A dog owner was in critical condition with a fractured skull after a man knocked him to the ground with one punch during an argument over his Chihuahua, neighbors and sources said. Paul Martone, 49, was at the corner of West 55th Street and Ninth Avenue about 6:30 p.m. when a blond man stepped over his dog — which neighbors identified as Peanut — to get onto the sidewalk, offending the victim and sparking an argument, sources said. The suspect shoved Martone before punching him in the face, knocking him to the ground and then fled the scene. The attack was caught on surveillance footage. Martone's head struck the pavement, fracturing his skull and possibly causing swelling to his brain, police said. He was in critical condition at New York-Presbyterian/Weill Cornell hospital, police said.