Digging an old bison skull out gravel in a shallow creek.

200 years ago, 50 to 60 million bison roamed the prairies of North America. The average lifespan of a bison is 15 years resulting in a possible number of 400 million bison having lived on the plains during the 18th. Multiply that by 10 centuries and you have 4 billion bison. Multiply that by 10 millenia and you have 40 billion bison having had lived and died since the end of the last ice age.