Riding Shotgun

I was curious earlier today about the origin of the phrase, and found a good article on its history:

Thus, the sequence seems to be that the usage "shotgun guard" on a stagecoach in the Old West (say, the 1880s) evolved to "riding shotgun" in popular fiction about the Old West in the 1920s and 1930s, from there made its way into movies and television, was applied to automobiles in the 1950s, and finally was shortened to "shotgun" in the 1960s.