i was at the natural history museum in NY and noticed that only some of the animals on display at the bones exhibition had a collar bone. from what part of the anatomy did the clavicle evolve from and why do some animals have it and others dont? Thank you!

see
http://www.ncbi.nlm.nih.gov/pubmed/16894572
http://www.npr.org/templates/story/stor … &cc=fp
https://en.wikipedia.org/wiki/Clavicle#Other_animals