The largest and heaviest animal to ever live on the Earth, as far as we know, is the blue whale, which is extant today. The blue whale is larger than any dinosaur, even the giant sauropods. The average weight of a blue whale is 160 tons, with the largest specimen being 190 tons, and 110′ 17″ (33.58m) long. The largest sauropod, Argentinosaurus, weighed up to 110 tons. The reason the largest whales are bigger than the largest dinosaurs is simple – whales swim in the ocean, so they have buoyancy to help carry their incredible heft. The ancestors of whales were land mammal of modest size. It was only when they adapted to the water that they grew very large, and the age of gigantism among whales started about 4.5 million years ago.

At least that is what we thought from existing evidence. That is one of the interesting things about paleontology – a single specimen can upend our phylogenetic charts, the history of what evolved into what and when. Essentially we have scattered puzzle pieces that we try to fit together into a branching tree of evolutionary relationships. One specimen that fits outside of the branches of this tree forces scientists to redraw some of the lines, or add new ones.

That is what has happened with a new extinct whale species, discovered in Peru in 2010 but only recently described in detail. The species is appropriately named Perucetus colossus, and it is a whopper. Scientists estimate the weight at 85 to 320 tonnes, depending on assumptions about soft tissue like organs and blubber. If we take the middle of that range, 180 tonnes, that puts it at the upper range for blue whales. If we assume this is an average specimen (statistically likely but not a guarantee) then its size range may exceed that of the blue whale. Perucetus is not, however, longer than the blue whale, it’s a little shorter. But it’s bones are a lot heavier, they are denser and overgrown, which is an adaption found in other shallow water mammals. It’s the heavy bones that makes it potentially heavier than the blue whale, and regardless, this species has the heaviest skeleton known.

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