Newly discovered fossils of a giant whale that may have been the largest animal to ever exist

I'd always been told that the largest animal the Earth has ever seen is the blue whale: heavier even than any dinosaur.

But scientists have reported in Nature that they've found fossils of an ancient whale that they believe could have been even larger, Perucetus colossus.

Here we describe Perucetus colossus — a basilosaurid whale from the middle Eocene epoch of Peru. It displays, to our knowledge, the highest degree of bone mass increase known to date, an adaptation associated with shallow diving. The estimated skeletal mass of P. colossus exceeds that of any known mammal or aquatic vertebrate.

The blue whale weighs in at 190 metric tons, and the partial skeleton (vertebrae and ribs) of P. colossus suggested to the researchers the possibility of a weight up to 340 metric tons.

The Washington Post reports:

But not everyone is convinced this colossus, while undoubtedly big, is truly more massive than a blue whale. The research team acknowledges their estimates for the animal's body mass range widely, from 85 tons all the way up to 340 tons. The team exhumed only a partial skeleton without a skull, leading some scientists to say more fossils are needed before anyone names a new heavyweight champion of the animal kingdom.

"I don't think we know enough about this group of whales to really weigh in on which interpretation on its body weight is the right one," said Nick Pyenson, a paleontologist at the Smithsonian Institution. "I'm really skeptical of these high-end estimates."

But he added: "Clearly, it is really big."

I was curious about how the illustration in the posted tweet, taken from the Nature paper, could include that ridiculous-looking tiny head and those intermediate, vestigal limbs when the only fossils we have are vertebrae and ribs. In the journal, the illustration's caption explains:

Because portions of the skeleton are unknown, several aspects of the reconstruction are tentative: the overall proportions of the axial postcranium are based on a close relative Cynthiacetus peruvianus, which was scaled-up and dilated according to the elements recovered for P. colossus…; the skull and limbs were only scaled up; the tail fluke and forelimb use (bottom walking) are based on the manatee (Trichechus), the extant marine mammal with the closest degree of pachyosteosclerosis in the postcranial skeleton; the hind limb of P. colossus was not recovered, but the anatomy of its innominate indicates the presence of a reduced, articulated leg.