Gunsmoke was never a 'serial' in the accepted sense of the word, Just a continuing series with the same characters.In fact they changed very little over the whole 20 seasons,other than cast changes.
Curiously I had some conversation with a company here in the UK who have been putting out the 'Bonanza' series in regular installments. They send a DVD out every month or so together with magazine about the show to subscribers. All the editorial team are pretty young , nearly all in their early 20's so don't remember the series running.They wanted to know about story arcs & continuing characters & how they developed over the seasons. So many of todays shows do go in for this form of storytelling & are much more 'serialised'.. I had to explain that the old shows didn't much operate like this. Apart from the main cast, characters came & went within an episode & were mostly never heard of or mentioned again. They were quite surprised.