Simplify the following text for a Grade 6 native English-speaking person.
Original: Othniel Charles Marsh ( October 29 , 1831 -- March 18 , 1899 ) was an American paleontologist .
Simplified: Othniel Charles Marsh M.A PhD LL . D ( October 29 , 1831 -- March 18 , 1899 ) was one of the leading paleontologists of the 19th century .