Re: What is a Dinosaur?

["Jerry D. Harris" <dinogami@hotmail.com>]

> But *Megalosaurus* and *Iguanodon* have certain characters. They will share
> some of those, and many of those with certain other animals which are
> _thereby_ identified as dinosaurs, and none others. Diagnoses aren't
> abolished, as Nixon & Carpenter seem to think (I don't know their papers),
> they are still there, just that they aren't called definitions anymore.

  Yes, I do understand and recognize this.  My point is simply that an 
animal cannot be recognized as belonging to any particular group of 
organisms _without_ resorting to examing the characters it has.  It really 
isn't possible.

  ...well, it may be _possible_, but it's not very good science!  After 
all, I could sit here and, without looking at any actual organisms, proclaim 
that there is a clade called "Zorkae" that is the common ancestor of the 
zebra, the hummingbird, a trypanosome, and some organism that hasn't been 
discovered yet called a blarg, and all of its descendants."  Because I've 
said it, it is thus established.  You heard it first here, folks!  Now I go 
dig up a previously undiscovered fossil organism.  I sit and stare at its 
remains.  Do any of these remains say "I am a member of the group Zorkae 
_because_ I am a descendant of the most recent common ancestor of the zebra, 
hummingbird, trypanosome, and blarg!"?  Of course not.  What the remains 
_do_ say is "I've got this fossa, that process, the other limb:body ratio, 
etc., etc."  Once I've recognized those characters (and, hopefully, some 
autapomorphies!), I can start comparing it to other organisms and get a feel 
for what its relatives are.  (Very phenetic, yes.)  And lo and behold, it 
looks kind of like a blarg.  So now I can plug it into some analysis 
(cladistic or otherwise) where I include a zebra, a hummingbird, a 
trypanosome, a blarg, and other taxa that may or may not belong to the 
Zorkae.  Let's say that the results show that yes, indeed, this new animal 
is a member of the Zorkae.

  The point is, _I_didn't_know_ that it belonged to this group _until_ I 
examined the characters and plugged it into an analysis.  It would have been 
impossible to assess a priori whether or not it belonged in this group 
_without_ examining the characters.  In other words, whether or not a 
phylogeneticist wishes to pretend that s/he can divorce the phylogeny from 
the physical characters, the phylogeny cannot be assessed without them.  
Thus, the characters should be accorded as much philosophical weight as all 
the theory behind lineage-based phylogenies.  That's all I'm sayin' -- a 
dinosaur isn't a dinosaur (just) because it is descended from the same 
common ancestor as other dinosaurs, but (also) because it has 
characteristics of, or derived from, those of that ancestral organism and 
its descendants.  We can't find that out until we analyze those very 
characters.  Phylogenetics _is_ still character based, whether we want to 
admit it or not.  Not that this detracts at all an ancestor-descendant, 
nested-group-based phylogeny, which I think is wonderful and certainly far 
more representative of reality (whether or not the phylogeny actually 
reflects reality or not!) than any previous system.  Just that we need to 
quit pretending that phylogenetics can be done without the characters.  I've 
declared "Zorkae;" now wouldn't you love to be able to run through 
descriptions of various organisms to see if they fall into that group or 
not?  You'll need to see my character list!  ;-D

> Definitions are the stable things for theory, while diagnoses are the
> changing ones for practice. Some HP had in his signature in 1994 "In 
> theory,
> theory and practice are the same; in practice, they aren't" :-)

  I agree 100%.  Definitions are stable, because they are designed to be, 
and that's fine (and applies to far more things than just Life!).  But 
_our_understanding_ of the definition -- or, more precisely, the contents of 
the definition -- does indeed change, with every new discovery.  Dinosauria 
may be "the most recent common ancestor of XXXsaurus and YYYodon and all its 
descendants" (or the roughly comparable stem-based phraseology), but we 
don't always agree on what does and does not fall under that definition.  If 
one scientist says that "ZZZoides" is a dinosaur and another one says it 
doesn't, the definition of Dinosauria hasn't changed (and, I should point 
out, neither has the diagnosis), but the group pointed to by the label 
"dinosaur" most certainly has, either way.  In other words, the _content_ of 
the definition changes, either with opinion, with new discoveries, 
reanalyses, etc., etc.  In this respect, a definition really isn't all that 
stable.  Moreso than the diagnosis, perhaps, but hey, science is all about 
change and constantly readjusting and fine-tuning our perceptions and 
conceptions about life, the universe, and everything.

> What about "'the common ancestor of [...], unless the almighty Commission
> should ever rule otherwise ;-) '; and 'those animals happen to share, as 
> far
> as the scrappy fossils allow us to say at the moment, G, H, I, J, and K, so
> we think that animals 1, 2, 3 and 4 belong into this group while 5 and 6
> don't, unless we find convincing evidence to the contrary'?

  Works well for me!  I only picked one particular phraseology to type due 
to lack of time (read: laziness).  ;-D

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 573-8373
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
http://www.sas.upenn.edu/~jdharris

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