Already the article you're reading is several weeks late, and I had to make a real effort to finish it before those weeks became months. Anyway, I present here the penultimate in the 'month in dinosaurs (and pterosaurs)' series (for the previous articles please see part I, part II, and part III). The whole point of this little series was to discuss what happened during January 2009 in dinosaur and pterosaur research. So, we've done the dinosaurs (or, at least, the theropods and ornithischians). This time: pterosaurs. Before launching into the new stuff, I must note that a single, exceptional event in the pterosaur literature overshadows everything else*: namely, the appearance of the special pterosaur volume of Zitteliana. This collection of papers results from the Peter Wellnhofer meeting held at Munich in 2007: there's a lot of neat stuff in there, so much so that I'm not about to start talking about it now.
* Well, nearly everything else.
Pterosaur taxa come in thick and fast these days, and a great many (most) of them are from the Lower Cretaceous of China (usually Liaoning Province, the place where most of the feathered theropods come from)*. And yet another new Chinese pterosaur was named in January 2009: it's Ningchengopterus liuae LÃ¼, 2009 [shown above] from the Yixian Formation of Dashuangmiao, Nincheng County, Inner Mongolia. Ningchengopterus is reasonably well preserved and includes a complete skull and neck, an articulated wing, dorsal vertebrae and ribs, and a partial pelvis and hindlimbs. A portion of wing membrane is preserved and internal wing fibres are visible. Fibres are also preserved around the skull and neck; these presumably represent the integumentary hair-like structures now well known for pterosaurs. It's clearly a pterodactyloid, and it might be a ctenochasmatoid (but read on).
* Barrett et al. (2008) listed 19 Liaoning pterosaur taxa (though a few of the species they listed are probably synonymous, some new ones have been published since (e.g., Shenzhoupterus, Elanodactylus), and a few others await publication).
Anyway, the big deal about Ningchengopterus is how tiny it is: its long-jawed, toothed skull is about 40 mm long and the bones in its wing add up to about 100 mm (minor point worth noting here: when people estimate pterosaur wingspans, they usually do so by adding the lengths of all the respective bones together. In life, however, the wing bones were not held in a straight line, so the 'living' wingspan would have been shorter. Chris Bennett has drawn attention to this but it's rarely been mentioned elsewhere).
However, LÃ¼ (2009) admits throughout the paper that the only known specimen is a baby. This is supported not only by its small size, but also by the poor ossification present throughout the skeleton and by a lack of fusion between the skull bones (Dave Unwin has referred to pterosaur babies as 'flaplings', hence the word I used in the title for this article). Given the generally quite good evidence showing that morphology changes during ontogeny, is it possible then that this tiny baby is a tiny baby of a previously named taxon? Among Lower Cretaceous Chinese pterodactyloids, there are a few potential taxa to look at, including Eosipterus, Beipiaopterus and Gegepterus [shown here]. LÃ¼ notes that Ningchengopterus is different in various of its details from most of those taxa, but what's suspicious is that its wing bone ratios are quite similar to those of Eosipterus. This is enough to at least suggest that Ningchengopterus might be an Eosipterus flapling, though LÃ¼ excludes this possibility by saying that Ningchengopterus lacks the elongate mid-cervical vertebrae of ctenochasmatids (the group to which Eosipterus belongs). Furthermore, the Eosipterus holotype lacks a skull, so we can't say whether or not it was similar to Ningchengopterus in skull anatomy.
Of course, Ningchengopterus is far from the first, tiny baby pterosaur to be reported: there are tiny juvenile specimens of Pterodactylus from the Solnhofen Limestone, for example, that have wingspans of just 19 cm. We've previously looked at flaplings here, back when Nemicolopterus - also from Liaoning - was described in 2008.
Sailing the Cretaceous skies - - or not
Another, err, interesting pterosaur paper appeared in January. Among the most flamboyant and bizarre of pterosaurs are the 'antlered' nyctosaurs described in 2003 by Chris Bennett [his page on the fossils is here]. Referable to the nyctosaurid taxon Nyctosaurus, they differ from other nyctosaur specimens in possessing gigantic antler-like crests. The main spar of the crest projected mostly dorsally from the back of the skull, while an additional spar projected posteriorly from the main one (Bennett 2003).
Within the first few weeks of publication, people were already reconstructing nyctosaurs with wind-sails on their heads, and speculation abounded that these crests served some sort of aerodynamic function [there seems to be no end of sail-crested nyctosaur pics on the internet. The one shown here is from wikipedia]. While we know that the bony crests of various pterosaurs supported much larger soft-tissue crests, there is no evidence for this in nyctosaurs, nor indeed among ornithocheiroids in general. In fact Bennett (2003, p. 73) specifically considered the possibility of soft-tissue extensions on the nyctosaur crests and rejected it because the distinct, rounded edges of the crests are very different from the jagged, fibrous edges seen on those crests that did anchor soft tissue extensions. Bennett favoured the idea that the crests were visual display structures, and I agree (Naish & Martill 2003). In fact I personally have a strong dislike of the idea that weird crests and other such structures 'must' have evolved for functional reasons: in Mesozoic archosaurs, most of the evidence we have indicates that such things primarily functioned in display. This goes for the plates of stegosaurs, the frills and horns of ceratopsians, and the crests and horns of some theropods.
