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----- Original Message -----
From: <Vorompatra@aol.com>
To: <dinosaur@usc.edu>
Sent: Saturday, November 02, 2002 10:23 AM

> In a message dated 10/31/02 2:07:41 AM Eastern Standard Time,
> Dinogeorge@aol.com writes:
> << To me, two dinosaurs belong in different genera if the average dinosaur
>  paleontologist can tell them apart at a glance (assuming they're at the
>  ontological stage and are of the same sex), without resorting to making
>  detailed measurements or detailed examinations of skeletal anatomy.
>  Contrapositively, if they cannot be told apart at a glance, they very
>  belong to the same genus; and detailed anatomical examinations and
>  measurements (e.g., multivariate analysis) may turn up consistent
> differences
>  in the hypodigm of the genus that indicate the existence of distinct
>  within the genus. >>
> No offense, but this reminds me of the "Thomas Jefferson on horseback"
> Chip

Admittedly, the distinction of dinosaur genera is pretty fuzzy. However, I
do not completely agree with DinoGeorge's statement that generic distinction
is largely a "gut instinct." (I'm not using this term to belittle his
statement, but simply because "gut instinct" is a short and convenient
term.) As most of my experience in dinosaur research has been with
ceratopsids, I'll use them as an example. There is an *incredible* amount of
variation in ceratopsid skulls--just look at Chasmosaurus! I can easily
distinguish many of the individuals belonging to Chasmosaurus belli simply
by differences in horn proportions, snout length, or frill configuration. By
the "gut instinct" criterion, this means they could be separate genera. But,
all of these specimens have been referred to Chasmosaurus from the get-go.
It has taken very careful and thorough analysis to sort out ceratopsid
taxonomy and systematics, as many of the *species* were originally erected
on the basis of "gut instinct."

It could be argued that the "gut instinct" criterion doesn't apply to
ceratopsids because they show so much individual variation in the horns and
frills. Eliminating these structures from consideration creates an even
bigger mess--try distinguishing Triceratops, Torosaurus, and Arrhinoceratops
when you remove their frills and horns. Ceratopsid cranial conservatism has
given me many sleepless nights.

Additionally, how does one determine that the specimens are of the same sex?
Ontological stage is usually fairly easy, but sex can only be *inferred*
(never proven!) after extensive analysis of a whole mess of specimens.

Hadrosaurs have similar problems--e.g., Anatosaurus and Edmontosaurus.

So my main point. . .generic diagnosis is subject to the same problems as
specific diagnosis. While being able to distinguish the genera at a glance
is a good starting point, it must be supported by solid, consistent

Andrew A. Farke
South Dakota School of Mines and Technology, Box P301
501 E. St. Joseph St.
Rapid City, SD  57701

Phone: 605-394-2817

E-mail: andyfarke@hotmail.com