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EXTRAPOLATING TRENDS FROM LACKING DATA



Jaime Headden wrote: 
 <<Even with *Alxasaurus*, we do see a trend in the caudals to become 
 much smaller, and much sooner than in your other, run-of-the-mill 
 theropods (like carnosaurs, or dromaeosaurs).>>

Certainly, I am no denying that.  The only preserved tail to date known from
any therizinosaur does seem to somewhat shorter than the tails of similarly
sized "normal" theropods.  In theropods as a whole, there does appear to be a
"trend" in tail reduction from say coelophysoids to enantiornithines, a lot of
which has to do with the reduction of the caudofemoralis muscle and the
transition to a "calf-based" locomotor system rather than an MCF based system.

 <Anyways, I never thought a therizinosaur had a sloth-like tail. To me, the
image of 
 such a short-tailed theropod is utterly bizarre (well, more so than 
 other "bizarre" theropods).>>

And again, the image is unwarrented.  _Alxasaurus'_ tail isn't very short to
begin with and is illustrated to end where the vertebrae stopped being
preserved and not actually where they stopped growing.  In other words, the
tail is longer than is shown in Russell and Dong, yet is not reconstraucted in
their illustration.
 
There is no reason to believe there was any sort of trend in tail shortening
in therizinosaurs, the evidence is simply lacking.  To reiterate, all we have
is a single partial tail from a single species which is only marginally
shorter than that found in most theropods.  This could simply be an
autapomorphy for _Alxasaurus_ itself.  Without any other therizinosaur tails,
I would be extremely weary in talking about any sort of trends in tail
evolution, especially one so weird as Russell and Russell's sloth-o-saurs.

Peter Buchholz
Tetanurae@aol.com

Yes.  I did it.  I killed Yvette.  I hated her sooo much, it was like flames
on the side of my face...