However... having said all that, I suppose it would be wrong not to at least test the possibility of an aerodynamic function for the crest. Actually, Elgin et al. (2008) did exactly this for Pteranodon: they put various Pteranodon skull models in a wind tunnel and recorded the side forces, yaw, and drag acting on the skull. They concluded that "the overall aerodynamc effect is modest" and that "the crest most probably evolved independently of any aerodynamic function" (p. 167). Like Bennett (2003) and Naish & Martill (2003), Elgin et al. (2008) favoured the hypothesis that the crest's primary role was one of display, noting in the conclusions that "Animals today often sport astonishing structures for display purposes (e.g. peacock, lyrebird, red deer): providing the structure does not represent an overwhelming encumberance, the riotous functioning of sexual selection may far outweight the common sense of natural selection" (p. 174). Actually, however, some display structrures do "represent an overwhelming encumberance", and exact a heavy cost on their owners. Isn't that the whole point? This is known as 'peacock's tail syndrome'.
Incidentally, I know from a poster display I once saw that Ross Elgin and colleagues also tested the aerodynamic performance of various other crested pterosaur species too. Does anyone know if and when these results will be published? I need to know as I'd like to see a published smack-down of Frey et al.'s (2003) proposal that sail-crested tapejarids were wind-surfers.
Anyway, I digress. Xing et al. (2009) looked anew at the possibility of sail-crested nyctosaurs. In contradition to what Bennett wrote, they reconstructed Nyctosaurus with soft-tissue extensions on the crest, and after doing some mathemetical analysis (I will skillfully gloss over that section of the paper), they concluded that a sail-crested nyctosaur may indeed have been able to control thrust and movement by changing the pitch of its crest. I cannot assess the mathematics involved, but there are some problems with the analysis worth noting [adjacent CG model from Xing et al. (2009)].
As mentioned above, the idea of a huge head-sail is flat out not supported for these animals. Secondly, the authors probably have the shape of the nyctosaur crest all wrong. They show it as being shaped like a wind-surfer's or yacht's sail: very tall, short anteroposteriorly, and with a high aspect ratio. However, as is clear from Bennett's descriptions, the posterior spar of the nyctosaur crest is not short as assumed by Xing et al. (2009), but very long: Bennett (2003, p. 70), stated that, in one of the specimens, the posterior spar is at least 32 cm long while the superior part of the crest is at least 42 cm long. Given the correct configuration [see Mark Witton's restoration below, from here on his site], a hypothetical nyctosaur head-sail must have been long anteroposteriorly, and with an absurdly low aspect ratio. This would have radically altered the properties of any hypothetical head-sail.
One final comment on Xing et al. (2009): they also tested the aerodynamic properties of a nyctosaur crest that lacked the hypothetical sail-like extension, and in this case they found that the crest had no special aerodynamic properties. Assuming that their analysis is sound (and, as mentioned, I am not able to conclude whether or not it is), this is interesting as it mirrors the conclusion reached by Elgin et al. (2008).
More pterosaurs next - - to be published tomorrow, probably.
Refs - -
Barrett, P. M., Butler, R. J., Edwards, N. P. & Milner, A. R. 2008. Pterosaur distribution in time and space: an atlas. Zitteliana B 28, 61-107.
Bennett, S. C. 2003. New crested specimens of the Late Cretaceous pterosaur Nyctosaurus. PalÃ¤ontologische Zeitschrift 77, 61-75.
Elgin, R. A., Grau, C. A., Palmer, C., Hone, D. W. E., Greenwell, D. & Benton, M. J. 2008. Aerodynamic characters of the cranial crest in Pteranodon. Zitteliana B 28, 167-174.
Frey, E., Martill, D. M. & Buchy, M.-C. 2003. A new species of tapejarid pterosaur with soft-tissue head crest. In Buffetaut, E. & Mazin, J.-M. (eds) Evolution and Palaeobiology of Pterosaurs. Geological Society Special Publication 217. The Geological Society of London, pp. 65-72.
LÃ¼, J. 2009. A baby pterodactyloid pterosaur from the Yixian Formation of Ningcheng, Inner Mongolia, China. Acta Geological Sinica 83, 1-8.
Naish, D. & Martill, D. M. 2003. Pterosaurs - a successful invasion of prehistoric skies. Biologist 50 (5), 213-216.
Xing, L., Wu, J., Lu, Y., LÃ¼, J. & Ji, Q. 2009. Aerodynamic characteristics of the crest with membrane attachment on Cretaceous pterodactyloid Nyctosaurus. Acta Geological Sinica 83, 25-32.
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A few pertinent comments here methinks:
1.On wing 'length' I have commented on this before (ish) in a paper. It may not be great, but it is a great proxy since it'll be very hard to work out exactly how the wings were held in many if not all taxa, and they would likely have been pretty conservative in this in any case so the error would be consistent. Also the torso is not measured merely one set of wing bones measured and doubled, so we do at least have that value not taken into account, reducing the total value a little which will account for the flexion of the elbow and non perpendicular wing position.
2. With Elgin and others we did indeed test Nyctosaurus *with* a soft tissue sail crest and found no evidence for such results. However this paper is on a huge and long-term stall while we fix some errors. It appears that issues with the wind tunnel system being used meant the data is skewed. We *should* be able to compensate once the engineers have worked out how much the data is out. My understanding is that this should not affect the pattern of our results (all the numbers should be shifted by similar amounts, or proportional amounts in the same way) but we can't go with that into print until it's corrected.
A function I've seen ascribed to Pteranodon's crest is as a counterweight to the beak. By putting the head's centre of mass further back it'd allow the neck musculature to do less work, thus saving energy.
Might there be anything to this?
Andreas, we already took care of this one in the Pteranodon crest paper Darren mentioned. ;-) You can see find quite a lot more about it here: http://archosaurmusings.wordpress.com/2009/01/11/hello-girls/ The short answer, is no, it doesn't function as a counterbalance (or as anything else aerodynamic really for that matter).
It will be interesting to find out exactly what nyctosaurs were doing with their crests anyway. Most likely it was a display structure, but the question is exactly why and how did it have an impact on the animal's lifestyle.
