Avimimus in the Process of Revising Hou et al.

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

Mickey Mortimer (Mickey_Mortimer111@msn.com) wrote [in reply to me]:

<Why don't you look at Kurzanov's (1981) figure 1, showing the skull in
dorsal view.  You'll note he labels the lacrimal (La), which extends
posteriorly on the orbital rim to contact the postorbital.  To cover the
anterior half of the long orbital rim like that, it has to have an
elongate posterior process.>

 ... and in reply to something else I said:

<Perhaps it is part of the frontal and Kurzanov imagined a suture between
it and the medial portion of the element.  Lacrimal-postorbital contact is
rather odd in coelurosaurs after all, but there are virtually no sutures
in that area in PIN 3907/3, so who knows?>

  Tell me why I should believe that a complete neomorphous condition
without ananlogous correlates in nature (a divided frontal?) could be used
to support the idea of some sort of frontoparietal dome? However, to get
to the point of dividing this long post from _that_ long post (because
this is a _long_ explanation), *Avimimus* is (to quote Kurzanov) unusual.
It leaves no one scratching their head when you realize that _portentosus_
in Latin (momentous, extraordinary) translates into Russian as
_nyeobychnykh_ (strange, peculiar, as in the title of the 1981 paper, "_O
nyeobychnykh tyeropodakh iz vyerkhnyego Myela MNR [Mongol'skoy Narodnykh
Ryespublik]_") and _nyeobykhnovyennyy_ (which Kurzanov used as the
etymology for _portentosus_, and translates to both unusual, and
extraordinary).

  After some study in the last few months, I have had the opportunity to
come to a frankly annoying conclusion ... but hey, science marches on. But
to begin, a simple statement ...

  No definite lachrymal is preserved in the holotype (PIN 3907/1). What is
labeled as the lachrymal (Kurzanov, 1981, fig. 1, "la" on pg. 42) pertains
to an element that Kurzanov later leaves unlabelled (1987, fig. 1). In
comparison of the original figure to the _photo_ of the holotype in
Kurzanov (1987) and one I've seen, plus to the figure of the type in
Kurzanov (1987), shows that the original figure is a poor approximation of
the type, and does not in any way but superficially resemble the actual
material. The illustration in 1987 is far superior, and exactly matches
the shapes and even landmarks in the photos, so it more accurate. These
indicate several things in regards to anatomy and comparison between the
braincases that I will get to later on. First, though, any assumptions of
the presence of a lachrymal element must have been in error. On the
lachrymal, Kurzanov (1981) writes [my translation, pg. 41]: "The lachrymal
is situated in the rostral half of the dorsal region of the orbit. [It is
N]arrow, broad, and probably with a descending process. The nasal lies
anteriorly." In the text, it is clear that the bone is identified on the
basis of Kurzanov's assumption that the domed part of the skull is the
frontoparietal. I see ample evidence to consider this in error.

  Identification of the posterior domed element as a frontoparietal dome
follows Kurzanov's assumption of *Avimimus* as a primitive bird (1981,
1985, 1987), but comparative analysis indicates that the braincase as
preserved is much more caudal in the skull than originally suggested.
Preservation is poor in the rostral region of the holotype, but the
rostral element appears to the frontal, despite labelling to the contrary,
and the "frontoparietal" dome comprising only of the parietal. An element
lateral to the "frontoparietal" is elongated rostrally, and is labelled
the postorbital (Kurzanov, 1981, 1987; both, fig. 1); if this
identification is correct, then the braincase should then be much farther
cranial in the skull than indicated in Kurzanov's reconstructions. The
caudal end of the skull was interpreted as part of the transverse nuchal
crest, but this lies medial to a fossa that persists no further rostrally
than it does, and this fossa is analogous to the supratemporal fossa. The
caudal end, as in the rostral end, is eroded and broken, and does not
permit concise definition in identifying regions of the cranium. Only half
the braincase is preserved, the right, and this is clearly not a broken
margin; if so, then the parietals are not fused to one another, and a
"dome" is usually conditional with fusion of the parietals, rather than
development prior. Even bird chicks with incomplete ossification of the
cranial cartilage show a complete parietal and frontoparietal dome. Thus,
I question Kurzanov's identifications and labels; an orbitosphenoid
element would then be part of the laterosphenoid, and the rostral position
of the postorbital would be logically placed in a more caudal position,
rather than bearing a process that extends to above the middle of the
dorsal orbital rim (however, an alternative identification is offered
below); the fossa for the two foramina for the seventh cranial nerve (not
as in theropods or birds, which have one only) may them be pneumatic in
nature, and the fossa identified as the _recessus intracusticus_ would be
the sub-postorbitial fossa for the _m. pseudotemporalis profundus_. The
element rostrally is identified by a clear division that is not a
fracture; this division is extensive ventrally and pneumatic and comes
close to Kurzanov's orbitosphenoid (my laterosphenoid), which is not
something seen in a lachrymal element. It may be an ethmoid bone, but
this, too, is not apparent in the biological record for such a lateral
element; thus, the element then logically becomes the frontal.

  Alternately, the "parietal" is the frontal, and the "lachrymal" is the
nasal. This agrees with the "postorbital" element, and the supratemporal
fossae meet nearly on the midline, but are more likely in position with
the "postorbitals" than previously provided for. The lateral elements,
rather than "postorbitals" may also be supraorbital ossifications, seen in
birds, or eroded portions of the lateral parietal. It is clear this region
of the skull is not well preserved, however.

  In comparison to the referred cranium PIN 3907/3 (Kurzanov, 1983), the
holotype differs in the presence of a dome further caudal to the orbit, a
dome present on the parietal only, and in overall size is longer by almost
200% (the holotype braincase, comprising mostly of the parietal, is longer
than the entire preserved cranium of 3907/3, and is 150% wider than the
same region divided in the refered skull). Lack of fusion between the
frontal and parietal, as well as between both parietals, indicates that
this element cannot be an ontogenetic variant of PIN 3907/3, and that
*Avimimus* and PIN 3907/3 are two different animals. The skull of
*Avimimus* was a good deal larger than 3907/3, and in comparison to its
postcrania, *Avimimus* was a very large-headed animal, similar in
proportion to *Velociraptor*, and may have been rather raptorial or
ornithomimid-like in appearance with a largish snout. Conversely, only the
skull can be considered for PIN 3907/3, and this animal may have been more
oviraptorosaurian in appearance, as well as in phylogenetic affinity.
Previous use of the skull of PIN 3907/3 along with its cervical and dorsal
vertebrae is therefore in error. Reconstructions using this material are
not truly *Avimimus*.

  It is suggested that, along with Mickey Mortimer's phylogenetic studies
(unpublished), *Avimimus* may have had a basal maniraptoran position with
affinities to Alvarezsauridae on the basis of some hindlimb characters,
but otherwise femoral features are oviraptorosaurian, and the
scapulocoracoid element is advanced beyond that known for alvarezsaurids
and is closer to non-alvarezsaur/non-segnosaur maniraptorans in nature.
The humerus is similar to oviraptorosaurs, and the vertebrae appear to be
fairly generic among maniraptorans.

  Material referred to *Avimimus* includes five other specimens besides
the skull and vertebrae of PIN 3907/3. 3907/2, 5, and 6 are pelvic and
sacral only, and do not overlap any other specimen. 3907/4 is dorsal and
cervical vertebrae and compares only to 3907/3, without any overlap with
3907/1; and 3907/7 is an element that is probably a carpometacarpus
lacking most of the metacarpal shafts.

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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