[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: John Bois on Armadillos at the K/T!
On Wed, Oct 03, 2001 at 09:16:30AM -0700, James R. Cunningham scripsit:
> Graydon wrote:
> > It makes more sense to say that in the shore environments for which
> > we've got fossils at all,
> I don't argue this, but point out that Qsp and Qn were not found near
> seashore environments.
True, and I hope there are more.
> > On the other hand, the very largest flying birds known are barely
> > getting into 'medium' for pterosaurs; the flight mechanics have to
> > have been very different, on the basis of wing structure;
> Well, my take is that they are about equally interesting for both
> their similarities and their differences. They did some things alike,
> they did others differently.
Well, the laws of aerodynamics will be the same, but the mechanism for
generating lift in birds depends heavily on feather rotation slot
effects, and there's no analog in pterosaurs, so the wing stroke and
lift mechanism is going to differ, and that's the most obvious place to
look for the basis of the size differences.
> Pterosaurs certainly seem to have reached larger maximum spans, but I
> expect a bird wing could be designed to allow more span.
I don't think the bird wing will support the 40foot+ spans seen in the
largest known pterosaurs; feathers will start to hit structural
limitations. There's also that the basic shape -- big nose well forward
-- isn't found in birds, and will have had aerodynamic side effects.
> I'm not sure the bigger bird could have still launched if it had that
> larger span though. The big pterosaurs appear to have been much
> better at launching.
> > and feeding behavour for pterosaurs and early birds may have
> > differed sufficently to avoid direct niche competition.
> I'm not sure I agree with that. calories is calories, as are
> opportunities. The extensive pterosaur trackways at the Crayssac
> mudflat would appear to indicate similar feeding habitats.
Similar habitat, but dis-similar mouth morphology; the birds look like
opportunistic generalists, whereas most pterosaurs have relatively to
highly specialized feeding apparatus. They wouldn't necessarily have
competed for primary food intake, any more than gulls and puffins do.
> > It's entirely possible that smaller pterosaurs were outcompeted by
> > *larger* pterosaurs in the marine niches; something had to have
> > been driving the size increases, and it's plausibly related to
> > success as a skim-feeder.
> Not all pterosaurs appear to have been skim-feeders, though. Might it
> also have been that the apparently increasing turbulence toward the
> end of the Cretaceous may have provided favorable conditions for large
> soarers (more appropriately perhaps -- motor-gliders)?
Not all, no, and the South American finds seem to support the idea that
medium sized fish-snaggers and so forth weren't under the same pressure
to get large as the plausible skim-feeders.
The correlation with size that I think is most interesting is that of
travel distance; with current sea birds, ability to travel from land
correlates pretty well with size, with the larger the bird, the further
out it can go to feed.
That's not of all that much use with large oceans, since deep benthic
water doesn't generally have a lot of food floating around on top of it.
(there are exceptions, but those may depend on the energy gradients
caused by glaciated poles, and there's no obvious way for a flyer to
suddenly start feeding on krill hundreds of miles from shore, there has
to be a pathway to the food source.)
With something like the inland seas, though, they'd be relatively
shallow all the way across, with an active upper layer irrespective of
distance from shore. The ability to get out further would have
translated to reduced feeding competition from other shore dwellers; if
you can get to the school of anchovies 300 miles out, and all the
smaller pterosaurs and birds can't fly that far, you're *way* ahead on
food source competition.
To maintain the end is to uphold the means.