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THERIZINOSAUR TAILS



Aside from tiny plant eating dinosaurs with little tiny teeth, therizinosaurs
are my faves.

Jaime Headden 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.>>

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.

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 discovered.

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
otherwise.

Peter Buchholz
Tetanurae@aol.com

REFS:
Barsbold R and Maryanska, T.  1990.  Segnosauria. Pages 408-415 in:
Weishampel, D B, Dodson, P and Osmolska, H (eds) The Dinosauria.  University
of California Press, Berkeley.

Russell, D A and Dong Zh-M.  1993.  The affinities of a new theropod from the
Alxa Desert, Inner Mongolia, People's Republic of China.  Canadian Journal of
Earth Science 30:2107-2127.

Russel, D A and Russel, D E.  1993.  Mammal-dinosaur convergence.  National
Geographic Research and Exploration 9(1):70-79.