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Re: secondary flightlessness
> We certainly know that avian flight evolved at >some< point in the evolution
> of birds from theropods. The problem is, when? So far, we keep turning up
> these Cretaceous birdlike theropods that can't fly, but since we know that
> something like avian flight was already present in the Late Jurassic
> _Archaeopteryx_, we know that a volant lifestyle was already being exploited
> millions of years earlier. It seems most natural to me to conclude--or at
> least to entertain the hypothesis!--that the flightless Cretaceous forms
> descended from Jurassic fliers. This is because I believe that avian flight
> took tens or scores of millions of years to evolve, slowly and
> incrementally--allowing for the existence of thousands of potential flying
> ancestral species--and because I think it is extraordinarily unlikely that
> avian flight developed from the ground up, working on large, heavy animals
> >against< gravity rather than on small, lightweight animals >with< it.
I would have to disagree on this point (sorry George!). I think that
flight most probably evolved quite rapidly. The earliest pterosaur
fossils show highly specialised fliers. So too with the earliest bat
fossils. There don't seem to be any (AFAIK) supposed "transitionary"
forms for bats or pterosaurs.
Secondly, I don't think feathered flight was necessarily so unique that
it developed just once. Take bats and flying foxes for instance. Two
groups that are not all that closely related, but which evolved into
similar forms. Just because Archae had feathers and seemed capable of
flight does not mean that it contributed in any way to the line that
led to modern birds.
I supose it comes down to what you define as a "bird". It is my opinion
that the best biological group descriptions are based on living animals.
An animal is not just a skeleton - it is also behaviour, appearance,
and genetics (which presumably control much of the other two). If
it were up to me to define Aves I would have it as the most recent
common ancestor of all extant birds. I wouldn't be surprised if this
was some time in the Cretaceous. That is not to say that there weren't
feathered, flying dinosaurs before that time, but in my opinion I
wouldn't necessarily call these "birds" (assuming bird=Aves).
At least we agree on something. I don't think flight developed from
the ground up. There are too many features that would be useful for
both a climber and a flyer, and few that I can think of that would
be useful for a flyer to co-opt from a fully terrestrial animal.
FFDCF - Flying Feathered Dinosaurs Came First ?