[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Did pterosaurs feed by skimming?



comments inserted below. I gotta go back to work -- If I don't respond again promptly, it isn't because I'm ignoring you.

----- Original Message ----- From: "Mark Witton" <Mark.Witton@port.ac.uk>
To: <dinosaur@usc.edu>
Sent: Wednesday, July 25, 2007 10:01 AM
Subject: Re: Did pterosaurs feed by skimming?



Sorry Jim, I'm not really in agreement with you.

Mark, if we all agreed on everything, this'd be no fun a'tall. :-)

I've had fun playing
with the Quetzalcoatlus sp. neck vertebrae too and they seem to have
pretty limited ventroflexion to me,

It is limited, but laying it out in ventroflexion on a 6 inch background grid indicates that it is sufficient to serve the purpose.


though I think we all have to agree
that the Q. sp. material leaves a little to be desired when it comes to
forming a neat articulatory series.

It does, but with most of two similarly sized necks to work with, it is better than most fossil remains.


However, the exquisitely preserved
azhdarchid cervicals from the Judith River Formation show both stonking
hypapophysis and exapophyses as well as articulatory facets on the
posterodorsal side of the procoelus condyle only. To me, this all points
to an neck that was not well versed in the act of ventroflexion.

I'll leave it to Wann to publish the amount of quetz ventroflexion. He's the one who did the work and sent me the images (which I can't release). Again, it seems to me to be adequate for the job.


What's more, azhdarchid necks are famous for their cylindrical shape - there
are none of the enlarged processes we should expect in an animal that
exerts strong forces on it's neck. Skimming, as demonstrated by
Rynchops, is a violent feeding method that snaps the head backwards on
impact with a prey item or obstacle.

But, there is no indication that the larger skimming pterosaurs were searching for fish that were anywhere near the relative size ratio of Rynchops and its prey. Think relatively big predator, relatively small prey. I agree that the head will be snapped back upon hitting a large, massive obstacle. I feel that is to be avoided, and they seem to be well equipped to do that (avoid hitting large, massive objects).


The neck of Rynchops is short,
comprised of highly sculpted vertebrae and segregated into distinct
functional units to facilitate it's skimming lifestyle.

Yes, it is.

Azhdarchid necks
are none of these things and, in most cases, offer totally opposing
conditions.

Again, more than one way to skin a cat. It is possible that one of us is assuming different skimming styles for Quetz and Rynchops (I don't think they are anything alike).


As you've already read, this is enough evidence for me to
start questioning their skimming ability without a flume tank or
sailplane in sight.

As you should. I differ. I also differ with Matt Wilkinson on quite a few of his assumptions about pterosaur flight -- but, I strongly support his investigatigative attempts, as I do yours.


Furthermore, if Quetzalcoatlus - heck, all postulated pterosaur
skimmers - were skimmers, why the dickens does their jaw morphology not
reflect it?

It does, but not the Rynchops scenario. I'd be amazed if a pteroaur skimmer's neck and jaw strongly resembled Rynchops.


Not one pterosaur has a mandibular symphysis compressed like
that of Rynchops - not even close. Why aren't their jaw joints
reinforced to absorb impacts? Why are their mandibles so slender?

Because they aren't heavily loaded.

Where are the swollen jaw adductor muscles to hold the jaw in position whilst
skimming?

You see skimming loads as being far greater than I do, which leads me to suspect that you are positioning the lower mandible differently than I do when in contact with the water. I still haven't read the paper, but will do so as soon as I get past a deadline on my day job........


Bottom line: there is nothing, absolutely nothing, about the
pterosaur skeleton to suggest skim-feeding was a habitual foraging
method.

For some pterosaurs, I agree. For others, I differ.

We stressed it in the paper but it's worth repeating: there's no
reason to expect the exact same adaptations in alleged skimming
pterosaurs,

We do agree on that. Pretty strongly.

but adaptations to address the same functional problems
experienced by modern skimmers should be expected.

Yes. And there are at least two different ways to address those functional problems.


 As it is, if we
strip palaeontology down to its core, evidence based values, the
skimming hypothesis should be rejected outright on the basis that the
anatomy - the fossils themselves, never mind any scary flume models or
calculations - do not suggest nor support it.

Again, I differ. I think some of the fossils do support it.

A couple more points: Rynchops regularly smashes into things when
skimming, hence the abradable, hyporegenerating mandibular tips.
Equally, flying with eye heights 2 m above the water surface is not
conceivable for any pterosaur - even the biggest ones.

