This is being touted as a landmark episode of The Simpsons, and with very good reason. It’s episode number 636 since this animated Fox series began as a full-length prime-time show in 1989, expanding from interstitial slots on The Tracey Ullman Show to launch with a Christmas special, “Simpsons Roasting on an Open Fire.” And that means The Simpsons, with tonight’s episode, finally passes CBS’s landmark Western Gunsmoke as the scripted TV series with the most episodes in TV history. There remains room to nitpick, if you’re so inclined: For much of its run, TV’s Gunsmoke was an hour-long drama, so it ended up generating more hours of TV than The Simpsons. Also, because so many more episodes were televised each season when Gunsmoke moved from radio to TV in 1955, it took Gunsmoke only 20 years, until its cancellation in 1975, to amass 635 episodes – while it took The Simpsons just under 30. Oh, and finally, this important note: Even after  Gunsmoke was canceled, the show, and its star, James Arness as Matt Dillon, came back to TV in a series of four two-hour made-for-TV movie reunion sequels, starting with 1987’s Return to Dodge and ending with 1992’s To the Last Man. So even counting the extra time allotted for The Simpsons Movie in 2007, the Simpsons folks and the residents of Springfield still have some work to do to really lap the residents of Dodge City. But 636 episodes is an astounding achievement regardless, and will be noted, along with a Gunsmoke joke or two involving Homer and his dad, in tonight’s new episode. I expect The Simpsons to keep going for quite a while longer – and I fervently that when episode 666 arrives, it just happens to be with a Treehouse of Horror installment…