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Re: The iguanodont paper



On Dec 8, 2007 9:10 AM,  <GSP1954@aol.com> wrote:
>
> Currently a large group of ornithopods is easily named by a single word as
> hadrosaurs. This is an accident in that the group was taxonomically lucky 
> enough to
> have gone extinct before they left descendents.

Had the group left descendants, the group wouldn't have gone extinct.
That's a tautology, not luck.

> Had they done so then they would
> not be known under the cladistic system as hadrosaurs. Instead they would be
> known as say, nongeekosaur hadrosaurs -- i.e. at least two words would be
> needed to label the duckbilled dinosaurs.

I think that's fine -- it's important to distinguish groups based on
symplesiomorphies from groups based on synapomorphies. When you have
one naming convention for both, people consistently confuse them and
think that a taxon of one type is comparable to a taxon of a different
type. (You yourself used some interesting ad hoc conventions for names
of paraphyletic groups in _Dinosaurs of the Air_.)

> Iguanodonts lasted at least as long as hadrosaurs,

Much longer -- hadrosaurs *are* iguanodonts.

> probably inhabited more
> continents, and as the CR paper helps show with the increase in genera, were 
> at
> least as anatomically diverse. Yet there currently is no name that
> incorporates all the dominant large, spike thumbed, styracostenan ornithopods.

What's so interesting about that paraphyletic group as opposed to
something like non-lambeosaurine cerapods, or non-saurolophin
iguanodonts, or non-crested styracosternans, or pre-Late Cretaceous
predentates, or non-_Ouranosaurus_ ornithischians?

I'm not debating that some paraphyletic groups are interesting. But
most paraphyletic groups are just clades with one or more random
subclades removed -- that's not interesting or useful. ("Thecodontia"
is a great example of that, as are some -- but not all! -- classic
usages of "Dinosauria".) The paraphyletic groups that are interesting
and useful typically involve:

1) symplesiomorphy,
2) chronology, or
3) inferences based on information present in extant organisms but
lacking (or generally lacking) in fossils.

I agree that there should be ways of referring to these types of
paraphyletic group. But I disagree that the way should be names that
are formed the same way as clade names. Imagine the confusion if both
species and clades were written as single, capitalized words. Well,
that's no worse than the confusion that occurs when both clades and
paraphyletic taxa are written as single, capitalized words.

We already have pretty good systems for referring to the "interesting"
types of paraphyletic groups. A group delimited by a symplesiomorphy
can be referred to by that symplesiomorphy. Chronology-based
paraphyletic groups can be referred to using names of geological
units. And stem groups can be referred to with the "stem-" prefix.
Examples:

1) spike-thumbed iguanodonts; gilled vertebrates; featherless
amniotes; terrestrial mammals; limbed tetrapods; arboreal primates
2) Mesozoic archosaurs; Mesozoic dinosaurs; pre-Holocene homininans
3) stem-mammals; stem-avians; stem-crocodylians; stem-whales;
stem-bats; stem-humans

It does occur to me that you could make category #1 a bit more
succinct in some cases. It might be useful to have an informal prefix
for apomorphy-based clade names that specifies only the members of the
clade which retain the apomorphy. "Plesio-" might work. Thus,
"plesio-tetrapod" = "limbed tetrapod"*, "plesio-Ankylopollexia" =
"spike-thumbed ankylopollexians"**, and "plesio-Avialae" = "flying
avialans". Just a thought ... any takers? Haters?

* Assuming an apomorphy-based definition for _Tetrapoda_ -- or maybe
this could be permissible, anyway, as a contraction for
"plesio-apo-tetrapod".
** _Ankylopollexia_ currently has a node-based definition, but an
apomorphy-based one might be more useful.

> To be blunt, this arrangement is bizarre to the point of being wacky. What
> other classification system labels things as much but what they are not as by
> what they are?

But that's the honest way to name that group. If you call it
"Iguanodontidae" it looks like the same type of group as
"Hadrosauridae" -- which it certainly is not.

Once you do start referring to groups using a "non-X Y" formula, I
think you have to ask yourself -- what exactly is so interesting about
this particular paraphyletic group? The answer that usually comes back
is "tradition". If another answer comes back, there may be a better
way to refer to that group.

> Nonhadrosaur iguanodontoids is even harder for folks to grasp than is 
> iguanodontids.

But scientific nomenclature isn't designed to be easy for the lay
public to comprehend -- it's designed to be useful to scientists.
That's the entire reason it exists.

> Iguanodontids could be defined as iguanodontoids excluding hadrosaurids. No
> muss, no fuss. Thecodonts could be archosauromorphs excluding dinosauromorphs
> and pterosauromorphs (and perhaps crocodylomorphs, although it could be argued
> that they are the last surviving thecodonts).

But it can only be argued if "thecodont" actually means something. The
definition drives the interpretation, not the other way around.

> For one thing, the
> anti-dinosaur people are detached from reality anyway so they are going to do 
> what
> they do no matter what is done with the taxonomic system.

Fair enough!

> Also, basal archosaurs are also defined by the exclusion of dinosaurs and 
> pterosaurs, so there is
> no practical difference except that the current system is linquistically more
> awkward.

"Basal" is relative, not absolute. And calling an alligator -- an
organism living 240-odd million years after the ancestral archosaur --
 a "basal archosaur" is pretty absurd. (I wonder if Chris Brochu is
reading....)

"Basal archosaur" is a fuzzy group, opaque at the ancestral archosaur
and fading out into translucence the farther you go along any lineage
stemming from that ancestor. It's not the same as a precise group like
"non-dinosaurian, non-pterosaurian archosaurs" -- or at least it
shouldn't be. (The term "basal" does get abused an awful lot.)

> I am collaborating with others to continue to work on Brit iguanodonts. In
> particular am looking forward to describing and naming the mystery specimen 
> with
> the long jaw and super massive arm figured by Richard (bug eye) Owen that is
> so forgotten that its exact stratigraphic level is not currently known (other
> than below the high tide level at a location on the coast). I stumbled upon
> this while checking out the correct spelling of "I." hoggii in the original
> publication. "I." dawsoni and "I. " fittoni also need dealing with. It is
> important that from now on iguanodont specimens be assigned to a specific 
> genus or
> species only if it can be solidly justified, No more tossing European
> Barremian-Aptian specimens into either robust I. bernissartensis or gracile 
> "I."
> atherfieldensis, or anything outside Europe into Iguanodon unless it can be 
> shown to
> be the same basic beast as the specialized Bernissart robust genus.

Now *that* I can get behind. Really looking forward to future work on
cleaning up that mess!
-- 
T. Michael Keesey
Director of Technology
Exopolis, Inc.
2894 Rowena Avenue Ste. B
Los Angeles, California 90039
http://exopolis.com/
--
http://3lbmonkeybrain.blogspot.com/