Complex sentence: It was originally described in 1877 when Othniel Charles Marsh named the first known species, "Apatosaurus ajax", and a second species, "Apatosaurus louisae", was discovered in 1916.
Split into simpler sentences: It was originally described in 1877, when Othniel Charles Marsh named the first-known species, "Apatosaurus ajax". A second species, "Apatosaurus louisae", was discovered in 1916.