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Re: Stego/Ankylo limbs (really ceratopsian limbs)



From: Robert.J.Meyerson@uwrf.edu (Rob Meyerson)
 > 
 > Ultimately, one does not just take a track and from that make assertions on 
 > how
 > the animal moved.  One has to go back to the actual skeleton and see if it 
 > fits
 > the data. 

The two must be congruent.

However, since the trackmaking process is a behavioral process, while
skeletal fossilization is not, the tracks and traces are the only direct
evidence we have of behavior.

If we have a trackway (not just a single track), then we have sure
evidence that a certain behavior *actually* *happened*.

 > The point I tried to make from the trackway analysis I mentioned, is
 > that sprawling forelimbs don't necessarily fail to match the tracks.  I'm not
 > saying that this closes the book on anything, I am saying that sprawling
 > forelimbs must be considered a possible alternative, since it is possible 
 > that
 > even a sprawling mount matches trackway data.

I guess this depends on what you mean by "sprawling".  What is usually
meant is a stance with the pes/manus placed well outside the projection
of the torso.  This is NOT consistant with the trackway evidence -
it is fully rejected by the trackway evidence.

Now, if you include a stance with the feet tucked under the torso
but the elbows flexed outward in your definition of "sprawling",
then yes it is possible for a "sprawling" posture to match the trackway
data.  But most people do not consider this stance "sprawling".   This
is the stance I term "bow-legged".

There are four general types of stances:
        sprawling  = foot outside the torso projection
        bow-legged = foot under torso, elbow/knee flexed outward
        tucked     = foot under torso, elbow/knee flexed forward/backward
        full-erect = foot under torso, elbow/knee straight

The trackway evidence rules out the first, but is marginally consistant
with the remaining three. I say marginally because to accomplish the
second given the trackway data would require substantial outward rotation
of the wrist.

Simply put, the intermanual separation in the trackways is the same
as that of the "wide-guage" sauropods. Furthermore, the fore-aft axis
of the print is along the inner toe, or between the inner toe and the
next toe - the outer two toes pointing outward.  This puts the manus
under the outer edge of the torso, rotated outward. The posture I
called "tucked" above happens to produce this placement naturally
and automatically, without any twisting.

The full-erect posture would tend to be slightly narrower in guage and
to produce a more forward facing manus print. The bow-legged posture,
on the other hand, would tend to produce a manu print turned *inward*.
Thus, while with suitable twisting one can get the observed manus
trackways from all of the last three postures, the "tucked" posture
produces the observed tracks with the least effort.
 > 
 > Besides, how can we reliably match a track with a skeleton since the 
 > likelyhood
 > of the two belonging together is almost impossible.  ...

This is in fact the biggest step. However, the trackways in question:
        - match the foot shape and size of ceratopsians.
        - are *best* matched by the ceratopsians of known tetrapods
        - are the only known trackways that match the ceratopsian foot
        - occur in the same geographical areas and times as ceratopsians
        - are locally the most common trackways, just as the ceratopsians
          are often the most common in skeletal remains

Thus, the inference that they are indeed ceratopsian trackways is a
very solid inference indeed.

 >  People take trackways made by known small
 > theropods to deduce the lifestyle of _T. rex_.

Well, now that a handful of probable T. rex tracks have been found,
this is no longer necessary.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.