Re: secondary flightlessness (wordy)

[Jonathon Woolf <jwoolf@erinet.com>]

Dinogeorge@aol.com wrote:

> In a message dated 98-06-24 16:47:12 EDT, th81@umail.umd.edu writes:
>
> << Under phylogenetic-driven hypotheses, all that would be required to
> convince
>  me that _Caudipteryx_, _Protarchaeopteryx_, _Mononykus_, _Velociraptor_,
>  _Allosaurus_, _Apatosaurus_, and/or _Triceratops_ were secondarily
>  flightless is to demonstrate that the most parsimonius phylogeny places
>  these taxa within a clade whose ancestral state is unambiguously volant.
>  This is the same standard which evolutionary biologists working on other
>  groups of organisms employ. >>
>
> I, for one, don't think _Apatosaurus_ and _Triceratops_ were secondarily
> flightless. (Sheesh!) They probably, however, had a common arboreal ancestor
> with birds, way back in the Middle Triassic somewhere, when the first great
> cladistic split occurred in Dinosauria.
>
> Certainly life would be a lot simpler if it were always the case that we had a
> phylogenetic bracket. But in the evolution of avian flight--as in the
> evolution of avian endothermy, by the way--we do not. If we did, the case
> would already be closed in favor of theropod secondary flightlessness, and
> there would be no debate and no need for my attempts to convince anyone. (I
> am, of course, eagerly awaiting the discovery of a small, 50-cm-long Early
> Jurassic carnosaur with large forelimbs and feather impressions...)
>
> 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.

It's an intriguing hypothesis, certainly -- but how does it explain the
fact that
the birdlike Cretaceous theropods are all recognizably less derived than
_Archaeopteryx_?

> 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--

Why?

> 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 think the ground-up theory is rather absurd too -- but I also think
it's quite
likely that the small arboreal predator niche was colonized by dinosaurs
very
quickly and very successfully, perhaps as far back as the Triassic. 
These
tree-dwelling coelurosaurs were probably something like the "arbrosaurs"
in Dougal
Dixon's fanciful _The New Dinosaurs_.  Feathers then evolved first as
either
insulation or camouflage, then as display mechanisms, then became useful
for
gliding, and finally the arbrosaurs developed powered flight.

-- JSW