wing membranes to be studied.

I do agree that bat fingers are heavy, but the bones may actually assist in maintaining a suitable airflow, rather than being disruptive.

Again, I'm not saying that pterosaurs were bad fliers, I am simply not convinced that they would be any better than the average bat.

Re: floccus, OK, gotcha. I read it the wrong way.

We can't comapre them directly, but the membranes of pterosaurs show some extremely gigh levels of contraction and expansion in the fossil record, and given their preservation in water, this can't be a result of dessication, suggesting they are *very* elastic. In relative terms, bat wings do not appear to be as elastic.

As for the airflows / performace, of course we can't measure pterosaurs directly, and there is much we have to reconstruct with quite a large margin of error, but we can make soem comparisons. As I say, we are jut starting this work in Karlsruhe including modelling elsatic wings, tail vanes and more. Hoepfully we will have a better idea of pterosaur flight performance soon, but probably not for a few years yet. The wheels of science grind slowly! ;-)