*Sinornithoides youngi* and New Data on Troodontids

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

*Sinornithoides* is now as completely described as possible, as the specimen 
will not be prepared
more fully until it is removed from a travelling exhibit (Currie & Dong, 2001).

  Currie, P.J. & Dong Z.-m. 2001. New information on Cretaceous troodontids 
(Dinosauria,
Theropoda) from the People's Republic of China. _Canadian Journal of Earth 
Sciences | rev.
Canadienne Sci. Terre_ 38 (12): 1753-1766.

  The dorsal skull roof, several cervical and most of the dorsals, and the 
dorsal margin of the
sacrum and ilia, are missing, as is the distal end of the tail; and the hands 
are not complete.
The left first metatarsal is a diseased element, having an osteopathic "growth" 
on the flexor
edge.

  Some interesting features of *Sinornithoides*:

  1. The dentary is not medially curved to the symphysis, though the jaw and 
teeth are essentially
     the same as in other troodontids.
  2. There is a very large lachrymal foramen.
  3. All crowns are serrated on both mesial and distal carinae, with the 
mesial-most two or three
     premaxillary and dentary teeth having nearly conical crowns, and resemble 
the "leaf-like"
     teeth of segnosaurs and alvarezsaurids.
  4. The fifth metatarsal articulates high on the tarsus proximally, and 
contacts the proximal
     tarsus.
  5. There is evidence for the calcaneum to be present in a facet for the 
fibula on the proximal
     tarsal element (which is fully fused). This suggests that even the 
juvenile troodontids known
     for a complete proximal tarsus, both astragalus and calcaneum are present; 
this is the
     likeliest condition, as Currie & Dong present it.
  6. Midcervical and anterior dorsal vertebrae have two lateral pleurocoels, 
one of which
     invaginates the transverse process, and the other within a distinctive 
lateral fossa.
  7. As illustrated by Paul (1988), troodontids have three distinct types of 
haemal arches
     (chevrons): 1) proximal blade-like, persisting for about ten elements, and 
successively
     becoming shorter, to 2) about three to four short, sub-rectangular blades, 
then to 3) skid-
     like on all succeeding vertebrae.
  8. The coracoid is plesiomorphic, sub-rectangular, but not "twisted" as in 
dromaeosaurids and
     oviraptorosaurs, so that the coracoid was even with the scapular angle. 
Similarly, the
     glenoid has virtually no lateral component. Instead of a defined acromial 
facet on the dorsal
     surface of the scapula, there is a broad facet on a tall but 
non-projecting acromion process.
  9. Hypapophyses on the cervicodorsal and anterior dorsal vertebrae are very 
well-defined, more
     so than in *Deinonychus,* but similar to *Velociraptor,* and paralleled in 
*Avimimus.*
  10. The second metatarsal is much shorter relative to the third than is the 
fourth, and more so
     than in other troodontids.
  11. The first and second manal claws are subequal in size, and the second 
pedal claw is similar.
  12. The first pedal digit is subequal to the first third pedal phalanx 
(pdIII-1).

  Currie and Dong introduce CT scans to assist in study, and along with it find 
the following: an
extensive secondary palate, six sacrals, absence of a lachrymal duct (as so in 
other troodontids),
elongate olfactory bulbs, cervical ribs without rostral processes, the 
clavicles are broken at
their rostral extent and thus were most likely broken rami of a complete 
furcula, a complete
gastral basket, with each medial gastrale element contacting two others 
(underlapping the first at
the tip, and overlapping the following, both opposite elements); the forelimb 
is less than 50% the
hindlimb length.

  They also expand our information on troodontid synapomorphies, including 
features of the
lachrymal, and the distal articulation of the third metatarsal.

---

Currie & Dong indicate new troodontid specimens from Erenhot and Bayan Mandahu 
in Nei Monggol
Zizhiqu, China:

  AMNH 21751: the distal ends of both third metatarsals, but these elements are 
of different
colors and may be different specimens. From Erenhot, Iren Dabasu Formation, 
Santonian?, and
collected in the 1920's by the CAE.

  AMNH 21772: a metatarsal. From Erenhot, Iren Dabasu Formation, Santonian?, 
and collected in the
1920's by the CAE.

  IVPP V10599: the sacrum, partial caudal series, and pelvis. Referred to 
*Saurornithoides
mongoliensis.* The specimen has seven sacral vertebrae, the seventh of which is 
a second
sacrocaudal, diagnosed by having the articulation for the first haemal arch on 
the last sacral;
this is also the largest *Saurornithoides* specimen, and the fusion of a caudal 
to the sixth
sacral appears to be an age-related feature. From Bayan Mandahu, in strata 
equivalent to the
Djadokhta Formation, Cenomanian or Campanian, collected in 1988 by the 
Sino-Canadian Expedition.

  IVPP 230790-16: a metatarsal. From Erenhot, Iren Dabasu Formation, 
Santonian?, and collected in
1990 by the Institute of Vertebrate Paleontology and Paleoanthropology.

  These are not illustrated.

=====
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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