Re: Bird vs. theropod dinosaur tracks/trackways

["Ralph W. Miller III" <gbabcock@best.com>]

Among the differences between non-avian theropod dinosaurs and extant birds is
that the earlier theropods attached a good amount of thigh muscle 
(caudofemoralis)
to the tail, whereas modern birds attach more leg muscles to their broad hips 
and
caudally directed pubes; birds would tend to flex the knee more and the hip 
joint
less than non-avian theropods would.  The shortened, fused caudal vertebrae of 
the
modern bird -- the pygostyle -- are decoupled from terrestrial locomotory
functions, and are utilized instead in flying (except in flightless forms, of
course).   Pelvic remodeling is evident even in a comparison of flattened 
fossils
of non-avian theropods and early birds, in that the former are typically 
flattened
laterally and the former dorsoventrally.  So, in spite of homology, change
happens.  With the legs astride a broader body, and flexing at the knees more 
than
the hips, it is not surprising that the tracks of modern flying birds should
differ in some specifics from the tracks of non-avian theropod dinosaurs.  On 
the
other hand, the more primitive birds and the more bird-like non-avian theropods
should have tracks which are not so easy to distinguish.  I wonder what
_Caudipteryx_ tracks would look like.  See Paul's _Predatory Dinosaurs of the
World_ for a much better treatise on the subject of bird and theropod anatomy 
and
locomotion than I can offer.

How confident can we be when assigning fossil tracks to non-avian versus avian
theropods?  How can such interpretations be proven?  The footprint of a modern
rhea, if discovered in rocks of Mesozoic age (I'm obviously speaking
hypothetically here), with its narrower angles of phalangeal divarication, would
undoubtedly be taken to be that of a non-avian dinosaur.  And might not some 
small
fossil "bird" tracks in fact belong to non-avian theropod hatchlings?
Chatterjee's _The Rise of Birds_ displays bird tracks dating back to the 
Triassic
(in the case of _Plesiornis_).  Is this interpretation generally accepted?

-- Ralph W. Miller III       gbabcock@best.com

I invite all informed parties to post corrections to the above as needed.  It's 
a
dirty job, but...