I heard many speculations about the function of the two clawed small arms on the T-Rex.
I have my own idea.
They had no function, it was loosing them through evolution. The T-Rex main assets were his strong head and his powerful hind legs, and probably his mighty tail as well. The need for the forearms was lost, and as they say, 'What you do not use you loose', and that is what was happening to the T-Rex's forearms. They were getting smaller and smaller until eventually he would have lost them completly.
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That makes sense through evolutionary adaption0