The flapling is also interesting. Unwin also suggested that pterosaurs used niche partitioning, with juveniles being small generalists and the adults becoming more specialized animals. It would be interesting to see if his hypothesis is true, thought to be fair, it sounds a little out there in my opinion. I have a question though, is Nemicolopterus an adult pterosaur, or is it still a juvenile?
Gosh darnit, I think it's silly that juvenile animals are given their own names. Ontogenetic changes produce wildly different animals--look at Brachyceratops compared to Einiosaurus or Pachyrhinosaurus.
It seems that even without soft tissue that antlers that big would still have some effect on aerodynamics. Are the spars circular in cross-section? How strong would they have been? I suppose Nyctosaurs had to avoid trees and swinging dinosaur tails.
Ross, apparently, tested only a few of the possible uses of a crest, and such results, as rigorous as they may be, don't extend to other possible uses; more on this below.
To assume a sail on Nyctosaurus does not equate to assuming the sail extends the full length of the boom. One reason not to find jagged trailing and top edges of the preserved crest members would be if the sail was thick, like a proper wing, and pneumatized, so that the skin connected tangentially. However, there might be evidence where the sail's trailing edge attached, if those bits were preserved.
Usefulness for display doesn't preclude usefulness for other purposes; a practically useful structure may be hypertrophied by sexual selection and continue to serve its original purpose. The toucan still eats with its beak, and my wife's admirable gazongas fed our kids. As I noted on Dave's blog, a pimpmobile and Air Force One are also transportation.
Does the final image above illustrate using internal air sacs for buoyancy? I suppose, re: Frey, they could use their wingtips as a keel.
Regarding uses for a crest, Ross doesn't seem to have tested their value for flight stabilization, nor as a means to increase the pitch-rotational moment of inertia of the head. This latter might simplify neck linkage in a creature that snatch-feeds. I don't know how one might test for a sensory use, or preclude one. Arguments that the shapes ought to converge if they were useful don't apply to the rotational-moment argument; one shape is about as good as any other, for that. Likewise, for stabilization, once it's big enough, any extra for display purposes is just extra. I'll leave aside notions of symbiotic photosynthesis, for now.
Congratulations to Dave Hone on the term "flapling". Every paper that ever uses the term will be obliged to mention him. By the way, do immature bats have any better name than "pups"? I rather like "clinglings", which might as well apply to possums.
Finally, I take it as given that no word can be said about the Giant Boneless Aquatic Pterosaurs that terrorized early whalers â until the latter largely eliminated their food supply â and then disappeared leaving no physical trace of their existence.
In case I need add... :-)/2
The point has been made before that flamboyant structures on animal's bodies almost always serve a sexual function, but have almost never been interpreted in that way, until recently. I like the fact that the above-mentioned experiments discount the idea of pterosaur crests having a "utilitarian" function as rudders, which leaves the historically more plausible explanation of a feature to attract mates and/or intimidate rivals.
How likely is the membrane stretched between the tines of the crest in spite of roundness? That would be quite the advertizing billboard regardless of its aerodynamic usefulness. Much more so than just the crest itself. In the second drawing the crest looks so "empty" for a structure meant to impress. Know what I mean?
Just had an idea. I've no anatomical training, so it's probably wrong, but ...
Depictions of Nyctosaurus's crest outfitted with a membraneous sail have always struck me as implausibly large and unwieldy -- especially with the posterior spar at its correct length as shown in Mark Witton's restoration.
On the other hand, a completely unadorned crest strikes me as extremely bare, like the bare, skinny branches of a tree. Not very impressive, really. It looks about as visually striking as a bare willow branch.
However, what about something "in-between"? Specifically, we know that many pterosaurs had a hairlike or furlike integument. What if the "antlers" of Nyctosaurus were covered with skin and then fringed and tufted with long silky hairs like the mane of a horse? The wind could blow through them and they'd offer less aerodymic interference than a huge sail of skin, but they'd still enhance the visual aspect of the sail considerably.
Please see a rough mock-up of what I mean here. Unfortunately, my "hairs" ended up looking a bit more like feathers, but at least this gives you a rough idea of what I mean. And of course, this is only one possibly configuration for the "hairy plumage." The hairs could be longer or shorter, or run completely along all the edges of the crest, or be more concentrated toward the tips.
(I realize this may also call to mind David Peters' controversial restorations of elaborately plumed pterosaurs, but that didn't occur to me until after I did the mock-up.)
One question, though: If the crest were covered with skin that had long "hairs" anchored in it, would signs of that show up in the bone of crest somehow?
It belatedly occurred to me that when I said the unadorned crest looked oddly "bare," I was essentially repeating what Abyssal said right above me.
Steve: Gimme an "L"! Gimme an "L"! Gimme an "L"! "L"! "L"! and an "L"! and an "L"! Yaaay, "L"! ... Life in the lower Cretaceous must have got kind of monotonous sometimes.
The term Flapling is Unwin's not mine, I don't like it personally and when reviewing Dave's book prior to publication suggested he take it out. It's better than pterosaurologist though.
Darren, I don't have the Elgin paper, so I'd like to know: Does the study ALSO use the short-posterior-spar model for the crest, or the hypothetical long spar of Witton? Assuming Bennett is correct (and there's no reason to contradict him on this), and the spar is incomplete, it could be merely .05 to .5 meters longer, a BROAD range that may have aerodynamic effects. If Elgin and Xing both assess the same length for the posterior spar, then the differences lie in their maths and not the models they use, so this is a fairly important means to comparing the studies.
What was structure of Nyctosaurs antler? How much tension could it withstand?
If Nyctosaurus lowered its head, two branches would point forward and upward. The antler might be used in mating fights. Maybe males defended its turf from alighting rivals. The antler might be used for piercing wing membranes or air sacs of a rival.
