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Re: Avian Monophyly (Was Re: Sheesh)... :-)



> Convergent evolution is one explanation.  Another
> explanation is that 
> critters like _Buitreraptor_, _Unenlagia_,
> _Rahonavis_ and _Archaeopteryx_ 
> share a similar body plan because this body plan is
> ancestral for the 
> Aves+Deinonychosauria clade (Paraves).
> 
> The idea that _Rahonavis_ is a flying dromaeosaur is
> certainly alluring, but 
> I have a feeling that future analyses will put
> _Rahonavis_ back in the Aves, 
> closer to _Archaeopteryx_ and other traditional
> birds.

But what to make of Vorona then, the ultimate mosaic
perhaps?

> BTW, something else that came up on this thread...
> If theropods like 
> _Caudipteryx_ and dromaeosaurs are demonstrated to
> be secondarily flightless 
> (i.e., descended from flying theropods) this does
> not necessarily mean that 
> they belong in Aves.  Aves is not defined by flight
> ability (not usually, 
> anyway).  It may be that flight is primitive for a
> much more inclusive group 
> (say Paraves, or even Maniraptora).  Thus, we could
> have secondarily 
> flightless non-avian theropods.

The "Birds Came First" model in a nutshell.

What is fairly certain (as certain as something can
get without hard - i.e., early/mid-Jurassic in this
case - evidence) is that there was quite some to and
fro between adaptations which are necessary though not
sufficient preconditions for maniraptoran (i.e.,
feathered, as functionally and physics-wise - vortex
generators etc - opposed to pterosaurian e.g.) flight
until 105-100 mya, after which time there seems to
have been some secondary flightlessness but very few
new takes on powered flight (of which Rahonavis IMHO
was one, hence its primitive features. It does not
really look like it had flying ancestors in the more
recent past, it's too much Archie by other means for
that, and that would imply it was *secondarily flying*
after evolving a fine non-avian theropod skeleton,
with "raptor claws" even as it seems. From its looks,
I find it a good case of ground-up).

If the period of more than seriously dabbling with
flight goes back to the first theropods, I'd be very
amazed. BCF is a feasible argument on basis of the
available data, but the evolution of Aves "sensu
traditio" (i.e., including Confuciusornis,
Rahonavis,...) in the Cretaceous suggests it wasn't.
>From some 100 mya on, to me it seems there was not
much except
a) Enantiornithes
b) Neornithes
c) the occasional oddball derived bird (e.g. Vorona)
d) quite a number of secondarily flightless modern
birds in all these lineages (Gargantuavis is a current
darling of mine. I would love to see a complete
skeleton being found! What is this... thing? An
"Enantiostrich"?!)
e) some derived non-bird flying theropods
(Microraptor)
f) the occasional new, primitive take on feathered
flight (Rahonavis).

20 my before that, the flying modes were less
advanced, but there must have been a larger "jumble"
of them, with an inordinate combination and variation
of the required and optional features and concurrent
adaptations.

The main problem I see with BCF is that lineage f
contains forms that would require secondarily
flightless *ancestors* and then evolve flight anew.
Possible, but nor parsimonious (which is a weak
argument, I admit).

Regards,

Eike


        

        
                
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