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Re: Archaeopteryx and Flight
> > > 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
> > morphological varient of primitive feathered 'birds' that were able
> > 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.
Well this expert does not think it is untenable in the context I have
stated. Perhaps the use of the term 'hopeful monster' was incorrect,
but I stand by my interpretation on evolutionary processes. I have
seen enough evidence to satisfy me that 'mutations' can take over a
population if it is a large enough population and the conditions
change to favour that 'mutation'. I also never said that evolution was
an individual processes, far from it. The whole point about the
'mutation' process is that it is in the genes of the population to
produce this varient and that it occurs as a small percentage of the
entire population. If the population is large enough to produce large
numbers of these varients then it is possible that they will become
the more dominant variety in particular environments.
> 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).
Birds may now fill the niches that pterosaurs once held, but this in
itself is not evidence for direct competition between them. As far as I
know there is no evidence that birds did compete openly with
> > The fact that flight has been *lost*
> > in island species may be more to do with the lack of terrestrial
> > competition that allows the non-flighted varients (which may be
> > dominant) to become the sole form. The islands may not have
> > populations to allow enough (if any) flighted varients from
> > 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.
OK, I forgot about the predators, but the point still stands. The
non-flighted varients can become dominant again due to the lack of
terrestrial based competition and predators. It was probably the more
specialised birds that retained flight on the islands. Perhaps
someone can correct me if I am wrong in this assumption.
> > > 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
> > birds that caused their demise. The specialisation of the
> > hae been the cause of their downfall at a time when birds were
> > 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!
I'm sorry, I don't know when the 'fly-catching' pterosaurs died out.
Was it at an extinction event horizon or nearby? It still does not have
to be from competition with the birds as they were probably still
mostly generalists or opportunists at that time.
> 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.
I don't think that there are enough fossils to swing this debate one
way or the other at the moment. I would like to know why we don't
have more fossils of birds with fossils of pterosaurs if they were
fighting it out. Sorry, the last statement may seem a little crass, but I
think that without enough fossil evidence it is difficult to come to any
agreement on this debate. The problem with a lot of vertebrate
palaeontology (as opposed to arthropod palaeontology) is that there
just ain't enough fossils.
> 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.
I'm not sure about that one either, but I guess you have more fossil
evidence for that one. You see gradual take-overs in arthropods too,
but I don't think they were interpreted as competitive take-overs.
More, it seems, a varient that is better adapted to the changes in the
environment that takes over to become the dominant form.
Curator of Palaeontology
University of Glasgow
Mountains are found in erogenous zones.
(Geological Howlers - ed. WDI Rolfe)