Atlanta rockers Mastodon have grown steadily weirder since their gritty early days, and it sure hasn't hurt them. The burly crew's latest purist-baiting move sees them joining forces with Butthole Surfers vocalist Gibby Haynes, whose sardonic drawl graces Side A of this Record Store Day EP.  Atlanta's loose, garage-ready distortion and grunge snap recalls vintage Prong and White Zombie. What's more, it sounds like the most fun Mastodon's had in years. Sadly, Side B is an instrumental, which feels like a waste of wax: If you've got Gibby Haynes in the studio, you'd better damn well use him.