Complex sentence: The Southern Elephant Seal ("Mirounga leonina"), one of two species of elephant seal, is not only the most massive pinniped, but also the largest member of the order Carnivora living today.
Split into simpler sentences: The Southern Elephant Seal ("Mirounga leonina") is one of two species of elephant seal. It is not only the most massive pinniped but also the largest member of the order Carnivora living today.