Re: Dino Birds (was Re: Dinosaur = extinct animal)

[Chris Brochu <cbrochu@fmppr.fmnh.org>]

For a good discussion on the basis behind birds being dinosaurs, I
usually recommend:

de Quieroz, K.  1995.  Phylogenetic approaches to classification and
nomenclature, and the history of taxonomy (an alternative
interpretation).  Herpetological Review, 26(2):79-81.

Basically, there's a difference between historical entities delineated
by common ancestry and classes of organisms sharing traits, but not
delineated entirely by ancestry.  A taxon, in the strictest sense, is a
historical entity, whereas other named groups are ecological
descriptors.

The example de Quieroz uses goes like this - we have a family of
peasants named Bauer, which is German for "peasant."  One member of this
family goes to town and becomes an accountant.  He is no longer a
peasant, but we would still call him a member of the Bauer family.

The same is true for groups of organisms.  Some are ecological
descriptors rather than taxa - "fish," for example.  "Fish" are
vertebrates living in the water, with fins and gills, and lacking
digits.  Basically, they're non-tetrapod vertebrates. We no longer
recognize a taxon named Pisces, as some "fish" are more closely related
to tetrapods than they are to other "fish" - so we evolved from fish,
but are not fish ourselves.  This does not make the word "fish" useless
- it's much easier to say "fish" than to say "nontetrapod vertebrate"
when discussing vertebrates living in the water, with fins and gills,
and lacking digits.  But it does not describe a natural biological
entity, because though its lower bound is unambiguous and natural (a
common ancestor), its upper bound is subjectively defined by people.

The taxa Osteichthyes and Sarcopterygii have been redefined to include
tetrapods, but that does not make us fish - simply members of taxa named
Osteichthyes and Sarcopterygii, just as an accountant may be named Bauer
without actually being a peasant.

"Reptile" and "amphibian" have more confused meanings in the
literature.  Both have been redefined in the phylogenetic system, but
not all accept these redefinitions, preferring to use them as ecological
descriptors instead.  I don't want to get into that one right now -
suffice it to say, there are valid points to be made on both sides.

The overall point is that paraphyletic assemblages, above the species
level, are subjective, have no biological reality, and are not
recognized in modern systematics.  To make a supraspecific taxon name
biologically meaningful, it must be monophyletic - and we either
redefine taxon names already in existence (with the risk of making them
mean something very different) or coin thousands of new names (with the
risk of confusing everyone).  It's a tough call, as placing a name on a
taxon is always subjective (though recognizing the group by itself is
not).  

Let me know if you need a copy of the abovementioned article.  It
explains things better than I could.


chris


Carl Ramm wrote:
> 
> Hello All,
> 
> Please forgive what may be a dumb or worn-out question, but I'm afraid the
> classifying of birds as dinosaurs just doesn't make that much sense to me.  My
> knowledge of cladistics isn't very good, so that may well be the problem, but 
> on
> purely logical grounds it is hard for me to grasp the reasoning involved.
> 
> If we are going to classify birds as dinosaurs because of their descent, then
> would we classify mammals as reptiles, reptiles as amphibians, amphibians as
> fish, etc.?  If not, what is the difference in the case of dinosaurs and
> birds?  If the idea is that the birds never "left" the dinosauria, then how do
> you ever "leave"?   If in fact we should consider mammals as a form of reptile
> (and thereby also a type of amphibian, fish, etc.) haven't we stretched the
> meaning of the terminology past the breaking point?  Wouldn't it be better to
> come up with a new terminology?
> 
> Carl Ramm

-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice: 312-665-7633  (NEW)
fax: 312-665-7641 (NEW)
electronic:  cbrochu@fmppr.fmnh.org