The eternally laconic J Mascis isn’t anyone’s idea of the kind of frontman suited for the main stage, in an evening slot on the first day of a festival. There’s no extra showmanship, no real acknowledgement that Dinosaur Jr. is anywhere but at another one of their gigs. Even with what amounts to a greatest hits set for them — “Feel The Pain,” “Little Fury Things,” “Freak Scene,” “Sludgefeast,” and their cover of “Just Like Heaven” were all present — it’s unlikely that Dinosaur Jr. were winning a lot of new converts at FYF’s opening day. Still, if Dinosaur Jr.’s particular brand of grimily massive, chugging guitar work is the kind of thing that works on you, then FYF was just another Dinosaur Jr. gig — a firm reminder that they’re one of the most surprisingly vital reunion acts in an era full of reunion acts, and a reaffirmation that few other three pieces can capture the immense squalor that these three men wield so easily.