RE: Shoehorning (Augury and Alvarezsauria)

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

Ken Kinman wrote:

> The point is that "Alvarezsauria" should probably have never been 
> proposed to begin with.  I regard it was a form of *shoehorning* to
> group genus Alvarezsaurus with mononykiforms.
[snip]
> More recently Sinovenator was shoehorned into Family Troodontidae.

The word *shoehorn* gets bandied around a great deal in discussions on
evolution.  In essence, it refers to the (perceived) practice of assigning
unusual or poorly known taxa (taxonomic "oddities") to existing and
well-defined groups - even though the taxa in question may only show a few
features that recommend them to this group.  In other words, taxa with
peculiar morphologies are made to "fit" a given group - further examination
may establish that their placement in this group is unwarranted.  Then
again, it may be completely warranted.

The term *shoehorning* has been used on the odd occasion by Stephen Jay
Gould, particularly in his book _Wonderful Life_.  Gould had some strong
words to say on the referral of many of the weird-and-wonderful Burgess
Shale invertebrates to well-known phyla.  Gould avered that many of the
Cambrian arthropods from Burgess Shale were examples of unique body-plans,
and were not merely unusual representatives of the better-known groups
(Trilobita, Crustacea, Onychophora, etc) in which they'd been "shoehorned".

However, as it turns out, at least some of the Burgess Shale oddities *are*
most likely just strange members of well-known phyla.  For example,
_Hallucigenia_ wasn't a tentaculate wormy-thing that walked on spines (and
therefore deserving of its own special group or "phylum"), but an
upside-down peripatus (Onychophora) with spines down its back.

Keep this in mind when researchers place apparently peculiar or aberrant
taxa ("oddities") into existing groups.  It may not "feel right" to certain
people; but there might be a good (and wholly phylogenetic) reason for
putting them there.  Dodos are most likely just unusual pigeons (columbids);
most dino paleontologists agree that therizinosauroids (segnosaurs) are
weird coelurosaurian theropods; and _Sinovenator_ comes out as a basal
troodontid.

Over the past few decades many highly specialized taxa (especially
parasites) have been revealed to be highly derived members of other groups.
These "oddities" have merely diverged from the "typical" body-plan of that
group; their affinities and ancestry haven't changed.  (Or, in the case of
parasites, one could say that free-living taxa are peculiar relatives of
parasitic taxa.  Humans, in typical anthropogenic fashion, have a tendency
to consider "free-living" as normal behavior, and parasitism as an
aberration.)

> Whether one wants to put diadectomorphs in Class Amphibia or Class 
> Reptilia makes no big difference to me since they were making the 
> transition to basal amniotes (i.e. basal reptiles, in the traditional 
> sense).  

Deja vu all over again.   :-(  

> An Order Diadectiformes as the basal clade of Class Reptilia seems the 
> best current option and predict that it will remain in that position
> (thus promoting stability). 

Cassandra strikes again.  *sigh*  The "best current option" says you.  Once
again, this "stability" of which you speak is entirely artificial.  Anyone
can construct a classification that has the illusion of stability - but new
discoveries (i.e. new data) have the habit of shifting taxa relative to each
other.  This isn't a fault of cladistic methodology; it's just a result of
an improved and expanding data set.  Don't blame the fossil record if it
happens to throw you a curve ball.



Tim


------------------------------------------------------------ 

Timothy J. Williams 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 3163