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

Re: Did pterosaurs feed by skimming?



Sorry Jim, I'm not really in agreement with you. I've had fun playing
with the Quetzalcoatlus sp. neck vertebrae too and they seem to have
pretty limited ventroflexion to me, 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. 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. 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. The neck of Rynchops is short,
comprised of highly sculpted vertebrae and segregated into distinct
functional units to facilitate it's skimming lifestyle. Azhdarchid necks
are none of these things and, in most cases, offer totally opposing
conditions. 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. 

Furthermore, if Quetzalcoatlus - heck, all postulated pterosaur
skimmers - were skimmers, why the dickens does their jaw morphology not
reflect it? 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? Where
are the swollen jaw adductor muscles to hold the jaw in position whilst
skimming? Bottom line: there is nothing, absolutely nothing, about the
pterosaur skeleton to suggest skim-feeding was a habitual foraging
method. 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, but adaptations to address the same functional problems
experienced by modern skimmers should be expected.  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. 

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. 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 (producing low mechanical advantage) and not really provide much
scope for catching anything big. 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, thereby limiting their ability to see
submerged obstacles through water reflection and all that. We might go
as far to suggest that smaller postulated skimming pterosaurs, like
Rhamphorhynchus, are of such similar size to Rynchops that their visual
fields my be similar, and skimmers demonstrably only have limited ideas
of submerged objects. 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.

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 or, in this case, skim. Pterosaurs occur in
too many environments to be reliant on this most temperamental of
weather events to locomote or feed. 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. I appreciate that some
volant critters utilise wind currents in all manner of flight, but
plenty - perhaps the majority - don't. I trust you that certain models
can show windy-skimming would work, but I think the weight of the
evidence is really against them. 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.

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. 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.

Cheers,

Mark


--
 
Mark Witton
 
Palaeobiology Research Group
School of Earth and Environmental Sciences
University of Portsmouth
Burnaby Building
Burnaby Road
Portsmouth
PO1 3QL
 
Tel: (44)2392 842418
E-mail: Mark.Witton@port.ac.uk


>>> jrc <jrccea@bellsouth.net> 07/25/07 12:30 pm >>>
>although Thalassodromeus and Tupuxuara score points for being more
flexible 
>than most. Long-necked azhdarchids, by contrast, would've snapped
their 
>necks the moment they skimmed anything with more resistance than snot,
and 
>that's probably enough to shoot their skimming prospects down on its
own.<

Physical manipulation of the skeleton of the quetz neck shows that it
has 
enough ventral mobility to allow the jaw to tuck under and aft if an
object 
is struck.  This will also sweep the wings slightly forward and the cg

slightly aft, lifting the snout clear of the water.  And, you can see
most 
objects in the water in time to avoid them anyway.  I've spent several

hundred hours doing low level search and rescue flying in a Piper J3 on
the 
Mississippi and White Rivers in the southern USA.  Even though the
lower 
Mississippi is quite muddy, while flying the J3 at altitudes of about a
half 
meter (eye height about 2 meters) you can see most floating objects
ahead. 
Visibility for submerged objects is better at altitude.  If you look
before 
you dive (as I would expect anyone to do, even a pterosaur), you know
ahead 
of time which paths to avoid.

Colin McHenry, John Conway, Margot Gerritsen, and I have given several
talks 
about the feasibility for some crested pterosaurs to use a process I
call 
sailglide skimming to use wind for powering a skimming process with a 
non-rynchopian beak.  For the particular example we looked at, it
turned out 
to be feasible in mean annual windfields on the order of  7 knots.  By
way 
of comparison and perhaps coincidence, the US Navy has estimated the
mean 
annual windfield over the WIS during the appropriate period to be on
the 
order of 7 knots.

I think we should keep in mind that there may be more than one way to
skin a 
cat and a pterosaur  :-)

JimC