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Birds and dinosaurs



The following posting appeared on the paleo newsgroup sci.biol.paleontology
today:

As a cladist, I'd like to respond to some comments recently made in this
forum on cladistics, some of which were misleading.

According to strict cladistic methodology, only monophyletic groups are
recognized.  In other words, we only recognize groups that include the
common ancestor and *all* of its descendents.  Since the descendents of
the ancestral dinosaur include ceratopsians, ornithopods, tyrannosaurs,
and hummingbirds, all of these would be considered as members of a
monophyletic Dinosauria.

This does *not* mean that we no longer recognize birds (Aves) as a
separate group.  Birds are still birds.  They are also dinosaurs.  Aves
is nested within Dinosauria, just as it is nested within Reptilia.

I can understand the distress some may feel at this suggestion.  Clearly,
there is a big difference between a kingfisher and a brachiosaur. 
However, these artificial divisions only make sense if there are huge
gaps in our knowledge.  100 years ago, we had living birds,
Archaeopteryx, and a handful of generally very large and broadly
unbirdlike dinosaurs.  Given that data set, the gulf between bird and
nonbird was large, and it was simple to create two groups "at the same
level."  Since then, however, several key fossils have been found,
including the dromaeosaurids, oviraptorids, troodontids, Sinornis,
Mononykus, and the enantiornithines, all of which are beginning to fill
the gap between Archaeopteryx and (respectively) nonavian dinosaurs and
living birds.  We can no longer put our finger on the precise point in
phylogeny when nonbird became bird.  Just about the *only* thing
Archaeopteryx shares with living birds that are not known in nonavian
dinosaurs is feathers; hollow limb bones, the "bird foot," a wishbone,
and a characteristic wrist are all present at some level in nonavian
theropods, and such "modern bird" characters as a toothless beak and
clawless hand appeared much later.  Furthermore, we have no way of
knowing if advanced nonavian theropods, such as Velociraptor and Troodon,
had feathers or not.

Taking a group and separating it from its ancestors ("at the same level")
cannot be done when the gradual nature of these transformations is taken
into consideration.  Isolating mammal from nonmammalian therapsid,
tetrapod from fish, or bird from nonavian dinosaur becomes an exercise in
arbitrary splitting when the fossils are considered.

This brings up the issue of "rank."  It is true that some cladists -
including myself - no longer operate in the Linnean system.  However,
pegging taxa into ranks is *not* the stuff of systematics; what we are
interested in is relationships, and nature does not work "at the family
level" or "at the class level;" these are artificial constructs.  To
quote Jacques Gauthier, "species speciate, but genera do not generate." 
Cladistic classification is hierarchical, but we no longer recognize the
reality of supraspecific ranks.

I encourage all readers to read the new book on the AMNH dinosaurs -
which, contrary to comments elsewhere, is one of the best books on the
subject - or the recent all-dinosaur issue of Natural History.

chris brochu
dept. of geological sciences
university of texas at austin
austin, tx 78712

--------------------------------------------------------------------
Herewith, my reply:

This posting responds to Chris Brochu's comments on cladistic
classifications 7/7/95; quotes are taken from there, [bracketed] comments
in quotes are my own:

"We can no longer put our finger on the precise point in phylogeny when
nonbird became bird." Actually, we NEVER could, because even when there
were major gaps in the fossil record, all we knew was that that "precise
point" lay somewhere in the gap. It was always an arbitrary call; the
major gaps just made the call easier. A better way of distinguishing birds
from dinosaurs would be to arrive at a consensus about where to put the
dinosaur-bird transition point and then stick with it for a while. And if
a few creatures that we now call dinosaurs, such as _Deinonychus_ and
_Troodon_, happen to fall on the avian side of the transition point,
that's fine, too.

"Aves is nested within Dinosauria, just as it is nested within Reptilia
[which ain't a clade, by the way--old habits die hard, even among
cladists]." You could also say, "Dinosauria is nested within Aves," if you
broaden your definition of Aves to include the common ancestor of the
dinosaurs. The arbitrary nature of cladistic taxonomies comes out in the
assignment of well-known names to particular branch nodes in the
cladogram. Why is Aves assigned where it is, instead of, for example, at
the point where "dinosaurs" diverge from the rest of the archosaurs, or
somewhere else? I understand the "distress" some cladists might feel at
the suggestion that their taxonomy also carries this irreducible measure
of arbitrariness(!).

By the way, the AMNH dinosaur book constantly uses the irritating
construction "nonavian dinosaurs" when referring to those animals we all
know simply as "dinosaurs." This construction of course depends on where
the node Aves is located in the cladogram, which, as I noted in the
preceding paragraph, is arbitrary. When I talk about dinosaurs, I mean
those familiar Mesozoic beasts; I do not include robins and hummingbirds
in my discussion, and I do not like to be forced by an arbitrary cladistic
convention to have to account for these obviously different organisms.
Cladistically, _Tyrannosaurus_ is more closely related to robins and
hummingbirds than it is to _Triceratops_; but because it is not nearly as
far removed from the dinosaurian common ancestor, it is morphologically
closer to _Triceratops_, and so it makes more sense to consider them
members of the a single larger group.

"Taking a group and separating it from its ancestors (`at the same level')
cannot be done when the gradual nature of these transformations is taken
into consideration." Of course it can!  You just--DO IT. "Isolating mammal
from nonmammalian therapsid, tetrapod from fish, or bird from nonavian
dinosaur becomes an exercise in arbitrary splitting when the fossils are
considered." So what? Being arbitrary holds no terror for me.
"...[P]egging taxa into ranks is *not* the stuff of systematics..." Sure
it is! You bet!

"Nature does not work at the family level or at the class level." Of
course not; but my MIND does, and that is even more important to me in
terms of my ability to COMPREHEND the way nature works than the actual WAY
nature may or may not work.

This stuff can be debated endlessly, because of course it all boils down
to an issue of personal preference. If you are a cladist, then you believe
that taxonomies should be organized strictly by phyletic relationships.
But if you are a Linnaean, then you ALSO acknowledge the worth of keeping
organisms with distinctly different morphologies in distinctly different
taxa, and you also acknowledge that genuinely new levels of organisms can
arise from equivalent levels occupied by more primitive forms.


George Olshevsky
Dinogeorge@aol.com
PO Box 543 Central Park Station
Buffalo, NY 14215-0543

Comments, anyone?