Collective answer to Dora Smith...

[David Marjanovic <david.marjanovic@gmx.at>]

----- Original Message -----
From: "Dora Smith" <villandra@austin.rr.com>
To: <david.marjanovic@gmx.at>; "DML" <dinosaur@usc.edu>
Sent: Sunday, May 30, 2004 2:43 AM
Subject: Re: "Dinosaurs Died Within Hours After Asteroid Hit Earth..."

> What about them cow-sized herbivorous mammals?

:-) When you write twice within 13 minutes, and that when it is between half
past 2 and a quarter to 3 at night over here (Austria), then you can't be
surprised when I don't give you an answer! Well, now that I'm awake and have
some time... I've never heard of this idea, and it's obviously nonsense,
because monotremes, marsupials and placentals all survived the K-Pg boundary
independently (as shown by the occurrence of representatives or close
relatives of those groups in the K). There is also no evidence for a
cow-sized or even pig-sized mammal in the K.

>From other posts:

> But still it seems a stretch that just one line of coelorusaurian
dinosaurs
> survived when so many lines had evolved in very similar ways.   Apparently
> more than one sort of mammal survived!

Apart from modern birds (Neornithes -- the group to which all living birds
belong), no coelurosaurs whatsoever have so far been found in strata younger
than the K. So... it doesn't seem a stretch.

> These definitions of Aves that have been provided all define the class in
> terms of common descent from the unknown nearest common ancestor.   My
> question is, how do we know that this nearest common ancestor was far out
> some narrow limb of the coelorusaurian family tree?

To cut a not much longer story short: because birds share more derived
features with some coelurosaurs (like dromaeosaurs and oviraptorosaurs) than
with others (like *Compsognathus* or *Sinosauropteryx*).

> Did all toothed birds die out before the end of the Cretaceous?

The youngest known bird tooth forms part of an ichthyornithid fossil that is
barely older than the boundary. So IMHO it's more likely that they died out
_at_ the K-Pg boundary. But bird fossils are rare in general, so the
statistical uncertainty attached to this statement is not negligible.

> 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.

You mixed a few things up here. First of all, we can be reasonably certain
that *Archaeopteryx* itself was not a direct ancestor of any known species;
this is because it's highly unlikely to find just that fossil, considering
how rare fossils are compared to the amount of organisms that lived at any
one time, and because old Archie has a few derived features of its own,
which would necessitate the additional assumption that those features were
lost again when Archie evolved into other birds. Secondly, a few people do
think that *Caudipteryx* -- and all other oviraptorosaurs -- are more
closely related to birds than Archie, but this is independent of Archie
being a dead end or not.

> I know that all other theropods including caudipteryx and the two other
> bird-like ones

Which ones do you mean?

> differed more from modern birds than archeopterix did,

Many did, some may not have...

> but not by much;

yep

> what is more, they and several other therapod dinosaurs were
> undergoing structural changes like those that modern birds have;

Not in parallel. Their common ancestors with birds had undergone those
changes, and they just kept the resulting features.

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

There is no uncertainty that *Confuciusornis* is indeed a bird. Well, I
think there's one man on the entire planet who thinks otherwise... :-)

> Partly because their
> feathers and other features might make them necessarily descended from
> whoever the nearest common ancestor whose existence defines class Aves
was.

Most to all of those features (feathers are a good example) were simply
already present in their nearest common ancestors _with_ the nearest common
ancestor of Aves.

> Do we even know that Parrot-jaw belonged to class Aves?

Well. It seems not to be an oviraptorosaur. It seems not to be a turtle. It
seems not to be an ornithischian. I'd bet money it isn't a monotreme. I'm
almost as sure it isn't an ornithomimosaur either. At the moment at least, I
can't think of more possibilities...

> unless again it was one of those coelorusaurian dinosaurs who could
> also be descended from the mysterious nearest common ancestor of all
living
> birds that defines class Aves.

In this case it would by definition belong to Aves. As you yourself wrote
earlier...

> Oh.  So if the most recent common ancestor was at the root of the theropod
> line, then all therapods would belong to Aves.

Exactly.

Hope this helps! :-)