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Re: Exaptation



On Thu, 7 Dec 1995 Dinogeorge@aol.com wrote:

> In a message dated 95-12-07 06:52:50 EST, Robert.J.Meyerson@uwrf.edu (Rob
> Meyerson) writes:
> 
> >My biggest reservation for BCF is the unlikelyhood of the events. =
> > Essentially, what is being said is that an animal gains the power of=
> > flight, then gives it up again.  What's the adaptive value in that???  I=
> > mean, no one talks about secondarily flightless pterosaurs.  Once
> theropods=
> > learned to fly, a whole new set of niches opened up for exploitation, and=
> > it seems to me that the wouldn't be able to reverse the trend until the=
> > evolutionary momentum had run down.  Evolution may be random usually; but=
> > once a trend has started, the group is usually committed to that trend to=
> > the end.

Just a thought here--weren't pterosaurs already fully accomplished fliers 
by the late Triassic?  I seem to recall that _Preondactylus_, 
_Peteinosaurus_, and some others had already made a strong showing by 
this point (?_Eudimorphodon_).

If this is the case, then evolving protobirds weren't necessarily 
expanding into a whole new set of *unoccupied* niches by developing 
flight.  Perhaps secondary flightlessness appears so commonly (by BCF) 
because of competition with pterosaurs, who may have already been 
occupying those niches.

Flight is, after all, pretty expensive metabolically.  One might well 
imagine that it would be lost pretty quickly if it confers little 
survival advantage.

Just an idea...

--Dennis
dchwang@itsa.ucsf.edu

> But flightlessness occurs time and again among modern bird lineages! And
> these include some of the best-developed fliers in the animal kingdom. How
> much more likely would secondary "flightlessness" then have been in lineages
> that were not nearly as highly derived fliers as modern birds? In lineages of
> climbing and gliding animals?
> 
> Actually, as the central avian lineage approaches modern birds, the rate of
> appearance of flightless branches seem to decrease. There were
> hesperornithiforms, _Patagopteryx_, perhaps alvarezsaurids and avimimids.
> Compare this to the wealth of herrerasaurians, ceratosaurians, carnosaurs,
> dromaeosaurids, oviraptorosaurs, ornithomimosaurs, and so forth, which BCF
> asserts branched off mainly before the advent of powered flight. Your thesis
> may be more correct than you think.
> 
> Regarding secondarily flightless pterosaurs and bats: it's entirely possible
> that they existed but haven't yet been found. I've heard intriguing rumors,
> but nothing concrete yet. Meanwhile, you might want to give some thought to
> how you might identify a flightless pterosaur in the fossil record.
>