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Does the human or bird
sternum consist of multiple elements in any phase of the development? I mean,
you say that these elements fuse prior to ossification, but are there multiple
pieces of cartilage around in the beginning, or does the whole cartilagenous
structure appear in one piece >from the very beginning? I believe
Hinchliffe has shown that the cartilage giving rise to the semilunate carpal
in birds is a unified whole throughout its existence.
I don't know how it actually runs in
ontogeny; what I know is that phylogenetically there is a left and a right
sternum (often incompletely fused in whales -- pers. obs. from skeletons in
museums), and that placentals and mammals have fused at least the interclavicle
to it (still separate in Zhangheotherium).
So I still think the semilunate of birds
consists of the distal carpals 1 and 2.
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