Re: EVOLUTION OF BIRD

[smithjb@sas.upenn.edu (Joshua Smith)]

Jerry D. Harris wrote:
> 
>         Attempts in the past have been made to use specific measurements of
> tracks to classify them into one group or another, but, like most
> ichnological procedure, this is an indicator, not proof, at best.  There is
> a lot of statistical overlap between avian and non-avian theropod print
> statistics, just as there is between theropod and ornithopod statistics. 
> Track morphology is also used, but again, there's a lot of overlap.  (In
> addition to the capacity of any individual to move toes around with respect
> to each other, thus changing things like divarication angles, there are
> also problems presented by substrate integrity as well as behavioral
> interactions between the animal and the substrate...all complicating
> factors in interpreting a track!)  As has been stated numerous times, the
> only time a track can be definitively associated with a specific trackmaker
> is when the dead body of the trackmaker is found at the end of the
> trackway, something which has never occurred (to my knowledge) with _any_
> verterbrate, let alone a bird.  8-C  So, we're left with some approximate
> indicators, and little if anything solid.  In short, no, we can't tell
> (where tell = prove) that these Late Triassic and Early Jurassic prints are
> of avian origin.  From body fossils, we know that theropods _were_ around
> in the Late Triassic and Early Jurassic, and have the best known candidate
> pedal morphology for the tracks, but there's always the possibility that
> the trackmaker is hitherto unknown from body fossils (could be a bird,
> could be something else altogether that has a bird-like foot).  At present,
> the most parsimonious conclusion is that the trackmaker was a theropod,
> given that we do have theropod fossils but no other good candidates in the
> time in question.  At best, the bird-like tracks imply that there was at
> least one kind of animal in the time in question that had a foot capable of
> making bird-like tracks in the substrates in which the tracks are found. 
> The prints _do_ contribute to the overall knowledge of theropod track
> statistics and morphologies, but any speculations derived beyond these
> basic facts cannot be toted as proof.
> 
        It may be parsimonious out west, but it is not necessarilly that 
way here in the east.  The Newark Supergroup has not produced much in the 
way of theropods, and has never produced a single theropod pedal element 
to my knowledge.  Even in the prosauropods we get from the Newark, there 
is a distinct lack of pedal data.  Most of the work that I have done, 
with Jim Farlow and Paul Olsen, on this subject, by necessity used pedal 
data from outside of the Newark Supergroup because they are simply absent 
from within.  This work seems to suggest theropod trackmakers for most 
of the ichnotaxa examined more strongly than non-theropods, but strongly 
enough for me to want to base anything on it.  The work is still in 
progress in many respects, and I would have to say inconclusive.  And 
almost nothing has been done in the Newark on anything that someone might 
want to call avian.


-- 
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Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
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