Final answer:
Francis Picabia was the leader of the American Dadaists and a close friend of Marcel Duchamp. Together with Man Ray, they challenged traditional art concepts in the U.S., significantly impacting future avant-garde movements.
Step-by-step explanation:
The leader of the American Dadaists and a friend of Marcel Duchamp was French artist Francis Picabia. Arriving in New York, Duchamp and Picabia, along with American artist Man Ray, formed a nucleus for the Dada movement in the United States. Notably, the Dada movement was well-known for challenging traditional conceptions of art, often utilizing everyday objects in their works — a practice Duchamp famously contributed to with his readymades. One of the more provocative examples was Duchamp's Fountain, which was a urinal turned into an art piece. This act of redefining what could be considered art had a lasting influence, shaping future avant-garde and conceptual movements such as Pop Art and Fluxus.