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PALEOZOIC ERA

CARBONIFEROUS PERIOD

318-300 million years ago

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The shoulder was modified to become more mammal-like.

In the shoulder, synapsid reptiles evolved a second coracoid bone (and it is this coracoid which is retained as coracoid process in mammals). The cleithrum was reduced to a very thin bone in ophiacodont pelycosaurs and had become vestigial by the sphenacodont pelycosaurs. The clavicle expanded medially.