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



--- Tim Williams <twilliams_alpha@hotmail.com>
schrieb:

> Dora Smith wrote:
> 
> >I was wondering when it would occur to anybody that
> post-Cretaceous birds 
> >might not be just one line of therapods.
> 
> Alas, there's no evidence that any theropod group
> other than the Neornithes 
> (including lithornithids) survived into the
> Cenozoic.

To which might be replied that Neornithes is
monophyletic by definition, but ratite phylogeny is
still too badly understood to say exactly how deep the
paleognath-neognath split is.

Add to that the major enigma that is ostrich
paleobiography... I can live with paleognath
monophyly, but ratite monophyly (i.e. Struthioniformes
sensu lato), while I'd not actively advocate against
it, gives me a major bellyache.

Supposing the paleognath-neognath split goes back to
c. 100 mya (or even deeper?), then Neornithes becomes
a rather arbitrary assemblage united more by C-T
survivorship than by a robust cladistic model. What
they apparently did was to outcompete the transitional
paravian flyers in the Early Cretaceous (although the
Enantiornithes certainly had a larger share in that),
like Confuciusornithes and Archie's hypothetical
descendants, i.e. the downstroking flap-gliders.

4-winged Microraptor had no problem competing with the
advanced "true" avian flyers it appears. So 150-120
mya, there must have been a minor radiation of
primitive more or less bird-like, feathered theropods
because they were beginning to be displaced already by
more advanced flyers, both avian and non-avian (sensu
stricto) after that time.

Of course there is still no fossil record to make this
more than a hypothesis, and exactly the same goes for
the say 30 my before Archie. But it appears that the
entire theropod lineage and most so the maniraptorans
were extremely prone to produce lineages which evolved
various mosaics of various degrees of "bird-ness"
between roughly 170-160 and 110-100 mya. After that,
basically there was not much new... although there is
always Vorona.

And speaking of mosaics, has anyone ever advanced a
satisfying theory for the evolution of hesperornithid
dentition? Is there something even remotely alike? The
old papers at least don't suggest so.

Regards,

Eike


        

        
                
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