Re: What is a Dinosaur?

["Jerry D. Harris" <dinogami@hotmail.com>]

> There is a basal dinosaur: The Most Recent Common Ancestor. Recently it was
> debated on the PhyloCode mailing list whether such ancestors are an entire
> species or just one breeding pair.

  The problem with phylogenetic-type definitions (that is, everything is 
node- or stem-based) is that they don't help you with anything other than 
finding order to evolution.  Note that this isn't a _bad_ thing, simply a 
limitation!  After all, if I go out and dig up a bone, I can say that the 
bone (for example) has this and that trochanter, a head oriented this way, a 
crest here, a fossa there, etc., etc., etc.  Knowing that "Dinosauria" is 
"the most recent common ancestor of _Megalosaurus_ and _Iguanodon_" (or 
whatever taxa one prefers to put there) "and all its descendants" doesn't do 
_squat_ for helping identify whether or not the animal I've got is a 
dinosaur.  In the long term, it is _unavoidable_ that everything is defined 
(not just diagnosed!) by the physical traits.  If we _only_ had the 
aforementioned definition of "dinosaur," then we'd never be able to say 
whether or not this, that, or the other bone belonged to a dinosaur.  In 
other words, we _have_ to _also_ have those dreaded character-based 
descriptors, or else, as Nixon & Carpenter have pointed out, one could sit 
down and do phylogenetics without ever having to look at a specimen -- it 
would become pure theory and no data.

  Please note that I am _not_ advocating reverting solely to character- 
(apomorphy-) based clades.  Certainly, if dinosaur (and other taxonomic) 
studies in the last several decades has taught us anything is that mosaic 
evolution certainly does occur -- where one taxon may have character states 
A, B, D, and E, another may have A, C, D, and E, and another one B, D, and 
E, so having a taxonomic group that is "defined" (diagnosed, if you prefer) 
by "possession of A, B, C, D, and E" creates the sorts of chaotic messes 
that node- and stem-based taxa are intended to avoid.  I _am_, however, 
saying that we should quit pretending that the "diagnosis" is somehow 
inferior or subordinate to the "definition."  We _need_ both, and I 
personally see absolutely nothing wrong with saying "Our _current_ 
definition of Dinosaur is 'the common ancestor of XXXsaurus and YYYodon and 
all it's descendants' _AND_ 'animals displaying characters G, H, I, J, and 
K.'"  As more data comes to light, the latter part is subject to change, of 
course...but at least we can then deal with new discoveries (or re-evaluate 
old ones) based on something that we can observe directly.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 573-8373
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu
and     dinogami@hotmail.com
http://www.sas.upenn.edu/~jdharris

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