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Comments on *Mei*



  The new troodontid, *Mei long* Xu and Norell (2004), is a peculiar
animal for many reasons, not the least of which is it's avian "sleeping"
posture (discussed to death earlier). What is more peculiar is the
avian-like anatomy of the animal, from the large triangular head (an
unquantified condition), slender jaws, and the really, really long legs.
It even seems to look more like a "missing-link" between dinosaurs and
birds than is *Archaeopteryx,* both superficially and in the minds of some
researchers. Some of these avialian features are:

  1. Rami of the furcula diverge at less than 90 degrees, close to "U"
     shaped than "V" shaped.
  2. Radius subequal to the humerus in length.
  3. Nasal less than 1/3 skull length (caution: related to short snout, so
     that this may be a juvenile character).
  4. Retroarticular process of the mandible elongated posteriorly and
     tapered.
  5. Femoral caput semispherical.
  6. Extremely large orbit (caution: related to short snout, so that this
     may be a juvenile character).
  7. Snout half the length of the skull (caution: related to short snout,
     so that this may be a juvenile character).

  Of these, I would automatically eliminate three for being ontogeny
related (3, 6 and 7). As a parallel to the "triangular skull" bit, compare
the skull of the juvenile (near-hatchling) *Scipionyx* holotype with that
of, say, *Compsognathus* or *Ornitholestes,* animals that were closely
related, but obviously not "birds," and obviously lacking in "triangular
skulls" in the sense used to support *Longisquama* and *Megalancosaurus*
as pro-avians, or the use of this feature as a diagnostic "tell all" for
birdyness.

  Aside from support for being a troodontid (in Xu and Norell, 2004,
recapped below), *Mei* has the following features that do not relate to
Avialae (species closer to birds than to dromaeosaurs, in short form):

  1. Coracoid relatively short with a rounded distal (craniocaudal) end.
  2. More than 20 upper and 20 lower teeth.
  3. Distal end of scapula squared off, almost expanded.
  4. Forelimb less than 60% of the hindlimb.
  5. Skull less than 70% of the femur.
  6. Humerus less than 75% of the femur.
  7. Scapula subequal to the humerus, rather than much shorter due to
     humeral lengthening (Wow! A troodontid with a short arm, who'da
     thunk?!).
  8. Medial pairs of gastralia cross in imbricating pattern (forming a
     "basket").
  9. Sacral ribs long, separating the ilia from one another above the
     sacrum.
 10. Femur more than 90% of the trunk (dorsal column used here, but
     glenoid-acetabular length works, too).
 11. Hallux shorter than pedal phalanx pdIII-1, and less than 40% the
     length of pedal digit 2. 

  ... aside from the troodontid synapomorphies, of course, which are ...

 12. Dentary nutrient foramina lie in a groove.
 13. Rostral dentary and maxillary teeth strongly crowded (also in this
     taxon, the middle series of teeth, an autapomorphy for *Mei*).
 14. Interfenestral bar of the maxilla/nasal is dorsally flat (as in
     ornithomimosaurs and alvarezsaurs).
 15. Interfenestral bar of the maxilla/nasal is "T-" shaped, having a
     flange extending ventrally.
 16. Metatarsal II relatively much shorter than metatarsal IV, shorter
than
     IV by as much as IV is shorter than III (in other troodontids, this
     tends to be even shorter; also in some dromaeosaurids).
 17. Metatarsals II and IV bear longitudinal ridges on their
plantar/flexor
     surface, forming a "metatarsal sulcus" between them.
 18. Metatarsals II and IV constrict metatarsal III between them on the
     plantar/flexor surface (as in *Elmisaurus*).
 19. Mid- to distal caudal vertebrae lack neural spine, bearing a dorsal
     sulcus instead.
 20. Mid- to distal chevrons form boat-like skid-shaped structures (as
     broad caudally and anteriorly) that interlock (also in
     *Shenzhouraptor*/*Jeholornis*).

