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RE: Euryapsids



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Ken Kinman
>
> Thomas,
>      Is there anything online or in print about Merck's ideas on
> Euryapsid-Archosauromorph relationships?

At present, just a few JVP abstracts, and a mention and figure in Brochu's
recent discussion of the "temporal paradox" in bird origins (JVP 20:
197-200; the cladogram is Fig. 1E).  John hasn't gotten around to writing up
the latest version, yet: I'm going to try and force him to do that this
summer.

> Are they sister groups, or are
> euryapsids nested within Archosauromorpha (i.e. closer to prolacertiforms
> than are the rhynchosaurs or trilophosaurs).

Well, by definition Lepidosauromorpha is the sister group to
Archosauromorpha (i.e., Archosauria and all taxa closer to it than to
Lepidosauria).  Some have tried to use a compositionally-based definition of
Archosauromorpha, but the stem-based def. above is preferred by me and many
others.

In his latest runs, Merck finds a clade of Euryapsida and Thallatosauria as
the sister taxon to all other archosauromorphs.  (He's been trying to toss
in some taxa not included in his dissertation work, which is the source of
Fig. 1E in Brochu's paper).

>      I assume Euryapsida also includes the placodonts, and perhaps the
> thalattosaurs, but what about choristodera?

As Merck uses the term, Euryapsida = Ichthyopterygia + Sauropterygia.
Thallatosaurs are the sister group to this clade.  Yes, placodonts are
sauropterygians, and thus euryapsids.

Merck finds choristoderes are not euryapsids, but are instead the sister
taxon to rhynchosaurs (and outside a protorosaur + (trilophosaurid +
archosauriform) clade).

>                      -----Cheers, Ken Kinman
> P.S.  Do any of the euryapsids have thecodont teeth?

Checked with Merck about this: most euryapsids have a subthecodont condition
(which is the condition found in most non-archosauriform archosauromorphs).
A few ichthyosaurs have a thecodont dentition, developed independently of
archosauriforms.

                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): 301-314-7843