Re: dinosaur argument

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

><<are two wooden pterosaurs hanging from the ceiling.  Now, as far as I know,
>pterosaurs are not dinosaurs, even though they lived during the same era.  I
>told my supervisor of this, and he 1)laughed and 2) told me I was wrong.  I
>do not believe that I am, however, >>
>
>Depends on who you ask.  According to the generally accepted notion of what a
>dinosaur is and isn't; pterosaurs aren't.  According to Bob Bakker they are,
>but that is because he redefined what a dinosaur is and isn't.

Note this:  the division is exactly as severe as Bakker says yes, all the
other hundreds of vertebrate paleontologists say no.

>In any case,
>if you place dinosaurs + birds into a group with their sister taxon of
>Lagosuchids (I hope I am getting this cladistic nomenclature right.  Why'd
>you guys have to go and screw everything up and invent cladistics!?),
>Pterosaurs would be the  sister taxon of That taxon (Dinosaurs + Birds, +
>Lagosuchids).  IMHO, Dinosauria should be interchangable as Ornithodira
>(Dinosaurs, Pterosaurs, Lagosuchids, and Ornithosuchids), so in that case
>pterosaurs would be dinosaurs.  In any case, the consensus opinion of
>dinosaur paleontologists is that pterosaurs are not dinosaurs, but are
>closely related.

Here is the basic status of things:
Ornithodira are all archosaurs sharing a more recent common ancestor with
birds than with crocodiles.

As currently understood, Ornithodira has two major branches, Pterosauria
and Dinosauromorpha.  Within the dinosauromorphs, some of the old
"lagosuchids" are closer to dinosaurs than are others.

Dinosaurs share a suite of adaptations that all pterosaurs and other
dinosauromorphs lack.  Because of this, it is important to keep the terms
"Ornithodira" and "Dinosauria" separate, since they mean different things.

>P.S. What does Plesiomorphic mean, what is the opposite of Plesiomorphic?

Plesiomorphic means shared primitive characters.  For examples, orangutans,
chimps, and humans all have five fingers, but that doesn't help determine
which two are closest to each other, since the character "five fingers" is
ancestral to a much, much more inclusive group than just hominoids.

The opposite is apomorphic, or shared derived characters (things like the
furcula, which is found only in birds and other tetanurine theropods).

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.