Re: Archaeopteryx and Flight

[Stan Friesen <swf@ElSegundoCA.ATTGIS.COM>]

 From: NCLARK@museum.gla.ac.uk
 > Reply to Stan's comments,
 > 
 > > Well, to start with, flight is metabolically expensive.
 > > So expensive that it is often *lost* in island species.
 > 
 > This may be so, but 'hopeful monsters' could easily have been an extreme 
 > morphological varient of primitive feathered 'birds' that were able to take 
 > advantage of an enironment in which competition was minimal. 

Hopeful monster type evolution is generally considered untenable
by most experts.  For one thing, evolution is a population process,
not an individual one, and individual variants must spread through
the population to become fixed.

Second, there *was* competition.  Pterosaurs had long since filled
most of the niches now held by birds.  And, except during a mass
extinction, niches do *not* get abandoned, especially lucrative
ones like eating insects on the wing. (Several species of
Pterodactylus were clearly capable of flycatching, for instance).

 > The fact that flight has been *lost* 
 > in island species may be more to do with the lack of terrestrial based 
 > competition that allows the non-flighted varients (which may be genetically 
 > dominant) to become the sole form.  The islands may not have large enough 
 > populations to allow enough (if any) flighted varients from becoming 
 > re-established.

Actually, in most cases it is the lack of *predators* that is
critical in losing flight on islands.  Birds that rely on flight
to feed *don't* lose flight on islands - it is only ground feeders
that use flight mainly to escape predators that become flightless.
 > 
 > > Pterosaurs did not start to decline until *after* birds evolved.
 > > In fact it is reasonable to conclude that it was competition with
 > > birds that gradually eliminated most pterosaur groups.
 > 
 > Yes, birds had evolved a number of forms, some flighted and some not, before 
 > the demise of the pterosaurs, but it may not have been competition with 
 > birds that caused their demise.  The specialisation of the pterosaurs may 
 > hae been the cause of their downfall at a time when birds were perhaps more 
 > generalistic and opportunistic.

The point is, why did the flycatching pterosaurs die out? That
life style is quite viable still, and has been since about the
Carboniferous.  it is not as if all pterosurs were in one lineage.
Just because the Pteranodontid and Dsungapterid lineages shifted
to fish eating and scavenging respectively is no reason for the
Pterodactylid lineage to die out!

As far as I can see, only competition would have forced the
Pterodactylids to extinction - unless you can make a case that
the Early Cretaceous extinctions were far more extensive than is
generally believed, enough so so that it is believable that major
niches like flycatching were left empty.


The gradual loss of diversity on pterosaurs starting in the
Early Cretaceous looks more like gradual competitive takeover,
such as happened when the rodents replaced the multituberculates,
than like the results of a mass extinction.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.