Timely Comics (the company that later became Marvel Comics) continued to innovate by inventing the first superhero crossover. The above panel in Marvel Mystery Comics #7 (March 1940) is the first time comic book characters from different “strips” have hinted at a shared universe. The Human Torch and Namor, the Sub-Mariner would clash in the next issue and as a consequence make the mythos of superhero worlds way more richer for decades to come.
Marvel was the first to do it in comic books and the first to do it in feature films years later!
Also, Namor was a little bitch.