[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: EVOLUTION OF BIRD



Message text written by INTERNET:gbabcock@best.com

>>there are neither illustrations nor photographs
anywhere in the book to show the position of the _Protoavis_ bones in
situ<<

        This has been a long-time criticism of Chatterjee's approach to
presenting _Protoavis_ to both the public and the scientific community.  It
has been implied that Chatterjee removed all the bones from the matrix
before any sort of map was made of the taphonomic layout of the elements,
so such taphonomic information has been lost.

>>How confident can we be of the assignment of
pre-Archaeopteryx tracks to Aves?  On pages 137-138, Chatterjee depicts and
describes tridactyl and tetradactyl prints which show a wide divarication
angle between digits II and IV, caudally directed halluces, and slim claws.
 Among these footprints are _Plesiornis_ from the Late Triassic Manassas
Formation of Virginia and _Trisauropodiscus_ from the Early Jurassic
Portland Formation of Massachusetts.  Can we tell from the tracks alone
that these were birds?  Can we formulate valid generalizations regarding
the range of divarication angles of the phalanges of small non-avian
coelurosaurs?<<

        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.

                _,_
           ____/_\,)                    ..  _   
--____-===(  _\/                         \\/ \-----_---__
           /\  '                        ^__/>/\____\--------
__________/__\_ ____________________________.//__.//_________

                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 899-2809
                  Fax:  (505) 841-2866
               102354.2222@compuserve.com

-------------------- End Original Message --------------------