Or, alternatively, this might be colorful advertisement of fitness, like long tail of widowbird or quetzal.
BTW2 - there are many Nyctosaurs without antler. Is it possible that antler grew seasonally like deer antlers and was reabsorbed or broke off?
Almost certainly not. Even the wing feathers of birds don't always have quill knobs on the ulna or the carpometacarpus.
That alone is already news to me!
Since other people are speculating about possible functions of Nyctosaurus crests, I might as well also offer my own ill-informed notions.
While membrane-less crests may seem "bare" as far as display structures go, to me they still look like an impressive set of "antlers". Thus I wonder, what if they were used not only for visual display but also for some sort of inter-specific ritualized combat?
If a couple of these animals stood on the ground facing each other with their heads tilted down and forward, the dorsal spar of their crests could come into contact, and you could imagine various ways the two beasts could push or hit their crests against each other. While I have been told that the crests may be too weak to withstand actual impacts, it still seems like some rather spectacular shoving matches might be possible.
Anyway, it may be crazy, but still fun to imagine. I wonder how the torque from such jousts compares to the torque expected from the sails described in Xing et al.
I felt Nyctosaurus reminds me of something. And, indeed:
I like the hair example the best, as I think it makes the most sense (I mean, how could one see a thin, stick-like crest from so far off), but unfortunately it is still speculative until better fossils are found (but here's hoping!)
Anyway, it is very unlikely that these crests were used in ritualistic combat. The crests were so unwieldly trying to fight with them would be like those anime shows where the characters wield swords as big as their bodies. Looks cool, but not too functional.
Darren, great stuff! pterosaur weirdness par excellence.
A quibble, here: would the wing-bone ratio necessarily remain constant for any given species, during growth from flapling to adult? I am thinking that human and animal proportions do change toward maturity... sometimes significantly... from chubby short-limbed and big-headed to more (relatively) gracile (as well as absolutely bigger). As you know...
At a guess, by rule of thumb, I would say that the more distal wing elements might lengthen disproportionately.
Of course it may not have been so for pterosaurs' wings...
For Nyctosaurus' headcrest, I see already suggested here these various models, which we may playfully refer to as:
sail (floaty boaty)
hair (fluffty tufty)
bare (scantily antlery)
May I add the following:
harp (stringy thingy) - fibres stretched taut between them, the two divergent spars formed an Aeolian harp - tilted in and out of wind, or in flight, fibres perhaps individually tensioned/ slackened by erector pili type muscles, so musically 'singing' - used as a display/mating ritual. (How these fibres could form thus, attached at each end and in free air between, I leave for the reader as a fascinating exercise in speculative ontogeny).
snare (sticky picky) - an adhesive flypaper-like surface on the antlers, perhaps attractively-scented too, enabled these pterosaurs to fly through dense swarms of insects, then descend to pick off bugs from each others's crests - a cooperative feeding and social bonding technique.
It would of course be far too fanciful to suggest that the crests bore bioluminescent membranes for nocturnal animated displays, like Times Square advertising screens. That would just be silly.
(But remember - you read it here first!)
Simple. Nyctosaur crests were supports for tying banners. As the pterosaurs flew around with the banners flapping from their crests, they thus served as flying billboards.
'crests bore bioluminescent membranes for nocturnal animated displays'
You should get a copy of 'Save the Cosmos' story by Stanislaw Lem. I think its in the set 'Star Diaries'. It is humorous sci-fi story how human conquest of the outer space affects extraterrestrial fauna and flora.
There is, indeed, one crocodile-like carnivore which attracts prey with bioluminescent warts. In settled areas, the form evolved with glowing 00 pattern, pretending to be campsite latrine. ;-)
As usual i seem to have come in on the end of the discussion but perhaps i can offer something in addition to what has already been said. As Dave already mentioned any paper on the characteristics of large crested pterosaurs is sadly going to be a long way off, if indeed it ever sees the light of day. On the up side as i am currently flying more experimental models, this time looking at the whole animal, it is not much more effort to swap the heads around. Hopefully this will provide some much needed data.
While Nyctosaurus was one of the many models build, i was more interested in the larger azhdarchoids, and this was added as little more of an afterthought. Thus in error i simply skimmed over the paper to reconstruct the head profile. The model that we ended up using was the âshortâ spar version and it thus stands to reason that any results would be intensified by the addition of this missing spar. I believe the cross sectional profile of the crest was more a flattened oval rather than a sphere and thus would have been more aerodynamically streamlined rather than simply acting as a bluff body. The results as they stand for the moment â and i stress that this data should probably not be considered scientifically useable in its current form â suggest that the crest produced about 1/3rd the lift force that of a single wing and stands alone among pterosaurs in retaining the centre of pressure caudal to the occipital condyleâ¦ and is thus in keeping with the theory that the crest could potentially reduce some of the work needed to counterbalance the head. I unfortunately cannot comment on the paper in question (Xing et al 2009) as our museum and library conveniently donât receive the journal and i have yet to see the paper since it was last in for review.
Of course the likelihood that Nyctosaurus ever supported a large membrane sail should be subject to a little more thought and seems vaguely similar to Ross Steinâs paper on Pterandon in the 70âs (1975). Here a membrane was reconstructed running from the tip of the crest onto the dorsal surface of the neck allowing head turns to assist generating useful aerodynamic forces. Like Nyctosaurus though, this was a theory based on little to no hard evidence but rather another suggestion from the start of 20th century.
Readdressing at least one of Nathanâs points: the âusefulness for display does not preclude usefulness for other purposesâ is indeed a valid observation, however, as i have argued before, pterosaur crest diversity is quite staggering and the resolution of the lineage is sufficient that we should be able to see any trend towards a âusefulâ aerodynamic profile. Quite simply this does not happen. I am more than happy for people to suggest âusefulâ functions for crest â besides that of sexual display - however; this function should be targeted at a specific taxa or crest morphology rather than as a generalised statement. Even closely related taxa show very different crest morphologies and so whatever one is doing with its crest it is very likely that its sister taxa isnât.
