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Re: What Is Not a Dinosaur? (was re: Ankle Articulation in Pterosaurs)



At 04:34 PM 1/24/97 -0500, Dinogeorge wrote:
>Toward the
>beginning of the paper, Novas DEFINES Dinosauria to be the monophyletic group
>[...] But if he has already DEFINED Dinosauria to
>be monophyletic, then there is no reason to list characters that "support"
>its monophyly, as if its monophyly were something still in doubt, that still
>needed to be established.
        I believe that what Senor Dr. Novas was attempting to say (whether
he is aware of it or not) is "17 characters (synapomorphies) that support
monophyly of Dinosauria exclusive of other taxa in the study." or some other
such statement.
        While dinosaurs are commonly considered to be monophyletic exclusive
of the other taxa used in his study, many of the characters used to support
this conclusion are in doubt (as you point out).  It seemed to me that Novas
was try to review these characters and asses new ones in the hopes of coming
up with more solid character evidence in support of the exclusion of those
other taxa.  The excercise did feel a lot like the old papers which sought
to find diagnostic characters for taxa, and perhaps Novas himself may have
been a bit confused.  My head starts to swim when I work out the details.

>What Novas is really displaying are simply characters that seem to be common
>to the monophyletic group Dinosauria as he has defined it.
        Rather, they support the exclusion of taxa which are not commonly
considered to be dinosaurs.

>It seems to me that making the diagnosis of a taxon subordinate to its
>definition puts the cart before the horse:
        Q.V.  The oft cited DeQuiroz/Gauthier article, which goes to great
lengths to point out the undesirability of subordinating taxon definition to
diagnosis.  Most specifically in that diagnostic taxon definitions can lead
to paraphyletic or polyphyletic taxa.  Also, new discoveries can eliminate
all of your diagnostic characters.
        What do you do then?  Do you still have a taxon?  No, because the
taxon you defined on the basis of characters is not a real enitity, and
without it's diagnosis, it is nothing.  So you try to find new diagnostic
characters.  But in giving a new diagnostic definition, you are, in essence,
defining a new taxon.  Of course, the original way around this seems to have
been to define taxa based on diagnosis AND composition (which, as we all
know, changed constantly).
        So then you have a set of animals which you must find new diagnostic
characters for, in which case the diagnosis is subordinate to composition,
which doesn't sound at all right.  See what I mean?

>The devil with the characters; they're irrelevant.
>Toss them out, even though they're the only handle we have on whether a
>particular specimen might or might not belong to a particular clade.
        Yes, oh cauldron of irony :), but the characters only elucidate
phylogeny, they AREN'T the phylogeny itself.  By insisting that characters
define taxa, YOU are putting the cart before the horse.  I know it's an
hoary and easily dismissed example, but look at character states which are
convergent.  Should a convergent character be dropped from a list of
diagnostic characters?  Or should the diagnosis be dependant on having all
of the characters?  But then what if one of the other characters is
secondarily lost?
        Ok, so now you're going to say, well, we know that the fella who
reversed the character is decended from the common ancestor of this clade,
so we should include him.  But you can't do that, because you defined your
clade based on characters.  (oops, I forgot, George doesn't believe in
evolutionary reversals!;)
        So now you're left with making the diagnostic characters not
compulsory.  But how many have to be present?  What if a sister-taxon shares
some of them.  Will you simply whittle the character list back until there's
only one or two characters.  Oops, you run into the previous problem again.
And you also will end up with taxa which you can't place in any particular
group because they have some of the necessary characters, but not all, and
they clearly would form a polyphyletic group on their own.
        But don't take my word for it.  As Dr. Holtz and others have pointed
out, there is an extensive litterature on the subject  Indeed, whil I should
like to take credit for the above arguments, they are largely if not wholly
based on those of DeQuiroz and Gauthier (1990)

>Incidentally, I took some time to go over the characters that Novas lists.
>Most of them are >really< minor--ill-defined tubercles and bumps on certain
>bones here and there--and with such a poorly known distribution among
>dinosaurian and pre-dinosaurian taxa (as Novas himself repeatedly
>acknowledges) as to be practically worthless.
        Really, picking on someone for having small tubercles!  For shame,
George!
        I should note that:
        A)  Many characters are minor, and the best test of a character
seems to be how it is recieved by one's peers.  I would really like to find
out what Sereno (amongst others) thinks of the paper.
        B)  Poorly known distributions are a problem, and Novas certainly
does acknowledge this fact, but this is all we have to work with, and as
long as a properly conducted analysis shows that all other ingroups are
excluded from Dinosauria (Sauriscia sensu stricto, Ornithiscia sensu
stricto), you'll have to come up with a better complaint than that.

>Can't see why he bothered to publish the paper.
        I feel obliged to point out in his defense that the same has been
said about your work (although not by me, of course).
        :)
        Wagner
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| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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