Complex sentence: These are usually thought of as innovations which are likely to have developed after the breakup of Proto-Indo-European, though it is also possible that some of these are not innovations, but rather shared conservative features.
Split into simpler sentences: These are usually considered to be innovations, which are likely to have developed after the breakup of Proto-Indo-European. It is also possible that some of these are not innovations; it is possible they are shared conservative features.