Apologies for the length of this post, there is a great deal to say on this topic and attempting to condense it into the comments section of an article rarely does the topic justice.
Just to follow up for Graham, actually pterosaur wings are really really isometric during growth (i.e. the wings of juveniles have pretty much the smae proportions as adults along the whole growth curve). THis really helps us spot the potential taxonomic overlap b/w juvies and adults and sort out ontogentic differences which is nice. In the above example, since the wing proportions of the juvenile are different to other adults in the formation, it's actually quite likely that it is in fact different.
As a non-pterosaur worker, I am giving a big thank you to all of the pterosaur workers who have commented here. It is pretty fantastic to read a great post and then get commentary from the people actually doing the work.
And of course, thanks, Darren, for bringing the rest of us up to speed on another fascinating twig of the tetrapod tree. How you keep all this stuff in your head I'll never know. Keep 'em coming!
"If the crest were covered with skin that had long "hairs" anchored in it, would signs of that show up in the bone of crest somehow?"
Almost certainly not. Even the wing feathers of birds don't always have quill knobs on the ulna or the carpometacarpus.
Cool! That means my speculation hasn't been immediately falsified right out of the gate! I'm actually patting myself on the back that it passed the "David Marjanovic didn't immediately point out several excellent and irrefutable reasons why my idea is flatly impossible" test! ;) Seriously, I consider this something of an achievement.
I guess the next question is, would impressions of hairs be likely to be preserved in the substrate where Nyctosaurus remains were found? But I understand this is a question with no definite answer.
Meanwhile, I am now envisioning Nyctosaurus adorned with fluffy-tuffty, silky Princess Pony fringes on its crest. If I have time to do a better illustration, I'll put it up on my DeviantArt.com site and let you know.
Echoing Matt, thank you, Ross, for your enlargements and thoughtful comments. I don't think anybody here would complain about length when you have so much substantial to say. (I shall assume that by "spherical" you actually meant "circular").
I wonder why you demand convergence in the case that the crest is what Dave calls "mechanically" useful. If a crest of a certain size were to suffice for some mechanical purpose, but have been radically hypertrophied by sexual selection, surely we do not expect the extra growth to match interspecifically? Homo breast morphology, for example, certainly hasn't converged -- even across town -- but their (primary? secondary?) usefulness for infant nutrition has not been questioned.
I very much like the aeolian harp suggestion for Nyctosaurus, although I would expect it to be constructed of a stretched membrane with long oval holes. I await eagerly Graham's rhyming mnemonic suggestion for the symbiotic photosynthetic hypothesis, and for the sensory one. For bioluminescent displays, they might need cooperation from symbiotic hitchhiking tentacled marine cephalopods.
Seriously proposed N. crest sail forms should treat a subtriangular shape, with a deeply concave trailing edge, to minimize the possibility of flag-like flapping in flight. A pneumatically inflated sail that matches the thickness of the spars at the leading edge would minimize drag, and attached tangentially ought to leave no mark on them, being supported by tension in the skin wrapping the spars on the leading and lower edges. Would a pneumatic sail need one or more foramina in the spars?
Of course the presence of a sail does not preclude tufts of gaily colored fringe at the spar tips, or, indeed, iridescent sparkles.
To quote: "I wonder why you demand convergence in the case that the crest is what Dave calls "mechanically" useful. If a crest of a certain size were to suffice for some mechanical purpose, but have been radically hypertrophied by sexual selection, surely we do not expect the extra growth to match interspecifically? "
We do this becuase once the radical hypertrophinmg takes place in this case (pterosaur cranial crests) it is unlikely to be able to fulfil that function as effectively any longer. If the point of a small crest was to balance th skull say, then fine, but onse it becomes huge it will be much bigger / heavier / have a different mass distribution. It can no longer provide that mechanical function (again, speaking across all crests, of those derived from your hypothesises 'basic one' as say in an ancestral Pteranodon species) so that cannot be its function. The extra growth would have to match interspecifically if that fucntion would be maintained as the morphology would have to allow the continuation of that function and the variation seen in say ornithocheiroids does not fit that pattern at all (front crests, back crests, both and neither are all present).
Dave, Ross: Since it's easier to find examples in nature of sexually-selected hypertrophy and disparity not destroying utility as to find examples where it does, it seems bizarre to assume the latter. Do you have some particular reason, for the case of the pterosaurs?
On the possibility of the Nyctosaurus sailing:
1. Is is possible to estimate the maximum bending and torque moments that both spar and boom (the horizontal spar) could bear at each point along their lengths? This would provide a very useful constraint to any speculative aerodynamic models of the "sail".
2. The root bending (rolling) moment of the spar would need to be countered by the animal somehow to prevent rolling. A sailboat does this by having ballast at the tip of the keel, while the person windsurfing uses his/her overhanging weight as a counterbalance. Its also important to remember that angle between apparent wind and boat direction,
Sailing angle = atan[ (L/D)_sail ] + atan[ (L/D)_(keel+hull) ]
where L/D = Lift Force to Drag Force ratio of the aerodynamic surface in question is a measure of its aerodynamic efficiency. High L/D values typically need clean rigid wings with high aspect ratios and airfoil (wing cross sectional) shapes having thickness ratios in the 10% to 25% range (i.e. not membranous).
So, any useful upwind sailing requires an aerodynamically efficient keel/fin. Are there any proposals as to what one would look like on the Nyctosaurous? Unless it only sailed in a perfectly downwind direction while holding itself perpendicular to the wind.
