Complex sentence: The trilithons may have also represented a D shaped meeting house, also found at other Neolithic sites in Britain, this may have served as an imaginary meeting place for the ancestors of the stonehenge builders of which the monument was supposed to commemorate.
Split into simpler sentences: The trilithons may have also represented a D shaped meeting house of which similar structures have been found at other Neolithic sites in Britain. This may have represented a meeting place for the ancestors of the stonehenge builders of which their existence the monument was supposed to commemorate.