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Re: polyphyletic Alvarezsauria assemblage



>       I've been looking at the characters that supposedly unite
> Alvarezsauria, and all I can see is a bunch of homoplasic "noise".  The
> first two (small cervical epipophyses and neural spines) seem to be very
> subject to convergence and found in other groups.

Still doesn't automatically mean that that's convergent by default.
What about the keeled first caudal centrum?

> Procoelous caudals and
> sacrals pop up here and there in all kinds of archosauromorphs (incl. the
> bird Patagopteryx),

See below.

> and they grade into slightly procoelous-to-amphiplatyan
> forms in a variety of neotheropods.

Is that right?

> I find nothing convincing in this list
> of so-called synapomorphies.  The similarities appear to be pretty
> superficial and homoplasic.

A long list of characters that easily arise twice is still evidence of
monophyly. Fused frontals are a synapomorphy of Anthropoidea (IIRC) as well
as, I think, Carcharodontosauridae (I'm surprisingly tired today), and a few
other groups. It is just _less_ evidence than otherwise, but still, in the
absence of evidence to any contrary...

>      My conclusion is that Alvarezsaurus is fairly closely related to
> segnosaurians, and it would not surprise me if they form a clade.

Why?

> On the
> other hand, the differences between Alvarezsaurus and Mononykiformes are
> considerable, and I can certainly discuss those when I have more time.

Though I await this with interest, it's irrelevant for what they have in
common. Nematoda and Arthropoda have tons of differences and are still both
in Ecdysozoa, far apart from e. g. annelids. :-)

>      Order Mononykiformes, is much closer to birds, as is Family
Avimimidae.
>   It would not even surprise me if mononykiforms and Avimimus form a
> holophyletic group.  They are certainly far more alike than either is to
> Alvarezsaurus.

Please explain.

> It is really too bad Alvarezsauria is anchored on [...] Mononykus

(Is it really?)

>  This is a mess.  Sorry, but EVERYONE doesn't not
> think this is a holophyletic group,

But everyone _gets this result in their analyses_. Why is cladistic analysis
such a "powerful tool"? Among other things because it's not so easily
influenced by the personal opinions of those who do it.

> and what surprises me is that it has
> gone almost unchallenged after so many different cladistic analyses.

Because parsimony can't get easily around a list of _13 characters_ :-)