Skull switch

Boss Switches are items found in Super Mario Land and New Super Mario Bros., which cause the defeat of the boss they are in the room of. They replace the axe, found in the original Super Mario Bros. and Super Mario Maker, and function the same way.

''New Super Mario Bros.
In total, Boss Switches (bearing a skull design and Bowser-style collar around their base) are found in three boss fights; the first with Bowser, the second with Dry Bowser, and the third with Bowser and Bowser Jr. In the aforementioned boss battles, Mario's goal is to avoid the attacks of the bosses and hit the Boss Switch, causing the bridge which Bowser is standing on to collapse and send him plummeting downwards to whatever may lie beneath. When Mario sets foot on the safe platform with the switch, Bowser/Dry Bowser will run towards him and try to toss him back onto the bridge. Even though Mario can defeat Bowser with fireballs, he still needs to press the switch as in the original Super Mario Bros.

In later NSMB games, starting from New Super Mario Bros. Wii, the skull-iconed Boss Switches are replaced by ! Switches.