3. Sailboat and windsurfer sails are shaped like they are because they are additionally supported by standing and running rigging (this includes the person windsurfing) which help carry much of the bending moments. A truly cantilevered wing/sail structure needs to resemble a monoplane or at least bird or bat wing to be structurally and aerodynamically efficient. At the very least, it would require shaped ribs/stiffners attached to the membrane to enable it to hold a near airfoil shape. A (near) triangular membrane stretched between a rigidly fixed spar and boom with no rigging and no way to adjust sail tension would be functional, but would still be limited by bending and torque moment bearing capability of the spar, especially at the root. The leading edge spars of both windsurfers as well as membrane wing hang gliders are usually hollow tubes - to better bear bending and torsion. Is this the case with the Nyctosaurous' vertical spar?
4. Having a useful sail on its head would require the Nyctosaurous to always point at an angle dictated by desired direction of traversal. (Or into the wind when stationary.) This would limit the animal's vision and hearing sensory abilities quite severely, particularly in high winds. It would be quite inconvenient to have to Tack or Gybe just to be able to look around and the neck joint would be somewhat redundant.
5. Instead of a sail, if a head wing is to function as a fin/vertical stabilizer while flying, its center of pressure needs to be behind the animal's Center of Gravity to provide static stability. Alternately if it is ahead of the CG, it would be statically unstable, but could still be functional if actively controlled.
This sounds extremely cool. When you say flying, do you mean wind tunnel models, or are you talking of functional flying ornithopters in the style of Delaurier?
Stagyar makes a good point: to be useful for flight stabilization, the center of pressure of the sail surface would need to be well ahead of or behind the center of gravity, or at least extend well beyond it in one or both directions. I.e., any area directly above the CG can't contribute to yaw stabilization. To add to this, sail area too distal, vertically, creates a roll stabilization problem that must be (but could be) countered by the wings.
One stabilizes flight actively by displacing the control surface slightly, and then quickly returning it to the neutral position. Quick motions would be difficult with a big, high-moment sail. Passive stabilization requires the center of pressure be well behind the center of gravity, and the surface held steady -- easy with a tail surface, hard with a crest.
Anyone can do experiments by binding a couple of layers of poster board, cut to an appropriate shape and decorated appealingly, to the sides of a bicycle helmet, and riding around campus. Anyone who won't do that can't be said to be seriously interested in learning how pterosaur crests operate, aerodynamically. I look forward to seeing evidence of such serious interest appearing on youtube in the near future.
"Since it's easier to find examples in nature of sexually-selected hypertrophy and disparity not destroying utility as to find examples where it does, it seems bizarre to assume the latter. Do you have some particular reason, for the case of the pterosaurs?"
Yes we do. Since in this casse specifically the structres are supposed to convey a mechanical advantage which we can find no evidence for (that we have tested). If these crests are supposed to convey some kind of function and we cannot find a support for that function (at any size, remember we tested crestless / microcrested versions as well) then it is safe to assume that there is soemthing else going on. I would also disagree that it is 'easier' to find examples where function is not destoryed - that assumes that a) these are common, which I do not think is necessarily ture, and b) that disaply structures started out functional and then became co-opted for diaply without a functional change in most / all cases which is probably also not true. Once you take that lot into account, your premise that we are making the wrong assumption is strongly weakened.
In this contest people have specifically claimed that crests give benefit X or Y. We find no evidence for X or Y, and given the a) extravagant nature of them, (also weight, cost to build, maintain, carry etc.) and b) the huge variety seen both within and between families, that when speaking in general terms about pterosaur crests, display is the most parsimonious explanation based on the evidence. To nick Matt Wedel's phrase, it should be the null hypothesis for crest function.
Some corrections to my earlier post -
The formula should read:
Angle between apparent wind and heading,
Apparent Sailing angle = atan[ (D/L)_sail ] + atan[ (D/L)_(keel+hull) ]
where L/D = 1/(D/L) = Lift Force to Drag Force ratio of the hydrodynamic or aerodynamic surface in question, is a measure of its aerodynamic efficiency.
For a boat/animal lacking a hydrodynamic lift producing device (keel), L_(keel+hull) = 0 and atan[ (D/L)_(keel+hull) ] = 90 deg. So it can at best sail towards a point lying within the 180 deg downwind arc, even with a drag free sail. As sail (L/D)_max worsens,
atan[ (D/L)_sail ]_min correspondingly increases, thereby further limiting sail-able directions closer and closer to true downwind.
Bottomline: Proponents of the head sail conjecture need to demonstrate the existence and efficiency of a keel or explain why it was useful to the animal to only sail in downwind directions. Additionally they need to show how it prevented itself from tipping over and how it could look around and scan its surroundings in high winds.
David Hone and Ross Elgin:
I'm really curious about what is known about the structural properties and specifically bending moment and torsion bearing capacities of the two spars.
Stagyar: Modern sailing vessel designs dispense with deep weighted keels, substituting underwater wings providing active correction. While I agree the yarosaur notion is far-fetched, and more fun than practical, we shouldn't impose artificial demands on it.
Dave: The null hypothesis for any anatomical feature must always be that it has more than one function. Nature doesn't care how concise our descriptions are. Investigation may rule out this or that hypothesized use, but can't, even in principle, demonstrate that the animal did not find other uses. It would be unscientific and (frankly) unprofessional to suggest that you can list and test all the potential uses for a structure.
Not sure what you mean by 'Modern sailing vessel designs' and I don't see how 'active correction' of any form can resolve the rolling moment balance problem. Sans illustration or example, I'm inclined to guess that you are talking through your hat. Could you provide examples?
One class of competitive sailing vessels considered to be at the leading edge of technology are America's Cup vessels. All of the recent extant examples of this class certainly do have deep weighted keels. See for example here.
