Although lots of evidence nidicate that birds evolved from theropod dinosaurs, some scientists remained unconvinced.
Arguments:
1/ theropods appeared too late. the oldest known bird A appears earlier than NM, the closest known relatives of birds.
Against - i. Investigators have uncovered bones belong to NM dating to the time of A.
ii. Failure to find fossils doesn't rule out their existence
2/ the fused clavicles of birds differ from the unfused clavicles of theropods
Against - fossilized theropod clavicles look just like the clavicles of A.
3/ the complex lungs of birds could not have evolved from theropod lungs
=> can't be falsified yet since no fossil lungs are preserved in the paleontological record.