Re: Monolophosaurus

["Jaime A. Headden" <qilongia@yahoo.com>]

Jack Conrad wrote:

<<I was just wondering if anyone out there has seriously questioned
the placement of _Monolophosaurus_ as basal Carnosauria. I feel like
it is possibly basal Coelurosauria.  I would be happy to discuss this
with anyone, on or offlist, if there is any interest.>>

Tim Williams wrote:  

<I'm not so sure it is basal Carnosauria. Before I go on I think it's
worth mentioning that "Carnosauria" means different things to
different people. As I understand the term, the "Carnosauria" is a
paraphyletic assemblage of non-coelurosaurian tetanurines
(megalosaurids, allosauroids, maybe spinosauroids). Others regard the
Carnosauria as a valid clade - the monophyletic sister-group to the
Coelurosauria (and the basal clade of the Avetheropoda) including only
the allosauroids and maybe a few closely related genera.>

  To clarify, Carnosauria is all taxa closer to *Allosaurus* than to
*Ornithomimus*. If *Monolophosaurus* has a character or two more
similar to Allo than to Ornitho M., than he's a carnosaur, and not a
coelurosaur. But the real line comes when he lacks those characters,
and those of the coelurosaurs. Then he's neither.

<Confused? It gets worse. I believe the original description of
_Monololophosaurus_ regarded it as a "megalosaur-grade" theropod -
essentially a basal tetanurine or basal "carnosaur".>

  Actually, megalosaur-grade tetanuries can be considered basal to
Avetheropoda, making an un-named node, and may include such wonderful
taxa as *Baryonyx* and *Spinosaurus* and the indominatable
*Torvosaurus*.

  It may go something like this:

 Tetanurae
 |
 +--unnamed node
    |
    |--Megalosaur-grade theropods
    |
    +--Avetheropoda
       |
       |--Coelurosauria
       |
       +--Carnosauria

  There are a few refs for this, but I don't have them handy as yet.
The Archives have them, though.

<But subsequent revisions (sorry, don't have the papers handy) have
moved  _Monolophosaurus_ further up the tree and put it somewhere
among the Allosauroidea. Under this scheme, the Allosauroidea is the
sister- group to the Coelurosauria.>

  In my view, Carnosauria has three grades: sinraptors,
carcharodontosaurs, and allosaurs, all of which have so many
similarities they might be included into one suprageneric taxon.
*monolophosaurus* has two immediate similarities to the sinraptors: a
pneumatic snout and maxilla, and a very large lachrymal foramen, which
is diagnostic for Tetanurae (Holtz, (one of those years)).

<Future cladistic analyses could push other "carnosaurs" - like
_Baryonyx_, _Proceratosaurus_ and _Gasosaurus_ - into the
Coelurosauria.>

  I don't beleive any of these has been considered a carnosaur in the
post-1990 (post-_the Dinosauria_) studies or descriptions.
*Gasosaurus* is just plain weird, *Proceratosaurus* might be a
megalosaur-grade animal (more later), and *Baryonyx* needs more work
and specimens. Concensus seems to show Bary as a megalosaur-grade
theropod.

  Elongate jaws, dentary with great distal enlargement in front of the
shallowest part of the bone and corresponding enlargement of the
teeth, nasal/roof of the skull ornamentation, ischium as long as the
pubis or nearly so, elongate mid-caudal neural spines, short hind
limbs, and I think one or two more.

  These characters link Mono to Bary and Torvo, as well as a few other
megalosaur-graders.

  Tom Holtz may bash me for this, but I have tried not to think about
carnosaurs for a while, I really have!

  Jaime
_________________________________________________________
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com