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

Yixianosaurus longimanus, a Peculiarly Normal Yixian Theropod



So what's new out of Liaoning? Yet another bird-like theropod with
feathers detailing yet another new way flight could have evolved? How
about a new twist on how dromaeosaurids evolved?

  Nope.

  Xu X. & Wang Xiao-lin. 2003. A new maniraptoran from the Early
Cretaceous
    Yixian Formation of western Liaoning. _Vertebrata PalAsiatica_ 41(3):
    195-202.

  *Yixianosaurus longimanus* is named for, as previoauly shown, a partial
skeleton that for once does not included most of a badly preserved
skeleton, or an entire skelton with integument. Naw, its just a pair of
arms with some ribs and gastralia. And the integument? Just
*Sinornithosaurus*-like fluff. As a note, the caption to figures 1 and 2
have the misspelling of *Yianxianosaurus.*

  Also heard of the supremely long hands of this critter. No wonder it
compares well with *Epidendrosaurus*. But then, the rest of it would
suggest this is convergent. It's the second finger which is largest in
*Yixianosaurus*.

  IVPP V12638: partial postcranial skeleton; both nearly-complete scapula
(the right is missing the ventral margin and glenoid area, the left is
preserved showing only the ventral edge and not preserved in side view,
missing the distal end), both coracoids, complete right and partial left
humeri (missing the distal end), complete right and partial left ulnae and
radii (missing the proximal ends), complete left carpus, partial right
(missing much of the surface and the proximal ends) and complete left
metacarpus, nearly complete left and right manus, lacking only left
phalanx mdIII-1, proximal mdIII-2, distal second claw, and most of the
proximal right second claw; several portions of left and right ribs,
including proximal portions and shafts, and three pairs of ?middle
gastralia and another right gastrale. Integument is found on the right arm
and extends about 50mm from the ulna in the best preserved region, where
the ulna is 64mm in length.

  Measurements: (in mm)
          length: left    right - height: left    right
Scapula           45+     65
Coracoid           ?      22.5            26+      33
Humerus           61+     89
Radius            63      63
Ulna              62      64
MCI               14.5    13 est.
MCII              36      35 est
MCIII             34      31+
mdI-1             33      34
mdI-2             26      25              14       13.5
mdII-1            26      25
mdII-2            38      36
mdII-3             8.5+   28              12        ?
mdIII-1            -       9 est.
mdIII-2            7+      8
mdIII-3           24      22
mdIII-4           22.5    23              10.5      9.5
right rib shaft   59
left rib shaft    64, 67
left rib head     34
gastral rib       29.5, 24+

  There is a unique combination of features in the arm, including the long
hand and the advanced proportions of the ends of the fingers. The pre-claw
phalanges are longer than their fingers' metacarpals, a feature found
otherwise only in *Epidendrosaurus* and *Scansoriopteryx*. The carpus
includes a distinct semilunate bone, but it seems to be composed, if the
semilunate of other theropods would be distal carpals 1+2, of just the
first distal carpal; the element is not distinctly rounded, and only caps
the first metacarpal, but does contact the edge of the second metacarpal.
This is a feature not found in maniraptorans, but is known in other 
coelurosaurians (such as *Nqwebasaurus*, *Coelurus*) as well as tetanurans
*Acrocanthosaurus*, *Allosaurus*, etc. The claws are very deep, almost
half the length at the flexor tubercle; the flexor tubercles are
triangular and do not underlie the articulation, something found in
ornithomimids and caenagnathids, which are distinctly derived claws whose
features are otherwise not seen in *Yixianosaurus*; the claws also bear
two lateral grooves, rather than one, until they combine and terminate
near the tip, otherwise seen only in some caenagnathid claws.

  The radius and ulna both are bowed, features found in maniraptorans and
the possible maniraptoran *Coelurus*.

  The humerus is said to have a very short and rather rectangular
deltopectoral crest, as in *Deinonychus* (pg. 42) and it would seem this
to be true; the humerus is sigmoid in lateral view, so that the humeral
head inclines backward, and the distal end forwards, as occurs in
maniraptoriforms with a derived, folding arm; the shaft of the humerus is
straight, rather than curved, however.

  The coracoid is deeper than long, but has a rounded cranial margin, and
dorsal margin, and appears rather trapezoidal in view, similar to
*Caudipteryx* and *Sinosauropteryx*; the biceps tubercle is lower than
half the coracoid height, rather only one-third up the height, and the
"cranioventral" process is long and distinct; the glenoid appears to be
turned slightly laterally.

  The scapula is preserved in two views: the left scapula is shown in side
view, but is missing the lower half, preparation indicating it is not
buried in matrix; the left scapula is preserved in ventral view and
twisted compared to the in situ coracoid, and the glenoid is preserved in
ventral view, and does not show that the lateral glenoid was any more
laterally oriented than, for example, oviraptorids. The distal end is
rounded, and slightly expanded, rather than flat-ended or tapering.

  No furcula or sternum is preserved.

  The ribs are preserved as portions of the proximal ends, as well as the
shafts, and include the expanded ends that show they attached to
cartilagenous sternal ribs, so these were likely the very first few ribs;
the preserved rib head indicates an anterior to middle-series rib, because
the capitulum and tuberculum are at right angles, and the capitulum is at
a right angle to the distal end of the preserved shaft portion, broader
than is present in more anterior ribs, which are more similar to cervical
ribs.

  Integument is preserved along the right arm, and its orientation
suggests that it was present attached to the distal humerus, along the
ulna, and along the hand, probably following the orientation of the second
finger. Claw sheaths are also present, and only appear to extend the claw
length about a 1/3 longer than the bony claw is preserved. The claw
geometry is not fairly strongly curved, forming about 90 degrees of arc on
the first claw, almost 100 degrees on the second, and about 80 degrees on
the third.

  Overall, the material seems to indicate a non-avian, if it weren't for
the very large hands. The scapula was shorter than the humerus, and the
manus at 124mm is almost equal to the length of the rest of the arm, where
the humerus + radius is 153mm. These measurments do not include the
carpals in their calculations. The coracoids do not appear to have been
angled ventrally or more than even with their scapulae, and the scapulae
do not have distinct processes for the furcula, as in maniraptorans.
Rather, this complex is nearly identical to *Sinosauropteryx*. All in all,
this form seems to be a basal maniraptoran, possibly more basal, even
though the hand is protracted to such a length. Theoretically, and without
any other analysis, this can be considered a convergence to using the
hands to climb with, as their curvature would indicate. The simple
integument makes it unlikely *Yixianosaurus* was a glider, but may, as
conjectured for *Sinornithosaurus*, leaps among branches much as modern
galagos, spider monekys, and sifakas do.

  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)

__________________________________
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software
http://sitebuilder.yahoo.com