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Troodontidae and All That Jazz

  Yeah, I like a little jazz now and again :)

  Nick suggested I right up something on troodonts,
and I'm going to start off by saying I am not an
expert on dinosaurs, or theropods, or whatever. I
certainly don't know much on these critters, but I can
compose a passing essay on them.

  Needless to say, troodonts are one of the
understudied groups of theropods that have recently
received more attention due to the nitpicking of
people like Dale Russell, Mark Norell, Peter
Makovicky, and the rest of the MAS-AMNH team (MAE).
*Byronosaurus* is the tip of the iceberg.

  A few taxa were based on teeth, and these are
generally referred to as nomina dubia, but as Currie,
Rigby, and Sloan (1990) showed, are diagnostic
individually, and different genera _can_ show
differences based solely on dentition. *Koparion
douglassi* from the (LJ) Morrison F. is one such
example, based largely on the bulbous base of the
tooth and low height/length aspect ratio. *Pectinodon
bakkeri* was synonymized with *Troodon formosus* based
on extreme similarities in that they fell within the
same expected range of morphology for the same jaw, by
Currie et al. (1990). Ciski and Grigorescu (1999)
suggest that *Pectinodon* may be a distinct species of
*Troodon*, *T. bakkeri,* based on another set of teeth
from Romania, including another suggestion that
*Euronychodon* is a troodont. I have no diagnostic
opinion of *Saurornithoides/Troodon asiamericanus* and
*S./T. isfarensis*.

  Other troodont taxa are based on pedal elements
only, such as *Borogovia gracilicrus* and *Tochisaurus
nemegtensis*, but then again, these pes differ
markedly from those of *Troodon*, *Saurornithoides*,
and even *Sinornithoides* (pers. ob.). Short pedal toe
2 in Borogove with the relatively strait claw is
strange and unexpected, but also found in the
deinonychosaur (sensu recens) *Adasaurus*. It's a
troodont, though. But unlike other known troodont pes,
*Tochisaurus* had a very short mtII which, given the
average relative length of digit II to the metatarsal,
would have prevented the toe from touching the
substrate, and making it functionally didactyl, as in
ostriches, hence the name (_toch'_ in Mongolian refers
to ostriches).

  The taxa represented by more complete material
including parts of several regions of the body,
include *Saurornithoides*, *Troodon*,
*Sinornithoides*, *Byronosaurus*, and another troodont
from Ukhaa Tolgod. These taxa differ from each other
in more significant ways, based largely on the more
complete material, and are the main diagnostic tools
for the group. A complete skeleton is known only for
*Sinornithoides youngi*, and proportions are similar
to ornithomimosaurs in many respects, especially the
skull with *Pelecanimimus*:

  check out

  The skulls of *Byronosaurus*, *Troodon*, and
*Saurornithoides* are less complete, but are still
complete enough to compare, and all are relatively
more derived and closer to each other than to
*Sinornithoides*, and this suggests that Sinorn is
more basal. The skull is longer snouted with a deeper
"nose" and less triangular profile in the last group,
and the form of the pes is not indicative of any
derived form. The pelvis is propubic with a derived,
oviraptorosaurian-like anterior pubic process with the
ilium, so that the pubis is slightly offset under the
acetabulum then curves forward. Very short ischium
with no elongate post obturator process in Sinorn and
Saurorn, as far as I can see. Long tail lacks
stiffening structures except for possible
craniocaudally long chevrons in the distal half. Manus
of the dromie/caenagnathid form, with a shorter third
finger than second, and semilunate does not contact
the third metacarpal [could be wrong on a few cases].
Long, long necks, very slender. Coracoids are
intermediate between the ovate ornithomimosaur form
and the everted rectangular form of oviraptorosaurs,
closer to the latter than the former. Sinorn preserved
furcular splints, or true clavicles, whereas the
condition appears to be unknown in all other

  Taxonomy suggests that *Koparion,*
*Sinornithosaurus*, and *Byronosaurus* are successive
outgroups of the group formed by *Saurornithoides* and
*Troodon*. *Borogovia* and *Tochisaurus* could lie
anywhere along the line. *Ornithodesmus* has been
hypothesized as a troodont, and it too, may lie
anywhere, as a comparison of sacra has not been done
so far on troodonts, and I am unaware of the full
condition in Sinorn.

  Hope this helps, and I would enjoy corrections on
any errors or any additions that could be made to

  Data comes from Russell and Dong, 1994; Currie,
1987, 1990; Barsbold, 1976; Makovicky et al., 1999;
Chure, 1994; Currie et al., 1990; Russell, 1970;
Osmólska, 1988; Osmólska and Kurzanov, 1992; and Glut,

Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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