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Re: Cladistic definitions of Dinosauria, Saurischia, Sauropodomorpha



On 9/19/05, Phil Bigelow <bigelowp@juno.com> wrote:
> 
> > > Congruency of an idea over time has its good points, no doubt
> > about that.
> > >  But should that be one of the prime factors in defining a clade?
> > > Shouldn't the communication of new knowledge also be a factor?
> 
> > No! Then we'd have to change our definitions every time new data was
> > discovered.
> 
> I don't see why that would have to be the case.  Provided that there was
> enough empirical evidence used in defining the clade the first time, then
> there would be no need to worry about any new discoveries messing things
> up later.  Certainly you would agree that the dinosaurian characters in
> birds are voluminous.

I would; but so are the dinosaurian characters of ornithischians,
sauropodomorphs, coelophysoids, ceratosauroids, spinosauroids, etc.,
etc., etc., and those groups have been considered dinosaurian a lot
longer than birds have, as well as never having been placed *outside*
of _Dinosauria_ (again, except where _Dinosauria_ is not considered a
valid taxon). Birds have nothing on any of those.

> And to be clear:  I *loath* the idea of "redefining" a clade, and I hope
> that Phylocode comes up with rules that roughly parallel the rules for
> gen. sp. naming in the ICZN.  Once a clade has been defined in the
> literature, it remains that way forever (unless it has broken some other
> naming rule, such as priority, preoccupation, etc., etc.).

That's essentially how it's meant to work.
 
> Example: If Padian and May's (1993) definition is ever formerly adopted,
> and if birds are later found to be outside of Dinosauria, then I would
> advocate just leaving Padian and May's clade name in place, and then
> creating a new clade that excludes birds.  Call it Eudinosauria, or
> Owendinosauria, or something similar.

But it's ridiculous to name an "Owendinosauria" when you could just
stay true to Owen in the first place. What's next, "Seeleysaurischia"?

> > _Cetacea_ has been recently found by some studies to lie within
> > _Artiodactyla_. If this becomes an established hypothesis, would you
> > ever advocate using _Balaena_ as a specifier for _Artiodactyla_?
> 
> Ahh, interesting.  Well, I wouldn't dismiss it outright.
> I'm not opposed to the idea of using "extreme" examples of members of a
> clade to define the clade.

That's a bad practice, in my opinion, because very derived forms often
turn out to belong elsewhere. Very basal forms, on the other hand,
tend to have less unresolved branching patterns. The most stable
specifiers fall in between.

Furthermore, we should recognize that no phylogeny is set in stone.
Anything *could* change; we can't anticipate everything. All the more
reason to be conservative when picking specifiers.

> But I would only advocate such a thing provided that all of the following
> criteria are rigorously met:
> 
> 1) Artiodactyla is indeed monophyletic.
> 2) Artiodactyla hasn't already been cladistically defined (priority of
> publication date).
> 3) The empirical evidence for cetaceans being members of Ariodactyla must
> be *overwhelming* (such as is the case with birds being dinosaurs).
> 
> Since #3 isn't rigorously met, I would not advocate using a cetacean
> specifier for the Artiodactyla.  And until Phylocode is formalized, #2 is
> still up in the air.  But if the evidence was overwhelming, and if #1 and
> #2 have been addressed to my satisfaction, then I might give the idea
> some support.

But even if #3 were satisfied, there would still be no good reason to
use a cetacean specifier. It's just unwarranted.
 
> But mistakes can happen, and science is forced to live with them.  Such
> naming problems crop up regularly in the ICZN world, yet life goes on.
> But I don't believe that birds will ever be proven to be non-dinosaurs,
> so Padian and May's definition will never be shown to be unwise.

I don't either, but I still consider it improper.
 
> Just for the record, I suspect  that the final version of Phylocode will
> require that a clade definition that uses a Linnaean name *must* use as
> specifiers only traditional members of that Linnaean group (you appear to
> believe this too).  So this discussion will probably be moot in a few(?)
> more years, and Padian and May will be SOL (unless their definition gets
> "grandfathered in" as being valid).

Yes, that's precisely the recommendation the example was illustrating:

"Recommendation 11A. Definitions of converted clade names should be
stated in a way that attempts to capture the spirit of historical use
to the degree that it is consistent with the contemporary concept of
monophyly. Consequently, they should not necessitate, though they may
allow, the inclusion of subtaxa that were historically excluded from
the taxon. To accomplish this goal, internal specifiers of converted
clade names should be chosen from among the set of taxa that were
considered to form part of a taxon under either the original or
traditional ideas about the composition of that taxon, and they should
not include members of subtaxa that were not historically considered
part of the taxon."

—Mike Keesey