At this point its worth commenting on the high vs low aspect ratio crest issue raised by Darren. A low aspect ratio sail brings the center of pressure through which the lift force acts on the sail lower and reduces its rolling moment contribution. This reduces but does not eliminate the need for ballast at the tip of the keel to counterbalance it.
Some catamarans and trimarans eliminate the need for keel ballast by shifting enough ballast at the surface (read crew) to the windward side to counterbalance the rolling moment generated by the sail and keel. Windsurfers do the same. Is there a proposal to suggest that the Nyctosaurs did something similar? At any rate, solving the rolling moment balance problem is fundamental to any sailing vessel design and a solution has to be part of any proposed model for sailing Nyctosaurs.
PS: Darren, I'm interesting in reading Xing et al. (2009), and would be grateful if you could email me a copy. I might even be able to help with the mathematics.
Stag: If you don't understand the utility of underwater vanes in maintaining roll stability at speed, you may ask someone else to explain them to you. Current AC vessel design is sharply constrained by a variety of rather arbitrary rules that would be unpersuasive to a mother pterosaur of yester-eon, but underwater vanes were all the AC rage just a few years back.
For an amusing look at the underside of a keelless vessel, see http://yachtpals.com/hydroptere-4036 Links there lead to more conventional shots with the sail above the water, and the hulls too.
A hydrofoiling, sailing pterosaur, by the way, would be quite a find. It would would seem like the best of all possible configurations for (dare I say?) skim-feeding.
Nathan: Please allow me to apologize for the testiness of my previous post. But the only way your comments make sense to me is by concluding that you have substituted the issue I raised, which is the basic problem of Force and Moment equilibrium, with the somewhat secondary issue of "Stability" - i.e. staying at that (possibly unstable) equilibrium despite perturbations. The latter is amenable to "active correction"; the former is not.
A number of recent attempts at sailing speed records solve the roll balance problem sans keel ballast by using configurations with the sail and/or keel canted and/or located off-center such that the their respective aero/hydro-dynamic force vectors are aligned (or pass through the craft CG) resulting in zero net (roll) moment. The distinction between keel and underwater wing/vane/hydrofoil in this context is also a little artificial as it depends mainly on the symmetry of the surface's airfoil and its angle of cant.
Canonical examples of this class are Sailrocket, Monofoil and kiteboards, although the Hydroptere comes close. I'll concede here that craft that use hydrofoils to lift the hull up may use active systems for trim and ride height control, but passive systems (including Hydroptere's V-foil are also common.
I'll end by reiterating that it is insufficient to merely show the presence of a crest sail to demonstrate sailing in a Nyctosaurus or any other animal. A hydrodynamic surface analogous to a keel producing a leeway resisting side force equal and opposite to that generated by the sail, moment balance, and ability of proposed structural members to bear the loads in question also need to be demonstrated.
Stagyar: I welcome your gracious apology. You seem to be pointing out that without establishing the means to stay upright at and near rest, there would be no way to attain the speed needed for underwater control surfaces to apply any useful force.
L'HydroptÃ¨re seems to attain its static stability with buoyant outriggers, which at speed lift out of the water, their role subsumed by the hydrofoil vanes. I take this opportunity to link to an image of l'HydroptÃ¨re upright, and flying: http://yachtpals.com/boating/fastest-sailboat . Note the right hydrofoil lifted entirely out of the water, failing, at that point, to apply any useful force, and the crew applying what little weight they can rally.
A hydrofoiling Nyctosaur would have no problem controlling its submerged wing surfaces, but, as Stagyar notes, sufficient structural and muscle strength at the wrist has not been demonstrated. A pneumatic-sac pontoon float at the joint, for buoyancy and stability at rest, ought to present no ontogenic or phylogenic problem. I imagine N. starting out by sailing cross-wind, its crest turned nearly parallel to the wind and its wingtips serving only as keels, until it is moving fast enough for its wingtips to get some bite on the water. Neck joint morphology might not permit such a configuration, providing an easy test for the notion. Maximum speed is probably limited by how much force the neck joint can transmit to the, er, hull.
In closing, I must admit I have little direct experience with sailing, never mind sailing-vessel design, so have no choice but to talk through my voluminous, cathedral-ceilinged hat.
No. and Ouch! Its hard to be sure across the internet, but you seem to be speaking from a position of somewhat greater ignorance than I had assumed.
Let me try again. Sailboats sailing in upwind directions (efficient designs may sail 30 degrees or closer to the wind) have sails aligned with the fore aft direction of the hull, or very close to it. Angle of attack is small and the major force produced is lift, acting perpendicular to the apparent wind direction and hence close to perpendicular to the sail surface (and hence the fore aft direction). A boat lacking a keel/vane/canted hydrofoil would in such situation begin to slide sideways and would no longer be traveling in the desired direction, thereby necessitating the presence of one of these.
The keel/vane/hydrofoil acts as a hydrodynamic surface analogous to the sail, producing predominantly a lift perpendicular to direction of hull traversal in water. This hydrodynamic lift when vectorially subtracted from the net (lift+drag) sail force leaves a resultant force acting in the direction of boat traversal, which is equal and opposite to the combined hydrodynamic drag produced by the hull (if submerged or planing) and keel/vane/hydrofoil.
This now brings us to the problem of balance. The net sail force is equal and opposite to the total force produced by the keel and hull, but in a conventional monohull vessel at least, they are not aligned. One acts below the CG and one above. A net moment results which except at very low speeds exceeds the stability provided by buoyancy forces and would cause the vessel to capsize unless countered. Therefore the keel ballast, as well as the shifting of surface weight, including crew, etc.
