[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: *Utahraptor* and Polyphyly of Recent Dromaeosaurids



I wrote:

<<The condition is the opposite in oviraptorids (the rostral
process is covered by the subnarial process of the premaxilla),
and the caudal process is an extension of the medial extension
on the cranial roof, so they are not analogous.>>

Mickey_Mortimer (Mickey_Mortimer@email.msn.com) wrote:

<I'm not sure I know what you mean by "an extension of the
medial extension on the cranial roof",>

  By the dorsal extension of the lachrymal (not analogous to a
horn) turned medially. This exists in few theropods, one
possibly being *Monolophosaurus* and another *Dilophosaurus.*
But then, oviraptorid braincases are something else, aren't
they?

<<*Dromaeosaurus* does not possess complete lachrymals or
profrontals even, so the condition is very problematic. My boo
for using this as a dromaeosaurid character, but it seems
equivocal either way.>>

<...but what do you have to say regarding the triradiate
lacrimals of Caudipteryx? These look very dromaeosaur-like to
me.>

  Good case for an elongated caudal process. The angle between
the rostral and ventral rami is tight and close to 45, as in
oviraptorids, but unlike a majority of theropods with vertical
ventral rami; those with inclined rami (ornithomimids and
troodontids, for instance) have this angle as a result of the
ventral rami, not the rostral process, being transformed from
the primitive, vertical position. Only one other theropod
approches the condition in oviraptorids and *Caudipteryx*: the
type skull of *Acrocanthosaurus*, unlike the one labelled
"Fran," has an unusual rostral ramus that is inclined and curves
ventrally along its length.

<What I was getting at is the low number of taxa this character
has been examined in. Basal avians and troodontids have yet to
be examined, so it's more parsimoniously seen as a paravian
character until proven otherwise.>

  I agree. I still don't like limited unique characters like
this, Mickey, including neomorph bones that pop up in disparate
taxa. Only means the distribution or form of that element is
misunderstood in other forms. Ossified cartilage can form a
supradentary or antarticular, and simply not ossifying doesn't
mean it's not there. Relative ossification and pneumatization in
*Allosaurus* for instance, shows us how difficult such
conditions as basipterygoid or basisphenoid recesses are when
used as a diagnostic feature when all you have to go on is a
single braincase. Basipterygoid recesses are known in
*Gallimimus,* *Archaeopteryx,* *Velociraptor,* and *Allosaurus,*
to name a few. Basisphenoid recess in *Itemirus,*
*Velociraptor,* and *Allosaurus* also contend with basicranial
plateaus with pneumatic chambers, as in *Tyrannosaurus,*
*Erlikosaurus,* *Saurornithoides,* and *Gallimimus.* Otic
recesses across the basispenoid and prootic strut [lamina] are
present in *Saurornithoides,* *Erlikosaurus,* and *Gallimimus.*
What does this show? Distribution of seemingly key innovative
pneumatic features are so plastic as to be unreliable
(*Allosaurus* differs in the first two characters both
ontogenetically and individually, and there is well-documented
asymmetry (Smith, 1999, _JVP_). For most of these, we have one
or two specimens with which to compare others, and as such
specimens indicating individual variation or ontogeny are not
present so as to test relativity.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

__________________________________________________
Do You Yahoo!?
Yahoo! Auctions - Buy the things you want at great prices.
http://auctions.yahoo.com/