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I wrote:
<<They had quite short tails, as did some of the larger ornithomimids
(slightly longer relative to body size, but comparatively short 
nonetheless). Also, therizinosaurs.>>

Peter Buchholz wrote:
<I really must protest to the current restorations of therizinosaurs as 
being short-tailed sloth-o-saurs. Everyone must understand that no 
complete caudal series for any therizinosaur has ever been found. The 
most complete is that for the type of _Alxasaurus elesitaiensis_ which 
has 19 vertebrae preserved in articulation and 2 more nearby. Russell 
and Dong's reconstruction of the whole animal stops at caudal 22 even 
though it had to have at least 23.>

  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).

<Simply from an aesthetic perspective this seems unlikely because it 
ends blunter than the vertebrae and chevrons would suggest it should.
Additionally, no other non-avian theropod (or dinosaur for that matter) 
has such a hypertrophied tail, although it has been suggested for 
caenagnathids where, again, no complete caudal series has been 

 As the caudal trend continues, there is an extreme shortening of the 
haemal arches, and the neural spines, plus the caudal centra also reduce 
in length far sooner than the above average theropod. This suggests to 
me that the tail (of at least *Alxasaurus*) ends 20% sooner than [again] 
the above average theropod.

<This is taken even further in a paper by Russell and Russell where they 
present a composite therizinosaurid reconstruction based on 
_Therizinosaurus cheloniformes_ (forelimbs and feet), _Erlikosaurus 
andrewsi_ (skull), and _Nanshiungosaurus brevispinus_ (vertebral 
column). In their reconstruction they shorten the length of the linearly 
scaled up tail (based on the incomplete and already too short tail of 
_Alxasaurus_) by 75% because they believe that there is some sort of 
morphoclinic trend of tail reduction in therizinosaurs. I cannot see the 
underlying logic in this because, again, all we have is a partial tail 
from a single individual: how that indicates a morphoclinic trend in 
tail reduction is beyond me...

I will give you that _Alxasaurus'_ tail is smaller than is seen in most 
theropods, but not to the extremes seen in Russell and Dong or Russell 
and Russell. Addittionally, although _Nanshiungosaurus'_ pelvis does 
seem to show the caudals departing the sacrum at an angle of about 25%, 
which would seem to indicate a higher walking stance than what is seen 
in most theropods, that does no mean _de facto_ short tail. 
Therizinosaurs, most likely, had fairly long "normal" theropod looking 
tails, there simply isn't any reason to think

  I look at those therizinosaurs, and think "How would the animal best 
be balanced, long or short tail?" I would consider that the 
therizinosaur had the ability to extend its neck horizontally, perhaps 
even while walking, and thus would require a longer tail. 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).

  The condition, BTW, of shortening centra, haemals, and neural spines, 
is seen in the *Ingenia* at Nakasato, so a condition for short tails 
(not by a sloth's standards, but short nonetheless when compared to 
other [again] average theropods) goes into the realm of oviraptorosaurs, 
though all may not have had them, and I actually think caenagnathids had 
longer tails than otherwise shown, and that therizinosaurs, 
ornithomimosaurs, and oviraptorosaurs (oviraptorids) all developed a 
trend for tail shortening, independantly of each other.

Jaime A. Headden

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