Complex sentence: As a graduate student, he accompanied Lieutenant Colonel George Armstrong Custer's 1874 Black Hills expedition as a naturalist but declined a similar appointment to the ill-fated 1876 Little Big Horn expedition.
Split into simpler sentences: As a graduate student, he accompanied Lieutenant Colonel George Armstrong Custer's 1874 Black Hills expedition as a naturalist. He declined a similar appointment to the ill-fated 1876 Little Big Horn expedition.