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Climbing Dromaeosaurs (RE: a little background)



Tim Williams (TiJaWi@agron.iastate.edu) wrote:

<Try: http://qilong.gq.nu/Sinornithosaurus%20in%20a%20tree.jpg
  
(where are the legs?)  Nice picture though.>

  Thanks. The legs are there, but raised up so that the foot is level with
the pectoral region. This is possible is the femur is elevated sagittally
as most birds and reptiles can acheive ... but is rarely restored. Greg
Paul shows probably the only other examples of this in recent paleoart.
Here, the mechanics I propose for dromaeosaur climbing are such that the
animal acts like a pole-climber, using the arms to straddle the trunk, and
the pedal claws as pitons. The feet would probably be best used for this
purpose as the pull of gravity would cause the claws of the manus to
resist lateral bending forces in the only position they could be employed
in. I did an earlier drawing where I had the arm raised and held parallel
to the sagittal plane, but realized that scapular glenoid would not allow
this position for the humerus, as the humeral caput is not offset from the
shaft, and the glenoid is oriented laterally and posteroventrally.

  It's then a question of stability in the arms, with the brute strength
for climbing to be acheived in the legs, and the tail may be used as a
brace (which could explain elongation of the caudal verts in early
dromies, and stiffening of the shaft -- also as a result of aerodynamic
spar stiffening). Persistence of tail and pedal anatomy in smaller forms,
reduction in larger forms (most troodontids, *Adasaurus*) suggests that
these features were continuing to be employed in even large-bodied forms
for the original purposes of their development. Loss in birds may occur as
a decoupling of a scansorial capability, and either a purely terrestrial
launching ability, or minimalized arboreal living. One does not need to
have any scansorial ability to be arboreal, and this is implicit in many
arboreal capable birds that are extremely adapted to other environments,
including vultures, cranes, and duck, both of which are noted for
non-scansorality.

<Don't forget, birds can fly too, alleviating the need for specialized
tree-climbing equipment in order to reach the tree-tops (unless the bird
is specialized for vertical climbing, such as woodpeckers).>

  That said, scansorality should be restricted to the quality and
adaptations know for climbing, and climbing only. The ability for
squirrels to twist their angles and crawl down a trunk is one such
feature, and that of the strongly curved claws of woodpeckers and the
stiffened retricial ramus are two others.


=====
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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