[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: a rose by any other name(was fish & dogs)

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> philidor11
> Simply,
> the majority
> idea is that birds are very different from anything else extant, and
> 'deserve' to have a separate designation.  The resemblance to
> comparatively
> obscure dinos can be considered more an evolutionary history issue than an
> essential element of grouping.

But what if you use a grouping scheme whose binding principle IS
evolutionary history?
> When HP Holtz (by the way, if I could do over my post on 'It's
> the economy,
> stupid!', I would substitute HP for Mr.; sorry)

No problem.

> said:
> What anatomical features or morphological gap separates birds and
> their kin?
> Most of the classics (feathers, furculae, pygostyle, toothless
> beaks, etc.)
> are either
> more widely distributed among theropods than previously known or are not
> present in _Archaeopteryx_ and other basal forms classically considered
> "birds.">
> I wonder if this argument from fuzzy examples at the margins should be
> considered controlling?

These problems CAN be controlled if your taxonomic scheme uses some sort of
organizing principle that can deal with the otherwise-fuzzy margins.

> Are there any other animals which might or might
> not fit into a different, more universally accepted group?

Oh, my, yes!!  Any time you get near the base of a well-known lineage there
is some question as to whether a particular taxon belongs in that named
group or not.  (This is less problematic if the taxonomic scheme has
rigorous organizing principle, and more problematic when multiple (or no)
organizing prinicple is used).

For example:
Are conodonts vertebrates or aren't they?  If they are, are they "fish"?
Are morganucodonts mammals?
Are _Pakicetus_ and _Ambulocetus_ whales?
Is _Euparkeria_ an archosaur?
Is _Lagerpeton_ a dinosaur?
Are pterosaurs dinosaurs?
Are _Ichthyostega_ and _Acanthostega_ and _Panderichthyes_?
Is _Archaeofructus_ an angiosperm?
Are archaeocyathids sponges?

An advantage of a taxonomic scheme with a rigorous organizing principle is
that one can test whether a critter belongs in a taxon or not.  This is true
for systems (like phylogenetic taxonomy) where the principle is statements
of receny of common ancestry: for a given cladogram, a taxon can be seen to
be inside a particular clade or outside that clade.  This is also true for
systems that might use a particular suite of defining (rather than merely
diagnosing) traits: if the critter has these traits, it is a member of that
clade; if not, it ain't.

On the other hand, for "floating point" schemes where the organizing
principle is "what the guy with the biggest CV says is a taxon in his
masterpiece", the discovery of a taxon near the base of that group proves a
problem.  Would Prof. Big (who may be dead, making him difficult to reach)
include _Nova taxon_ in his Taxomorphoidea or not?  Do we expand
Taxomorphoidea to include it, or create a taxon of equivalent rank for it,
or what?  This has been a big problem with the traditional, authority-driven
systematic schemes: they did not have an algorithmic aspect to them.

> By the way (again), I would appreciate clarification on why
> Archaeopteryx is
> included in the bird definition.

Historical reasons.  However, and I'm afraid we have to get techincal here,
"bird" is still a vernacular term.  We could argue whether "bird"
technically applies to the clade Avialae sensu Gauthier (in which case
_Archaeopteryx_ and _Sinornis_ and _Hesperornis_ are all "birds") or if
"bird" applies to Aves sensu Gauthier (aka, the clade comprised of all
descendants of the most recent common ancestor of living feathered
vertebrates, in which case the Mesozoic taxa I listed are NOT birds).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796