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

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 an
epiphyseal ossification center within its cartilage), but this is considered
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.