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Synapomorphies (& the Clade Cemetery)





Tom,
I agree, but only if such synapomorphies are more specifically characterized, and also it should put us on alert for the possibility that some of the other so-called synapomorphies have probably arisen more than once. A predentary in Hesperornis (a bird) certainly makes me wonder if the "ornithischian predentary" arose more than once (I'm not yet convinced one way or the other).
Synapomorphies of Dinosauria are what brought me to DML some 15 months ago, and although I've learned a lot about dinosaurs in that time, that question still has not been answered to my satisfaction (so I will restart a thread on that).
In any case, Ornithodira probably isn't the only thecodont clade going to the cemetery. Therefore I will continue to recognize a (semi-)paraphyletic Order Thecodontiformes. As I stated in my classification of that taxon, the {{Pterosauriformes}} marker may need to be moved away from dinosaurs to the prolacertiform grouping of families (this will almost certainly be done when I post an updated version).
But the main point is that the contents of my Thecodontiformes have pretty well stabilized, and when Merck or Dilkes or others present new archosauromorph cladogram topologies, I will simply shift the families and markers around and recode them. I won't be sending lots of clade names to the cemetery, since I do not formally recognize intermediate clades with names. Coding and informal clade names makes a lot more sense to me. However, as I have stated before, if thecodonts can be formally cladified in a relatively stable fashion, I will finally split Thecodontiformes (Thecodontia) and abandon it. Until then, I find this taxon very useful (far more useful than Ornithodira).
--------Ken
*****************************************
From: "Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>
To: <larryf@capital.net>, "Ken Kinman" <kinman@hotmail.com>, <dinosaur@usc.edu>
Subject: RE: Archosaur Origins (& the Clade Cemetery)
Date: Fri, 24 Aug 2001 08:48:22 -0400


> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Larry Febo
>
> The characteristics of
> preorbital
> fenestra and advanced mesotarsal ankle are just convergences. I always
> assumed these characters were strong definitions of the group "Archosaur".
> Do we now eliminate these characters as synapomorphies of this
> major group?
>
Just a few word of caution:


On-going, EXTREMELY comprehensive work is strongly suggesting that the taxa
in "Prolacertiformes sensu Peters (or most everyone else)" represent a
polyphyletic assemblage within Diapsida. There does seem to be a core group
of true prolacertiforms (containing the famous batch: Prolacerta,
Protosaurus, the tanystropheids), but many of the other guys included in
Peter's website & recent papers are showing up at various other points in
the diapsid reptiles.


Also, just because the pterosaurian preorbital fenestra may be
non-homologous with the archosauriform antorbital fenestra doesn't mean that
these aren't valid synapomorphies for their respective groups. For example,
loss of hindlimbs is a valid synapomorphy for caecillians, for derived
cetaceans, and for derived snakes even though this state (loss of hindlimbs)
evolved more than once in the history of tetrapods.


The pterosaurian ankle is apparently much less mesotarsal than typical
considered, as related by Dave Unwin:
http://www.cmnh.org/fun/dinosaur-archive/2000May/msg00518.html

> Also,...is Ornithodira dead without a fight? I assumed there would be some
> kind of rebuttal to Dave`s phylogeny. Aren`t there still some
> (Padian?) that
> still think lagosuchids and pterosaurs are closely related?....


That remains to be seen.  Don't know who else is specifically working on
pterosaur origins these days.

              Thomas R. Holtz, Jr.
              Vertebrate Paleontologist
Department of Geology          Director, Earth, Life & Time Program
University of Maryland         College Park Scholars
              College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone: 301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661      Fax (CPS-ELT): 301-405-0796



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