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Re: Did pterosaurs feed by skimming?



"Mark, Your paper begs the question, if, say,  Pteranodon did not
skim-feed, 
then how did it feed?  It seems to me that  there are very few
alternatives. DV"

Well, I don't know - I think there's lots of alternatives, but I think
a lot of them are hard to test with Pteranodon bones being so horribly
flattened. I think the idea of Pteranodon being a pelagic seabird
analogue is pretty established and probably correct. With such stunted
hindlimbs, the wing shape, regardless of brachiopatagial attachment
point, is narrow and well suited to long-distance dynamic soaring, and
the discovery of over a thousand Pteranodon species 200 km from the
nearest palaeoshoreline is pretty good taphonomic evidence for this, as
well. Hence, I guess we shouldn't expect Pteranodon to be grubbing
around on beaches or floodplains for edibles, and the drastically
overbitten premaxilla of this form lends some credence to this idea -
it's hard to eat your Chinese takeaway with your chopsticks at all
different lengths, after all. Of course, all this is a rather mute
point: Pteranodon gut content (regurgitated into its gular pouch) is
known and shows a select diet of small fish, and last time I checked
non-tetrapodan fish outside of water were pretty exceptional cases. 

So, how can you catch small fishies out to sea? Well, for me at least,
skimming is out for reasons that needn't be repeated here - check out
Humphries et al. or my message to Jim yesterday for that can of worms
(http://dml.cmnh.org/2007Jul/msg00291.html). What does that leave? I
guess we're left with plunge diving, surface diving, dip-feeding or
dull-old alighting on the water surface to grab fish being bait-balled
by marine reptiles below. On diving: maybe, but not in the manner of
sulids - these fellas have all sorts of diving signatures across their
skulls and necks (rounded skull profile in anterior view, streamlined
skull in lateral view, super-robust cheek bones and neck vertebrae),
none of which we see in Pteranodon. If Pteranodon did dive, don't think
they'd do it from great heights or penetrate that deep - more tern like,
I guess. Oh, and I don't think anyone still buys into that 'pelican like
gular pouch' idea, but if they do, stop it: Pteranodon had jaws nothing
like a pelican and wouldn't have fed like one. So there. With dip
feeding; again, maybe. Pteranodon appears (and I stress 'appears' -
there are no good, three-dimensionally preserved cervicals to play with
for this animal) to have had a relatively flexible neck and was probably
a skilled flier, so why not? My only real concern on this hypothesis is
that mentioned by Chris Bennett in his monograph: Pteranodon lacks a
frigate bird-like hooked bill, and I wonder if this might be important
for dipping. If anyone knows any different, let me know.

What's next? Surface diving: although many albatross surface dive
nowadays, I'm not sure how plausible this is for any pterosaur, really,
as their wings don't appear to fold up as neatly as those of birds.
Swimming with only half-folded wings underwater would be a real drag (ho
ho), so I might put it on the 'doubtful' pile (sadly, this does mean
that the wonderful image of an exposed Nycotsaurus crest being steered
silently through the water to the theme of 'Jaws' wouldn't have
happened). However, Pteranodon feet are certainly very big for their
body size and suggest to me that they'd be absolutely ace at punting a
floating Pteranodon around the water surface. Their feet were almost
certainly webbed, so big feet in any pterosaur would be handy for
pedaling their way through the water. This is my favourite idea,
actually: Pteranodon alighting on the water surface, feeding simply by
dunking its long jaws into the water to snatch up little fish and other
pelagic critters. It's as speculative as the others, but suggesting
Pteranodon regularly alighted on bodies of water explains their foot
morphology as well as the apparently flexible neck: if Pteranodon only
used its feet for standing on when it visited land, it might've had
smaller, azhdarchoid-like feet instead. I guess I also like this idea
because it brings pterosaurs out of the skies: some pterosaurophiles
seem to think that everything took place on the wing, and I suspect this
is one reason that skimming became so popular as a feeding hypothesis. 
Oh, and before anyone mentions that places like the Cretaceous Interior
Seaway were full of big predators that could've turned a water-alighted
pterosaur into a between-meal snack, I you have a word with the
thousands of seabirds that swim on and around predator-filled waters
today, often with said predators only metres from their feathers. 

That's my thoughts on Pteranodon, then. I'm afraid I don't have a
straight answer (just a long one) - because skimming is so specialised,
it's relatively easy to test. With the exception of extreme plunge
diving and, to a lesser extent, surface diving, these ideas concern more
generalised feeding strategies and aren't so easy to pinpoint. Maybe
that's significant in itself: the lack of obvious adaptations to any one
lifestyle may mean Pteranodon wasn't a specialist feeder of one way or
another. I guess, on some issues at least, it's much easier to identify
what Pteranodon couldn't do than what it could.

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