  So at this point, using parsimony, there are more features to suggest
that *Mei* is NOT closer to birds than is *Archaeopteryx,* and many of
these can be used to support an exclusive affinity with troodontids. Make
special note that the arm of the basal oviraptorosaur *Protarchaeopteryx*
(along with the 1/3-length forelimb of *Caudipteryx*) is nearly the same
length as the hindlimb, as in *Microraptor,* but in *Mei* this is NOT the
case, suggesting two things: either 1) the limb elongation is apomorphic
of the oviraptorosaur + deinonychosaur + bird clade (in which case,
*Yixianosaurus* is a member, given as it has several features in common
with several groups at this point, many especially with oviraptorosaurs
and some with segnosaurs), and reversed in basal oviraptorosaurs and prior
to the onset of troodontids, OR 2) limb elongation was secondary in
*Protarchaeopteryx,* later oviraptorosaurs, segnosaurs, and
dromaeosaurids, and perhaps in addition even birds, whereas generally the
forelimbs were shorter as in ancestral ornithomimosaurs and alvarezsaurs.

  To Greg Paul's delight, I'm sure, *Mei* possesses a semispherical
femoral caput, but which lacks a distinct neck (as in *Microraptor* or
*Sinornithosaurus*) and also appears to be elevated by some 5-10 degrees
at most (it's not clear given the photos). This would perhaps give the
femoral around 10 degrees of lateral eversion or so, not counting what
modifiers for the possible iliac contribution. The legs are extremely
long, more so in the distal segments. The hallux is short, even though it
is distally placed, and was not reversed in orientation given articulation
of phalanx pdI-1; the digits have shorter distal phalanges than proximal,
and the claws were not particularly curved, so the leg was almost
definately terrestrial in adaptation (as have all troodontid pes been
known to be, to date). These features suggest that no basal troodontid had
"flight" adaptations of the like seen in dromaeosaurids and avialians,
oviraptorosaurs, etc.

  While no trace of a sternum is present, the coracoids are preserved so
that the craniodistal margin is turned medially; and while this
orientation is partially due to flattening of the anterior dorsal region
(as told by the nearly parallel coracoids and sternum, axially rotated
dorsals, and strongly backswept ribs, along with the extremely shallow
"chest" compared to the posterior dorsum), it suggests that the coracoids
were likely articulated to grooves along the anterior edge of a sternumk
similar in angle to oviraptorosaurs and dromaeosaurs.

  Xu and Norell (2004) state that the pterygoid flange of the quadrate is
distally placed; however, the preserved portion of the flange and the
qudrate height show it to be above or just at the mid-height of the
qudrate, and is actually more proximally placed than is the same flange on
*Herrerasaurus.* Xu and Norell also note that there was no contact between
the quadratojugal and the squamosal, yet the pair of bones are not
preserved completely enough to demonstrate this lack of articulation.
Indeed, the qudratojugal is preserved as a tiony splint of bone, which may
be broken from a larger piece, and the squamosal is separated from the
quadrate by matrix and shows incomplete margins, so this condition is
unknown in *Mei.* In addition, while they state that the postorbital was
not preserved, they use this to propose there was no contact between
postorbital and jugal, supported by lack of an ascending ramus on the
jugal. Unlike the Ukhaa troodontid, *Mei* possesses a deep posterior jugal
body and a flange ascends dorsally and caudally, with irregular, likely
broken margins, suggesting that there was at least a portion of ascending
ramus, and possibly suggesting there was a more complete postorbital bar.
They also note that such a contact is absent in *Archaeopteryx,* but this
is misleading: the contact between postorbital and jugal are not preserved
at all, the ends of the rami in question being irregular and broken,
implying they were more complete and that, in fact, there was such a
contact present.

  With these concepts in mind, it is thus not hard to conceive of *Mei* as
a troodontid, with no more a special affinity to birds than does
*Sinornithoides.*

  Cheers,

=====
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.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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