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AVIMIMUS IS NOT A HADROSAUR



Darren Naish wrote:
 <<Once again I will mention the possibility that the _Avimimus_ premax
 does not go with the cranium of the genus, but may instead be from a
 hadrosaur.>>

This is extremely suspect.  I believe Darren is going off of Dave Norman's
statement in The Dinosauria where he writes that the premax resembles that of
a hadrosaur (in fact, it doesn't really), not that it may in fact be from one:

"The premaxilla is very reminiscent of the form seen in ornithopod dinosaurs
(lambeosaurine hadrosaurids)." (page 284 of the paperback volume)

Note, that he does not suggest that it is from a lambeosaurine hadrosaur.  In
fact I would be quite surprised if it came from a lambeosaur, or anything but
an oviraptorosaur period.

The fragment is tiny, about 1cm x 2.5cm.  Compare this to the  premaxilla of
the juvenile Hypacrosaurus stebingeri illustrated in "Eggs and Babies" (page
314) which is roughly the same size.  Not only is the form of the fossa
completely different, with the anterior border in the hadrosaur being
undefined, but in Avimimus being well defined; the hadrosaur completely lacks
premaxillary crenulations as a juvenile and the angle of the beak base to
dorsal border in lateral view is different, with Avimimus being closer to 90
degrees and the hadrosaur being less than 45 (in other words, the hadrosaur's
beak slopes more and Avimimus' is a lot steeper).

<<This may or may not be correct. But Pete's contention that a
crennulated premax would be suggestive of oviraptorosaurian affinities
are a little suspect aren't they?>>

This is not the only reason I suggest that it is an oviraptorosaur, and is in
fact the vaguest reason.  First of all, the fragment is obviously that of a
rostrum.  There is a depression that is on the posterior portion of the
fragment.  This depression (fossa) could be one of two things, either the
anterior portion of the anteorbital fossa, or a fossa formed from the external
nares.

I believe that this depression is the anteorbital fossa for two reasons.
First off all, dinosaur nares rarely have fossae or any sort of depressions,
and if they do, as in the case of hadrosaurs, the sharp, defined edge of the
fossa is on the posterior edge of it, not the anterior one.  

Additionally, Paul and Chatterjee have illustrated the rostrum fragment as
possessing a small foramen within the fossa, which I suspect is the maxillary
fenestra of theropods.

All this means is that the fragment has a very low and large anteorbital
fenestra and an extrenal nares that has migrated up above the AOF.  The only
known dinosaurs to have this condition are oviraptorosaurs and diplodocoids
and I think everyone can agree that it is not a diplodocoid.

<<A vaguelly crennulated premax has been figured for _Oviraptor mongoliensis_,
but I'm not convinced these represent proper crennulations (which should be
denticulate). And are they present in other oviraptorosaurs? Not as
far as I'm aware.>>

Every oviraptorid skull, except that of Rinchenia, I have seen figured has a
kinda bumpy, wavey ventral margin which I take to be crennulations of some
sort.

Jaime Headden wanted a piece of the action with this suggestion:
<<ceratopsians like *Triceratops* have nasal fossae with holes. Darren Tanke,
or any other ceratopsian worker, do you know if any juvenile or sub-adult
ceratopsians have these?>>

It is a possibility, but the juvenile Triceratops that Goodwin et al presented
at SVP 97 (and I think also at that UK paleo conference Darren attended) show
a rather smooth, non-crennulated ventral margin to the beak.  Additionally, if
it was a juvenile ceratopian, the rostral bone would clearly be deliniated
from the premaxillae, and not fused as it is in the rostrum fragment.

So in conclusion, I believe that the posterior skull, which is the holotype of
Avimimus portentosis to be that of an unknown oviraptorosaur because of the
extremely slender proportions of the jugal and postorbital.  

I also believe that these two bones contacted eachother as in other non-avian
dinosaurs, and not in birds.  Both bones are broken splints and could easily
have made a nice triangular lower temporal fenestra.  The idea that they did
not meet is an invention of Kurzanov's most likely based on his assumption
that this head belonged to a probably chimaeric skeleton made up of birds and
other coelurosaurs found scattered around quite a large area, which he thought
was a very bird-like theropod.

The rostrum is that of an oviraptorosaur because the depression is most likely
the anteorbital fossa since nares don't make depressions like that and there
is a small foramen that is likely the maxillary fenestra, which is a feature
found in theropods within the anterior part of the anteorbital fossa.  This
also means that the external nares have migrated up and dorsal to the AOF.
This combination of characters, along with the crennulated beak are only found
in oviraptorosaurs.  

Arguments that it may be from a hadrosaur or a ceratopid are shown to be
false, and based on incorrect interpretations of the taxa, especially in very
small individuals.

I also suspect that the rostrum and the posterior skull belong to the same
taxon.  Although this is un-knowable with the material available at present,
it is likely since both pieces are completely fused and obliterate sutures
indicating adult-hood, both roughly the same size, from the same area, and
that are both demonstratably from an oviraptorosaur.  Obviously though, we
need to find a skull with the missing pieces.

Would anyone like to give me a grant to go to Russia? :-)

Peter Buchholz
Tetanurae@aol.com

Right on!!