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Re: Dinosaurs to birds



Matt    <Alien4240@aol.com> wrote:

> ... if dinosaurs evolved
> into birds, then they wouldn't be dinosaurs anymore, they'd be birds and you
> could just say that. Unless, of course, there's some other type of bird that
> DIDN'T evolve from dinosaurs, then I'd see a reason for it. Can someone please
> clarify? Thanks, and thanks to the responses to my last letter.

Thomas R. Holtz, Jr. could probably give you a proper course in the subject, 
but I'll give you the basic concept.

We evolved from earlier animals, vertebrates, amniotes, mammals, primates, 
apes, hominids...  We are highly derived (considerably modified
through the process of evolution over time) from the ancestral animal, the 
ancestral vertebrate, the ancestral amniote, mammal, primate, ape,
hominid,...  None-the-less, we are still animals, vertebrates, amniotes, 
mammals, primates, apes, and hominids, are we not?  Yes, we are.  (Note:
I'm leaving out many clades for the sake of simplicity throughout this post).  
Likewise, in spite of a bat's (chiropteran's) being very different
from the earlier mammals which ultimately gave rise to the bat family, a bat is 
a mammal!

And a modern bird which is descended from maniraptoran theropods would by 
definition also be a maniraptoran theropod saurischian dinosaur
archosaur diapsid reptile amniote vertebrate animal itself.  It would be a 
maniraptoran; it would be a theropod; it would be a saurischian; a
dinosaur; and so on.  So, not only are birds dinosaurs, but they are reptiles 
as well!

If you look at a cladogram (a modern "family tree" which arranges plants or 
animals according to synapomorphies -- shared derived features, or
significant inherited adaptations), you will see labels such as Theropoda, 
Saurischia, and Dinosauria which label the clades (complete groups of
related taxa which share the defining features).  The system is hierarchical, 
that is, the clades closer to the base of the "tree" are more
inclusive; they have more members.  There are more animals than vertebrates, 
more vertebrates than amniotes, more amniotes than reptiles, and so
on, because the latter groups are but subsets of the former categories.  Think 
of all the animals.  Not all are vertebrates, so obviously  the
taxon "Animalia" holds more members than the taxon "Vertebrata."  Likewise, the 
Reptilia include not only the turtles, lizards, snakes,
crocodilians, and the tuatara that now live on earth, but all creatures -- 
living or extinct -- which are descended from the original reptiles,
and this group also includes the mosasaurs, ichthyosaurs, plesiosaurs, 
pliosaurs, pterosaurs, dinosaurs, and the birds (to name a few).  Although
there are a few scientists who dispute that birds are descendants of theropod 
dinosaurs, the biologists and paleontologists who do accept this
genealogy (and these scientists are legion) and who use the modern system of 
systematics would tell you that, yes, birds are theropod dinosaurs,
as well as being reptiles, and that you are an ape (no offense).

There are online sources which can help you with this, including 
<www.ucmp.berkeley.edu/exhibit/phylogeny.html>.  Thomas R. Holtz, Jr. and M. K.
Brett-Surman coauthored a paper entitled "The Taxonomy and Systematics of 
Dinosaurs" which is in Farlow and Brett-Surman's _The Complete
Dinosaur_ and Currie and Padian's _Encyclopedia of Dinosaurs_ has lots of 
information on this topic, too.  For the novice, I would highly
recommend the book, _The Mistaken Extinction: Dinosaur Evolution and the Origin 
of Birds_, by Lowell Dingus and Timothy Rowe, which has many
illustrations and diagrams explaining systematic concepts, and deals 
specifically with the origin and radiation of birds.

Hope this helps.

Ralph W. Miller III   <gbabcock@best.com>