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Re: Development question
"The clavicle is the sole membrane bone of the shoulder girdle in mammals.
Its ends are preformed in cartilage (and in fact the medial end develops
epiphyseal ossification center within its cartilage), but this is
a secondary development acquired in mammalian evolution."
Both ends? This is interesting. The medial end is already thought to be an
endochondral bone in therian mammals -- the procoracoid, homologous to the
one & only coracoid of dinosaurs. But the lateral end?
This would imply that the bird clavicle is strictly
an intramembranous ossification.
Somewhere -- I believe in the journal Evolution and Development, but I could
be confusing this with an article on clavicles that I've definitely read
there* -- I've read that the clavicle (IIRC not specified of what animals)
is a "secondarily endochondral" bone, means, while it starts to ossify,
cartilage appears that later ossifies, too. This could, I used to think,
explain why Feduccia and/or some of his colleagues in the BAND have found
cartilage in the development of the furcula of I think a quail**. In any
case it sounds intuitively like a good explanation for why the clavicles of
most amniotes look like long limb bones while all other dermal bones I can
think of are plate-shaped. (Erm... except fin rays.)
* It mentioned a boy without clavicles. Otherwise healthy, but no
collarbones. They are simply switched off. The boy can make his shoulders
touch in front of his chest; there's a photo of this in the paper.
** And then went on to say that this endochondral part was the
interclavicle, proving that the furcula consisted of the clavicles plus
interclavicle, and thereby proving BAND because dinosaurs always lack an
interclavicle. Problems: a) the interclavicle is a dermal bone; b) in
*Hesperornis*, at least, the clavicles are unfused but touch in the midline
with a broad surface of contact.