Since fluid (aero or hydro) dynamic lift and drag are produced only when the fluid in question is in relative motion wrt the body immersed in it, this analysis applies only to situations where the hull/keel/foil is moving through water and when air is flowing over the sails.
This document contains some figures illustrating this if you care to take a look. Figures 1 & 2 on page 8 illustrate this force and moment equilibrium (again, for a moving, not stationary yacht).
Figure 7 shows how quickly hull drag rises with speed and so why its better to a) be light and b) suffer the much lower lift dependent drag of a canted hydrofoil (as in Hydroptere) or airfoil (as in Sailrocket and Monofoil) instead if you want to sail fast. Hydroptere's canted hydrofoil performs both functions here - generating sufficient side force as well as supporting weight.
Finally a comparison of the upper right corner of Fig 1 with the links for the Sailrocket and Monofoil that I provided above should illustrate how these craft use position and cant of sail and foil to solve the moment equilibrating problem sans ballast.
Once you have figured out this equilibrium of forces and moments, then you can ask the question as to whether this is a stable or unstable equilibrium and think about active control in case of the latter.
Stagyar: Your further details remain elementary. My point was that suitably arranged, steerable underwater planes can apply, at speed, all the forces that a traditional weighted keel provides (and if needed, more) to counter and redirect forces applied by the sail. L'HydroptÃ¨re, if I understand it correctly, demonstrates that a clever arrangement of non-steerable partially-submerged planes suffices. A sailing or hydrofoiling pterosaur would be equipped to steer its underwater wingtips as needed, just as its cousins did in the air, but the example of l'HydroptÃ¨re suggests that it might have had less need to do so.
Nathan: Actually, all my points can be considered elementary. What you would expect to encounter within the first week of a class on Yacht design, I expect. Nevertheless many, if not all of the "Head Crest as functioning Sail" proposals seem not to have taken them into account.
Your idea that Nyctosauruses hydrofoiled on their wingtips in a configuration similar to the Hydroptere would require flipper shaped extensions to the wingtips that could be lowered into water. These flippers would need to be rigid, of high aspect ratio,and of significantly smaller chord than the wings. They would also have to be rigidly held by what are mostly believed to be membrane wings. A configuration of this type imposes very different and greater bending loads all along the wing in comparison to those required for flying. This needs significant extra reinforcement, particularly near the wingtips. The extra weight would degrade flight performance, particularly if it's carried in a propulsive element. Lastly, I would would expect both the flippers as well as the stronger wings to be clearly discernible in fossils, but don't think that such has been reported.
There's a very good reason why we don't have (useful) flying cars yet. :-) Airborne performance is very unforgiving to even slight changes in form and loading made to obtain viable performance elsewhere.
Stagyar: Elementary is. As I understand it, pterosaur wingtips are very rarely preserved, so it would be easy for it to conceal aquatic adaptations.
The place where great strength would be required for hydrofoiling is at the wrist or knuckle joint, and the joint would have needed to bend downward (and, perhaps, underneath), not upward as is usually reconstructed. This would show. It would be fun to reconstruct N. as a living Hyroptère precursor, in full flight. In practice, though, it would probably suffice for it to float on pneumatic pontoons, and use its wingtips only as keels.
But is the same true of fins in fish or flippers in marine mammals?
The weight of a conventional airplane wing consists predominantly of the spar, which is sized to bear bending loads due to lift. The canonical lift distribution is elliptical, and most actual aircraft are not too far from this. So most aircraft wings are strong and sturdy towards the center and quite light and flimsy closer to the tips. I'm not intimately familiar with the dynamics of flapping flight, but I expect the loading to not be too radically different from this. Needing to bear a point load at the tip, which is at least equal to the weight of the animal would require far greater bending strength all along the length, not just the wrist. It would be like hanging from parallel bars so far apart that your arms are fully extended horizontally, instead of having the bars under your armpits. Do extant fossils of Nyctosaurus show significantly thicker wing bones compared to other Pterosaurs of the same size?
Yeah, but this would bring the balance and ballast problem - which foiling on one off-axis hydrofoil neatly solved - back up. Unless you limit yourself to very low winds.
In most pterosaurs I think most of the wingspan is beyond the wrist/knuckle joint, so it needs to be pretty sturdy.
I agree there's no point in talking about sailing Nyctosaurus at all unless we're talking about hydrofoiling N. The life reconstructions are no fun without it, and skim-feeding is not compatible with wallowing.
I'd still like to know if there are foramina on the crest spars, consistent with a pneumatic sail body.
On the question of how to achieve a righting moment to offset sail force, the leeward wing is operating at approximately the same dynamic pressure as the presumed sail, has a longer moment arm (aerodynamic center to center of rotation) and has the advantage of some ground effect. This would also lift the animal reducing hydrodynamic forces.
The keel effect for countering drift could easily be provided by the windward wing bent back to the rearward position. High aspect ratio is not required in the least for this. Presumably, all of the surfaces are capable of water immersion.
Its not difficult to imagine unmodified wing tips being useful for hydrodynamic purposes. birds utilize their wings for swimming, albatross often drag a wing tip in the water...It is hard to imagine any structure on a flying animal having a large drag penalty, and anyone flying RC models knows that a round tube of that size would seriously degrade performance. Its a mystery how any wind tunnel test could show otherwise. This is very basic aerodynamics. Another problem of a non-streamlined tube is Von Karmen vortex sheets which would buffet the head.
I dont see how a sail could provide a thrust vector continually in flight, but if there were wind variations, perhaps the shear and gradients from waves, these could be harnessed in the same way that dynamic soaring is accomplished. The bone does not seem as tapered as one might think...say compared to a wing tip, and this leads one to imagine a different or combined function.
Another possibility is that the bones were hollow and served as snorkels, perhaps the vertical one.