Apomorphy-based definitions - who needs them?

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Ken Kinman wrote:

>The apomorphy-based Mammalia continues (after many
>decades) to provide a clearcut boundary based on osteology (jaw and ear
>ossicles).  

The distinction may be "clear-cut", but it is nevertheless COMPLETELY
ARBITRARY.  The reason why Mammalia has been accorded its own "Class" has
nothing to do with how many bones make up the jaw.  The ear-jaw morphology
is just a convenient marker.  The typological definition of a mammal has
more to do with "warm-blooded-ness" and fur and suckling live young (the
reason for the name "Mammalia" was adopted in the first place).  

But all these "defining" characters are useless when applied to the fossil
record.  So, surrogate characters new had to be found.  These allow the
taxonomic composition of the Mammalia (more or less) to be retained, by
artificially co-opting the "three-ossicles" as a new "key apomorphy".  In
the fossil record, all creatures that possess three ossicles in the middle
ear happen to approximate what we intuitively think of a mammal.


>When we do find transitional forms that fill in the remaining gap, we
>simply take the very same characters and refine them a bit in light of >the
new evidence.  It's a very stable strategy. 

The "stability" is just an artifact of an incomplete fossil record.  When we
do find therapsids with transitional ear-jaw configurations, the taxa will
be caught in a morphological "tug-of-war" over whether it belongs to "Class
Mammalia" or "Class Reptilia".  

>      Class Aves on the other hand has not enjoyed this same kind of
>precise osteological definition, and we are suffering the consequences.
>The "true" semilunate carpal block might not be quite as precise as
>mammalian ear ossicles, but as far as I can tell, it is the closest we >are
going to get to it. 

Again, you're desperately trying to retain the old typological separation
between Aves and non-Aves by scrambling for a new "key apomorphy".  The fact
that something so obvious as feathers and flight are useless in defining
Aves should tell you just how USELESS apomorphy-based definitions have
become!  

It's like you're saying: "I know birds deserve their own Class - I just have
to figure out why!  Feathers have failed us, but maybe semilunate carpals
will do the trick."  Sisyphus has wiped the dust off his hands, and is about
to push that boulder again...

The reason why you can't find a "precise osteological definition" is not the
fault of birds.  It's because evolution doesn't work that way!  Wings and
feathers do not sprout out of nothing.  And nor do semilunate carpals!  You
can never maintain "precision" in apomorphy-based definitions, because the
apomorphies themselves are gradational and therefore imprecise.  Compare the
carpal configurations of _Allosaurus_, _Scipionyx_, _Sinosauropteryx_,
_Alxasaurus_, _Deinonychus_ and a pigeon, and you'll see how they grade into
each other.  And the fact that you yourself actually write:

>The "true" semilunate carpal block might 

..means that you're aware of this too.  

(By the way, if Dan Chure is right, the semilunate-carpal-like traits seen
in the carpus of _Allosaurus_ may not be homologous to the semilunate carpal
block of maniraptorans, and therefore evolved independently.)  

Under Ken's "new-and-improved (again) Class Aves", an altered range of
mobility in the hand and wrist now becomes the wondrous character that
allows a theropod to enter "Class" Aves.  You know as well as I do that it's
just a convenient proxy.  Why not obligate bipedalism or a furcula (as
George avers) or a pygostyle or uncinate processes.  

There's no reason to maintain the Linnaean concept of Class - heck, Linnaeus
was a creationist!  God made birds superior to reptiles, and he (?He) made
mammals superior to both!  GET RID OF CLASSES!  That nonsensical notion of
innate "superiority" has to be discarded by any classification system that
wants to be taken seriously.  The "Kinman System" is just perpetuating an
historical fallacy.

Ken will counter by saying that apomorphy-based definitions provide more
stability of taxonomic composition than a taxon-based definitions.  But if
the exact membership of the Aves does change (e.g. _Caudipteryx_ falls in,
_Rahonavis_ falls out) - who gives a muskrat's ass?  "Aves" is just a label
after all, which we happen to stick on a particular node of a cladogram
based on the relative position of its respective anchor taxa. 

 


Tim