Re: Dinosaurian Class (was: DROMAEOSAURS AND OVIRAPTOROSAURS ....)

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 05:22 PM 6/16/98 -0500, Dave Hill wrote:

>       Gorgonopsians and other protomammals gave uprise to the mammals we
>know today but they are considered reptilian, does this mean I'm a reptile?

As others have pointed out, most workers in the field do not consider
protomammals "reptiles" any more.  They are basal synapsids.

>We might as well throw everything back into the mix by saying that birds
>are reptiles.  I'm merely saying that we musn't abolish all the bounderys
>which give us diversity.

This is one of the major misconceptions about cladistic classifications.
The boundaries still exist: Mammalia remains its own distinct group.
However, it remains part of a larger, more inclusive group, that also
includes _Cynognathus_ and _Thrinaxodon_ and _Dimetrodon_ and so forth.

Put another way, cladistic taxonomy recognizes only "A" groups,
characterized by a particular property (in this case, common ancestry).
"Not A" groups, which lack a unifying property except for the fact they
aren't "A" are not formally recognized.  (For a non-taxonomic example,
"Canadian" is an "A" group, characterized by the property "citizen of
Canada"; "Non-Canadian" is a "not-A" group, and are only grouped together by
their lack on Canadian citizenship, not by any special characters of their own).

For a classic taxonomic case of this, Vertebrata form a single group based
on common ancestry, and can be recognized by many shared derived charactes.
On the other hand, the "Invertebrata" are simply any animals (and, in some
classifications, non-photosynthetic single celled eukaryotes...) unlucky
enough not to be vertebrates.  As such, Vertebrata is still a recognized
group, whereas Invertebrata is not formally recognized.  We can still talk
about "invertebrates" (lower case) as a shorthand for "non-vertebrate
animals", but we have to remember that this doesn't mean that invertebrates
as a group are characterized by unique properties.

For some recent reviews of cladistic classification, with a dinosaurian
spin, see the Holtz & Brett-Surman chapter on "Taxonomy and Systematics of
the Dinosaurs" in Farlow & Brett-Surman's The Complete Dinosaur, or Dingus &
Rowe's recent book The Mistaken Extinction.  (Our chapter is shorter; Dingus
& Rowe spread theirs out through a couple of chapters, and consequently have
more space for more illustrations).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661