Re: "Dinosaurs Died Within Hours After Asteroid Hit Earth..."

["T. Michael Keesey" <mightyodinn@yahoo.com>]

--- Dora Smith <villandra@austin.rr.com> wrote:
> The way I understand it, although many scientists think that archaeopterix
> is ancestral to birds, others think that they were an evolutionary dead end
> and not necessarily more closely related to birds than caudipteryx.

It would be better to say that many scientists think _Archaeopteryx_ is *close*
to the ancestry of _Aves_ (sensu Chiappe). Finding actual, confirmed ancestors
is pretty nigh impossible with the fossil record as it is.

> I know that all other theropods
(Assuming you mean non-avialan theropods?)
> including caudipteryx and the two other bird-like ones
Which ones?
> differed more from modern birds than archeopterix did, but
> not by much;
Depends on the theropod. _Coelophysis_, for example, differs pretty greatly
from birds.

> what is more, they and several other therapod dinosaurs were
> undergoing structural changes like those that modern birds have; in bone
> structure, foot structure, arm and wing structure, tail structure, and
> feathers.

The characters you mention are not examples of convergence, but of shared
ancestry. The traits we think of as "birdy" did not arise all at once.
Bird-like feet originated early on in _Theropoda_, heavily pneumatic skeletons
arose in early _Saurischia_, wings arose in _Maniraptora_, feathers arose
within _Coelurosauria_, etc. Certain non-avian theropods have these traits
because of shared ancestry with birds, not through convergent evolution.

Okay, *one* example you give is of convergence: tails with pygostyles appear to
have evolved twice, once within _Avialae_, and once within _Oviraptorosauria_
(although so far only known in _Nomingia_).

> I notice that, for instance, there is uncertainty that Confuciornis is a
> bird and not a non-avian therapod.

???
News to me!

(Well, okay, if you use Gauthier's crown clade definition of _Aves_, it would
definitely be a non-avian theropod, but then so would _Hesperornis_,
_Ichthyornis_, _Enantiornis_, etc. This is a semantic issue, not a phylogenetic
one.)

> In addition to uncertainty that
> caudipteryx and several others were not birds. Partly because their
> feathers and other features might make them necessarily descended from
> whoever the nearest common ancestor whose existence defines class Aves was.

I think it is now the general consensus that _Caudipteryx_is oviraptorosaurian.
There is a minority view that oviraptorosaurs are avialan, but they are more
commonly thought to be the sister group to therizinosaurs.


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