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Re: Barney vs Tweety



>Dear Group:
>     I have always had concerns with the relationship that birds have with
>dinosaurs.  It is difficult to pick up any dino literature which does not
>address theropods evolving into present day birds (I realize this is an over
>simplification).   Sure birds have phylogenetic trends similar to dinosaurs
>but why did evolution take this path??  Dinosaurs sizes where so diverse but
>birds today do not exhbit this diversity.  I understand that speciation
>events can lead to morphological changes (the horse for example) but it seems
>to me that when we look at fossils that we are seeing only "instant in time"
>so how can we be sure that these two groups did evolve separately and simply
>share common characteritics (homoplasy) because they lived in the same
>ecosystem??   Any thoughts??

This thought has been voiced for over a century.  It IS possible that the
similarities between birds and dinosaurs are all homoplastic, in the same
way that the shared characters between apes and humans, or placentals and
marsupials, or crows and ravens COULD be homoplastic.  However, this would
require, in all cases, fantastic levels of convergence, in some cases in
parts of the anatomy which are not directly relatable to similar modes of
life (relationships of some cranial bones, for example).  By far the most
parsimonius explanation is that birds arose within theropod dinosaurs.
Take a good look at an Archaeopteryx fossil and a dromaeosaurid fossil
sometime - they are truely, remarkably similar.

Fossils are only snapshots of evolutionary history, but they can tell us a
lot.  If you compared just finches and Diplodocus, for example, there isn't
much similiarity there.  However, our fossil record is a lot better than
that, and so we find much more bird-like dinosaurs and much more primitive
birds (dromaeosaurids and Archaeopteryx, for example).

Evolution takes whatever path is open (differential survival of variants in
a population, and all that).  The evolution of Archaeopteryx from a
primitive maniraptoran ancestor isn't that much more remarkable than the
evolution of the ceratopsians from their primtive marginocephalian
ancestors - for example, in both cases new structures arose (feathers and
rostral bones, respectively, and we really don't know that the ancestors of
Archaeopteryx WEREN'T feathered...).

Birds represent only a single line of dinosaurian stock, and do show quite
a bit of diversity themselves (humingbirds to penguins to Aepyornis to
Teratornis to woodpeckers to etc., etc., etc.).

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.