Complex sentence: American comics emerged as a mass medium in the early 20th century with the advent of newspaper comic strips; magazine-style comic books followed in the 1930s, in which the superhero genre became prominent after Superman appeared in 1938.
Split into simpler sentences: American comics emerged as a mass medium in the early 20th century with the advent of newspaper comic strips. Magazine-style comic books followed in the 1930s; the superhero genre became prominent after Superman appeared in 1938.