I agree. In a J3 I'd have to fly with the wheels submerged to lower my eye height any further, and the J3 doesn't have enough power or control authority to fly with the wheels submerged that far. But, I do believe that I have more time working close to the water than most pilots at close to pterosaurian speeds and have a fair practical idea of what can and can't be seen when near the surface. We does the best we can with what we gots .... :-)


In azhdarchids
with skulls getting on for 2 m long, that 2 m height would submerge the
bill tip, but require the head to be held perpendicular to the water
surface

I wasn't implying that I was flying at the same eye height as the pterosaurs -- but, if I went any lower in that plane, I wouldn't be flying at all after all the sudden loud noises got stopped. Again, doing the best I could with what I had, just as your tank limited you to unrealistically slow skimming speeds (and yes, I'm aware that increasing the speed increases impact energies and drag loads).


(producing low mechanical advantage) and not really provide much
scope for catching anything big.

Who wants to catch anything big? It's lots more productive to catch bunches of little stuff.


Besides, pterosaurs would have to skim
with their heads at an acute angle to keep the premaxillary tip clear of
the water. My point is that pterosaurs, even the biggest, could not skim
with such a field of vision,

Strange. Quetz, with its remarkably low orbit seems to have the field of view perfectly oriented for the type of skimming that I think they did.


thereby limiting their ability to see
submerged obstacles through water reflection and all that.

That ability is indeed limited. I don't see any reason why that should create a problem.


We might go
as far to suggest that smaller postulated skimming pterosaurs, like
Rhamphorhynchus,

I don't see Rhamphorhynchus as a skimmer. It might be suited for what I think of as tuck & pluck feeding, but I've not attempted to look at that as a possibility, so don't really have an opinion on it.


are of such similar size to Rynchops that their visual
fields my be similar, and skimmers demonstrably only have limited ideas
of submerged objects.

I agree.

On a similar issue, note that Zhejiangopterus has
a ventrally facing occiput: this does not set it up well for angling its
skull acutely into the water for skimming.

Again, I've not looked closely at Zhejiangopterus, so don't have an opinion.

Finally, the old issue of wind and pterosaur flight. I'm not a
subscriber to the idea that pterosaurs - at least, the vast majority -
needed wind to take off, fly

I'm of the opinion that even the biggest could take off in their own length in a dead calm (even in today's atmosphere). Sounds like you and I are in strong agreement about that, though we may differ on details of launch, flight, and landing.


< or, in this case, skim.

Go back and read what I actually said -- I think you are inadvertently misinterpreting me. There is a distinct possibility for any pterosaur with a crest similar to a Bermuda rig to extract flight energy when some portion of its body is in contact with the water and the wind vector is appropriate. Pterosaurs with crests similar in configuration to Crab's Claw sails should have the potential to do even better because of the Crab's Claw advantage. Calculations indicate the ability to fly in close and broad reaches that are respectable even when compared to efficient sailboats -- if a portion of the body is in contact with the water.


Pterosaurs occur in
too many environments to be reliant on this most temperamental of
weather events to locomote or feed.

I strongly agree.

In particular, the occurrences of
azhdarchid fossils are strongly biased towards terrestrial settings, and
I know these experience variable levels of wind because I've got an
equally strong terrestrial signature myself.

You're reinforcing my perception that you've misinterpreted what I said. I agree with your statement above. Azhdarchids seem to have been flapgliders (or flapsoarers, whichever you prefer), and obviously could make use of cloud streets and other sources of eddies, but they did not need 'wind' to fly, launch, or land. Nor did any other pterosaur that I'm aware of (durn those trailing prepositions...).


I appreciate that some
volant critters utilise wind currents in all manner of flight, but
plenty - perhaps the majority - don't.

I agree and never thought otherwise.

I trust you that certain models
can show windy-skimming would work, but I think the weight of the
evidence is really against them.

When we investigated it, we weren't attempting to demonstrate that any species did it. We were investigating whether it was possible. It was.


I hope it's not overstepping the line
to say that the windy-skimming hypothesis seems a bit like special
pleading against what, for me at least, is compelling fossil data.

Mark, you are a gentleman. I don't think you would ever overstep a line :-)
I just like to see if there are optional, alternative ways to tackle problems. There usually are.


Right, that's me done. Apologies if any of that comes across in a
patronising or rude manner - no offence was intended at any point.

And none taken. Buy you a beer next time we meet (or accept one, depending upon who offers first).


Oh, and thanks for reading my post on this, by the way. As is typical of
such things, the first direct quote of it is my discussion of snot. Oh
well.

Hey, I thought that was aptly said. I love distinctive visual images.

All the best,
Jim