Complex sentence: This led to an unusual development; phonetically, the ending was treated as the diphthong rather than containing a semivowel, and the sound was in many cases dropped; it did not participate in the sound shift from to.
Split into simpler sentences: This led to an unusual development; phonetically, the ending was treated as the diphthong rather than containing a semivowel, and in other cases the sound was simply dropped. We know this because it did not participate in the sound shift from to.