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Shoulder articulation (was" eaten words and food for thought")



 Colette H. Adams <cadams@hh.gpz.org> wrote:
> On the subject of chameleons I must disagree.  The oft-repeated
> "observation" that chameleons have erect postures is something of an
> exaggeration.

Right.  Old World tree chameleons do set down their feet directly under
their bodies when they walk along narrow branches-- eschewing the typical
reptilian sprawl -- but they do not walk with straight legs.  (Nor do
echidnas, for that matter).  Digressing a bit, it is interesting to note
that Dr. Robert T. Bakker has remarked on the unusual swinging shoulder
blades employed by chameleons (TDH, pp.220-223), which he compares to the
shoulder blades of some mammals and (presumably) dinosaurs.  

> Chameleon humeri deflect considerably from vertical,
> typically they hold them angled both laterally and caudally.  Their legs
> sweep outward while walking in typical lizard fashion and their trunks
make
> typical lizard machinations.

It may interest list members that paleolife sculptor and paleontologist
Stephen Czerkas brought up the topic of dinosaur shoulder anatomy and
movement at the Dinosaur Sculpture Panel Discussion at Dinofest '98 on
April 17, 1998.  He pointed out that skeletal mounts are often
reconstructed improperly, with the coracoids set much too far apart,
distorting not only the look of any restoration derived from such a mount,
but also leading to a misunderstanding of how the forelegs articulate with
the shoulder in quadrupedal dinosaurs.  According to Czerkas'
interpretation, the legs do contact the ground directly beneath the body,
in the basically erect posture which the trackways indicate, but the
forelegs are obligated to swing outward from the body in a semi-reptilian
fashion when the dinosaur walks.  He bases this view on his (unpublished)
studies of the humerus-shoulder joint anatomy.  No doubt this is a
controversial opinion.  Would anyone care to comment on the articulation of
the dinosaur shoulder during quadrupedal locomotion?

-- Ralph Miller III     gbabcock@best.com