[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Sinornithosaurus and Revising Hou et al. ... Not so Long

In *Sinornithosaurus*, I note many features which are illustrative to its
relationships. One of these is the monocapitate quadrate, but as I discuss
later, this is quite different from that of *Archaeopteryx*.

The lachrymal, to bring up an important structure, inserts caudally into
the rostrolateral edge of the frontal in a lateral slot in typical
theropods, seen in *Allosaurus* (Madsen, 1979), *Velociraptor* (Barsbold &
Osmólska, 1999), *Tyrannosaurus* and other tyrannosaurids (Carr, 1999).
The apparent lack of such a condition in the rostral frontals at which the
preserved lachrymals may contact, but the partial frontal-prefrontal
articulation with such a groove for the lachrymal in *Dromaeosaurus*
(Currie, 1995), suggests either incipient transformation of the prefrontal
into the caudal lachrymal process via fusion, opr reduction of the
prefrontal and incipient loss in *Sinornithosaurus* (Xu et al., 1999). No
prefrontal is present in *Archaeopteryx* (Elzanowski & Wellnhofer, 1997)
or *Confuciusornis* (Ji et al., 2000), and the condition is unknown in
*Microraptor* (Xu et al., 2000) for which a partial skull is known; these
upset the pattern of transformation relative to the ancestry of birds, and
suggest the possibility that birds either lost the prefrontal, or fused it
to the lachrymal as is incipiently-observed in *Dromaeosaurus* and is
indicated for *Velociraptor* (Barsbold & Osmólska, 1999).
*Sinornithosaurus* shows a small pair of posterior bones in place in the
lachrymal slots, but there is no other slot for the lachrymal apart from
these; it is then more likely to show that the condition seen in
*Dromaeosaurus*, where one prefrontal is separate from it's lachrymal,
whereas the other is fused, is an adaptation of the condition in
*Sinornithosaurus*; no prefrontal in *Velociraptor* may be the
pre-emminent condition; the prefrontals in *Sinornithosaurus* are likely,
therefore, to be caudal portions of the lachrymal processes bound by
missing cartilage to the lachrymose body. Based on present data, it would
appear that the preorbital anatomy of *Sinornithosaurus* is consisten with
the Dromaeosauridae, and that it does not indicate a condition variant
from than, but rather from that of *Archaeopteryx* and birds. Such a
condition may easily be plesiomorphic for the stem-Dromaeosauridae.

  Only one quadratojugal is known, but its quality as the right such
element is equivocal (cf. Xu & Wu, 2001); the element is preserved on the
left side of the skull, and overlies several medial and lateral elements,
including the cervical vertebrae. It lies near to the left quadrate, and
is convex on its exposed surface. If equivocal, then its ramal
organization and shape may be also explained as pertaining to the left
quadratojugal. and in this manner the element would agree with
dromaeosaurids, _contra_ Xu & Wu, 2001, wherein this element possesses a
blunt caudal ramus and elongated rostral ramus. This would require a
right-side element to rotate over to the left side and associate itself
atop medial elements, meanwhile displacing the associated left-side
element, which is now nowhere to be seen. If, however, the element was a
left-side element, some features become more distinctive and relate to
dromaeosaurids closely: the rostral ramus (or "caudal" of Xu & Wu, 2001)
would then be the more robust one, and the irregular margin suggests it is
broken; the caudal ramus (or "rostral" of Xu & Wu, 2001) is slender and
produces a lamina dorsally along its length, becoming flattened
mediolaterally distal to the ovate base. A typical rostral ramus possesses
a medial lamina that inserts into a groove on the lateral jugal and
between the forked processes of the posterior jugal ramus; otherwise, this
process is triangular in section, and this would be the expected, but
missing morphology in the present element, and indicates that the element
has not been flipped or rotated rostral to caudal.

  The quadrate is similarly important, and we are fortunate to have a
complete and dislocated left quadrate and partially exposed right quadrate
to draw inferences from. The left quadrate is preserved in rostral view,
lying on its caudomedial side, and is apparently monocapitate (Xu et al.,
1999; Xu & Wu, 2001), in that only one proximal capitate condyle is
exposed, and another should be present below and to the side of it. In
*Archaeopteryx* (Elzanowski & Wellnhofer, 1997) the quadrate is
monocapitate, but the condyle is offset medially from the lateral margin
of the bone, and it is presumed to be the otic head. In oviraptorids, the
quadrate is bicapitate, but both heads contact only the braincase, and the
proximal quadrate resembles that of *Sinornithosaurus* in profile
(Osmólska & Maryanska, 1996). In *Sinornithosaurus*, the single head is
laterally positioned and confluent with the lateral edge, and represents
the squamosal head; this provides a distinct variation from birds and
suggests that avian monocapitancy is unique at the level of
*Archaeopteryx* (Aves) and no further.


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.

Do You Yahoo!?
Yahoo! Health - your guide to health and wellness