Re: What is a Dinosaur?

["David Marjanovic" <david.marjanovic@gmx.at>]

I'm rather surprised of what you seem to think about cladists...

> >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.

100 % true, and I don't know anybody who had ever disputed this. Diagnoses
aren't abolished.

>    ...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!

Some people seem to call a group with this content "crown-group Eukaryota".
B-)
If I overlook the blarg -- an undiscovered organism doesn't have characters,
so nobody can use such a group, and PhyloCode wouldn't accept it.

> 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.

100 % true.

> 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.

_Now_ I start to disagree. A dinosaur _is_ a dinosaur only because it is
descended blah, blah, and it is _in practice recognizable as such_ because
it has dinosaurian characteristics listed in its _diagnosis_. We can't
_prove_ it is "descended blah, blah" (the only means of _proving_ it's a
dinosaur), but we can erect the hypothesis that it is a dinosaur based on
all known evidence. Science! :-)

> 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.

Every cladist I know admits it, and wants to admit it.

> [...] Just that we need to
> quit pretending that phylogenetics can be done without the characters.

See above.

> 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

No smiley necessary.

> >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.

Only of the contents.

> 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

yep

> (and, I should point out, neither has the diagnosis),

If ZZZoides is very basal, it _might_ have.

> but the group pointed to by the label
> "dinosaur" most certainly has, either way.

No. The _contents_ of that group :-) -- the group is only based on its
definition.

> In other words, the _content_ of
> the definition changes, either with opinion, with new discoveries,
> reanalyses, etc., etc.

yep

> 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.

The definition is absolutely stable (unless it becomes so confusing that
whatever Commission changes it). The contents of the group aren't, and
therefore neither is the diagnosis.

> >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

Erm... but that says something different from what you wrote, it's not just
the same in other words...