Perucetus colossus was an early whale of the basilosaurid family that swam in the oceans of the Eocene epoch between 39 and 37 million years ago. The study describing it estimated its weight as ranging between 94 and 375 tons, which would make it the most massive animal to have ever lived (for comparison, the modern blue whale has a maximum weight under 220 tons). So far, however, only parts of its rib cage and spinal column as well as vestigial hips have been found.