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Re: Therizinosaur track from Denali National Park, Alaska



Jaime Headden <qi_leong@hotmail.com> wrote:


> If Fiorillo & Adams mean four weight-bearing toes to refer to "forward 
> facing" toes, they should be including quite a large range of possible 
> candidates, including
> caenagnathids, troodontids, some odd short-sickled dromaeosaurid, etc.. Only 
> therizinosaurids, however, appear to have four _weight-bearing_ toes, with 
> hypertrophism
> of MTI and pdII. In this respect, oviraptorids, nor any other four toed 
> theropod, lack the morphology to easily produce the tracks.


Yes, I thought the mention of oviraptorids was a little incongruous.
A functionally tetradactyl foot (in a locomotory sense) is only found
in therizinosaurs among non-avian theropods.  In derived
therizinosaurs the pes was completely overhauled: metatarsals I-IV
became very short and very thick, and metatarsal I and its digit were
hypertrophied for weight support.  But metatarsal I still inserted
fairly high on metatarsal II - but the shortness of the metatarsus
meant that the hallux now contacted the substratum (ground) for weight
support.


The tetradactyl therizinosaur foot is very different to the
tetradactyl foot of _Epidendrosaurus_/_Scansoriopteryx_, where
metatarsal I descended to such a degree that the first toe inserted at
around the same level as the other three toes.  Although the first toe
was capable of contacting the substratum just as well as the other
three toes, all the toes were long and slender.  This *might* have
been a scansorial adaptation.  As for the tetradactyl foot of
_Balaur_... that's another story.


If large, graviportal birds existed in the Cretaceous they might have
left four-toed tracks.  (Unfortunately, the pes of  _Gargantuavis_  is
unknown.)


Brad McFeeters <archosauromorph2@hotmail.com> wrote:

> *Balaur* also has an enlarged, forward-pointing pedal digit I.  The Alaska 
> track is missing the tips of digit I and II, so we cannot be sure they did 
> not have the large raised
> claws of a similar dromaeosaurid.  Now THAT would be some fascinating 
> biogeography...


I agree.  Because of the large raised claws, I suspect _Balaur_ would
have left two-toed footprints, with the first and second digits held
above the ground duting walking.  Sickle-clawed dromaeosaurids such as
_Velociraptor_ and _Deinonychus_ would have held the specialized digit
II above the ground when walking.  In _Balaur_, both digits I and II
were highly specialized for predation. So in this sense the foot of
_Balaur_ was tetradactyl.  However, other dromaeosaurids and
troodontids could apparently use all four toes (including the hallux)
for grasping during predation (Fowler et al., 2011).  The large,
forward-pointing digit I of _Balaur_ obviously had a far more
specialized function compared to the typical dromaeosaurid condition.


An ungual-bearing hallux is present in most non-avian theropods.  This
digit lost contact with the tarsus, and migrated down the foot in the
course of theropod evolution.  Thus, the hallux might have had some
function prior to be being mobilized for more specialized functions in
various lineages (e.g., attack, grasping, weight support, climbing?).
So terms such as "functionally tetradactyl" often require
qualification.



Anthony Docimo <keenir@hotmail.com> wrote:

>> > (if less than two, how is that a third group?)
>> By the "other group" I meant birds in general. Sorry for not making
>> that clear. Pamprodactyly has evolved several times in the Avialae.
>
> Thanks for clarifying. So, saying "pamprodactyl groups" is just 
> paraphletics(sp) , rather than anything meaningful?


Pamprodactyly is polyphyletic.  So it is not *phylogenetically* meaningful.







Cheers

Tim