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Furculae, Form and Function



  The form of the furcula as a shock absorber requires certain
soft-tissue alignment, including the all-important clavicular
musculature. It also requires a morphology that also flexes in
only one plane, to optimize force during absorption of energy.
This is acheived in modern (Neornithine) birds by having
flattened rami, mediolaterally flattened, and the force is with
the shoulders contracting sagittally. In this state, the furcula
can act as a shock absorber. In more primitive birds, this is
not so, as the furcula is flattened craniocaudally, and would be
very difficult to bend in any way except to prevent the
shoulders from contracting towards eachother. The form of the
furcula generally is one where even craniocaudal compression is
reduced by the dimensions of the furcula, wider than
dorsoventrally deep, so that any force directed at the nadir of
the element must attempt to cross a broad C-shaped section and
compress it, or fail utterly. Such a shape, like an I-beam, is
very stably an anti-compression element.

  It is not likely early birds were using the element as a shock
absorber until about Neornithes or even Neognathae.... I don't
know about tinamous, sorry. :)

  Non avian theropods with a broad v-shaped furcula tend to bear
a ovate, sub-circular to circular, or ovate with a caudal
groove, section to the rami, and this can be considered along
the same lines as the primitive avian shoulder. However, the
rounder the shaft, the easier to bend. But in section, the
mediolateral section is elliptical through the furcula, and the
ovate rami even more so, minimizing any possible mediolateral
compression.

  I see this in only one possible function: to control the
positions of the shoulders relative to one another, and I feel
it likely that in coelurosaurus, this was derived to the degree
as to prevent any intra-scapular swing as seen earlier in
theropod evolution (coelophysoids have distally expanded
scapulae, and that in tyrannosaurids is mildly expanded, so that
it's probably swing was present...)

  Anyway, my thoughts.

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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