Correct me if I'm wrong, but I interpreted the original question to mean the roots of the actual melodic rock sound, not simply music that was inspirational. The Beatles were certainly the latter, but I don't think that they ever truly had a "melodic rock sound" in the sense that we're discussing here. Boston's debut album from 1976 is an excellent starting point. I would say that Equinox from Styx in 1975 is worth mentioning for at least a few of its songs. Foreigner's debut in 1977, Journey's Infinity in 1978, and REO Speedwagon's You Can Tune A Piano, But You Can't Tuna Fish in 1978 are all relevant as well.