Complex sentence: He began assisting Denis Lebrun on the "Blondie" comic strip in December 2002 and became head artist in May 2005, although this position remained uncredited until Sunday January 7,2007.
Split into simpler sentences: He began assisting Denis Lebrun on the "Blondie" comic strip in December 2002. He became the head artist of "Blondie" in May 2005, although he remained uncredited until Sunday, January 7,2007.