NEW YORK — It was a craze to rival the Hula-Hoop, and even less explicable. For a mere $3.95, a consumer could buy ... a rock — a plain, ordinary, egg-shaped rock of the kind one could dig up in almost any backyard. The wonder of it was, for a few frenzied months in 1975, more than a million consumers did, becoming the proud if slightly abashed owners of Pet Rocks, the fad that Newsweek later called “one of the most ridiculously successful marketing schemes ever.” Gary Dahl, the man behind that scheme — described variously as a marketing genius and a genial mountebank — died on March 23 at 78. A down-at-the-heels advertising copywriter when he hit on the idea, he originally meant it as a joke. But the concept of a “pet” that required no actual work and no real commitment resonated perfectly with the self-indulgent ’70s, and before long a cultural phenomenon was born.