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 superheroes became one of the most prominent genres 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, when the superhero genre became one of the most prominent after Superman appeared in 1938.