Photograph by We Are The Rhoads / Trunk Archive
At least we thought they were.
In a new study published in Nature last Wednesday, researchers reported a newly discovered species and a contender for Earth’s heavyweight title: Perucetus colossus.
Described from a 40-million-year-old fossil excavated from Peru’s Ica Desert, the ancient whale weighed, by the most conservative estimate, at least 85 tonnes. It might have reached 340 tonnes, about equal to a full Boeing 747 during takeoff and nearly double the blue whale’s world record. At 20 meters long, P. colossus was slightly shorter than a blue whale, but its skeleton was so densely packed that it might have outweighed the present-day titleholder. The massive finding highlights how much there is still to discover about our planet’s past.
“What is really amazing is that such titanic sizes and weights occur early in the evolutionary history of cetaceans,” said Dr. Alberto Collareta, a paleontologist and a coauthor of the study from Università di Pisa in Italy.
Before this study, scientists thought that whale gigantism first evolved very recently—two to three million years ago, culminating in the present-day blue whale. The prevailing hypothesis is called the “wind to whales” scenario according to Dr. Will Gough, a marine biomechanist who studies cetacean energetics at the University of Hawaiʻi at Mānoa and the Hawaiʻi Institute of Marine Biology.
Gough, who wasn’t involved with the recent study, said that five to 10 million years ago, strong winds began pushing coastal surface waters offshore and lifted nutrient-rich water from the deep sea. This phenomenon, called coastal upwelling, produced phytoplankton blooms, which in turn supported dense aggregations of zooplankton, krill, and fish.
Some baleen whales called rorquals, which include blue whales, fed on these aggregations by lunge feeding: They accelerated into prey clouds with their jaws agape, engulfing mouthfuls of water and filtering them using comb-like baleens. Larger whales gulped greater volumes and fed more efficiently, giving them an energetic and evolutionary edge. Additionally, they could swim more efficiently across oceans to reach patchily distributed upwelling areas and endure longer fasting periods between feeding bouts.
Modern baleen whales, then, grew larger and larger to efficiently roam the oceans and exploit prey. Meanwhile, upwelling provided the fuel for them to reach epic sizes, entrapping them in a positive feedback loop that led to today’s “time of giants.”
The discovery of P. colossus, however, shows that gigantism didn’t evolve just this one time a few million years ago; it arose a separate time nearly 40 million years ago—just 10 million years after the first cetacean returned to sea—in a different ecosystem, for a different function, and in a whale with a completely opposite lifestyle.
“It’s really cool to see that we’re still finding new species,” Gough said. “We sort of think we know how it worked, what happened—then we find one new species, and we have to reinterpret and reinform what we knew.”
P. colossus belongs to an ancient whale family called Basilosauridae. “They looked like weird, gigantic dolphins,” said Dr. Robert W. Boessenecker, a paleontologist at the College of Charleston and the University of California’s Museum of Paleontology who wasn’t involved with the new research. He added that they had cartoonishly small heads, vestigial legs, and arms that they used like a seal. These early cetaceans were already fully aquatic, but they lacked the streamlined bodies that today’s whales have.
The newly discovered species was described based on a partial skeleton of 13 vertebrae, four ribs, and a right hip bone excavated from the Ica Desert of southern Peru. The dig site, which is hours away from the nearest village and devoid of paved roads, has no vegetation and is one of the most arid places on Earth according to Collareta.
“It preserves an exceptional number of marine vertebrates: whales, dolphins, very ancient ancestors of cetaceans, but it also has penguins, crocodiles, birds, lots of shark teeth, and even shark skeletons,” he said. “But no one imagined finding such a bizarre thing [like P. colossus].”
The fossil was first discovered 13 years ago by Mario Urbina, a coauthor of the study at Museo de Historia Natural in Lima and a “pillar of Peruvian paleontology” according to Collareta. “He found himself in front of this incredible specimen. The skeleton was preserved at the foot of a hill,” he added. But discovery was far from the hardest part. Each bone took three to four people to move; it required a winch to pull the bones out of the ground. In total, it took over a decade to fully excavate.
Collareta, who joined the team after the fossil was discovered, recalls feeling “skeptical” when he first saw a vertebra—the largest of which spanned nearly a meter and weighed 160 kilograms. “It was really unlike any other vertebra I had ever seen. It was amazing,” he said.
“No one imagined finding such a bizarre thing like P. colossus.”
Like many ancient species, it is only known by this single, incomplete skeleton. Although the researchers had a hunch that it was a contender for the heaviest animal of all time, to know for sure, they needed to estimate the skeletal volume, skeletal mass, and body mass of a hypothetical complete skeleton.
To estimate skeletal volume, the researchers scaled up the fossil of a closely related but smaller basilosaurid, Cynthiacetus peruvianus, to the new fossil’s size—a suitable choice, Boessenecker said, because that species is known from one virtually complete skeleton. Then, they measured the density of the preserved P. colossus fossil that they uncovered. They used the volume and density to produce a ballpark range of the entire skeleton’s mass. By their best estimate, it would have been between 5.3 and 7.6 tonnes—two to three times greater than that of a 25-meter-long blue whale and heavier than any known mammal or aquatic vertebrate.
That’s because each bone was packed to the brim with compact tissue—a trait called osteosclerosis. The vertebrae and ribs also had thick layers of extra bone coating the outside, a phenomenon called pachyostosis. The ribs were practically devoid of a bone marrow cavity. The vertebrae, meanwhile, resembled a ginger root—lumpy and unwieldy compared to the sleek wing-like analogs in blue whales and other modern-day cetaceans.
“The vertebrae are almost unrecognizable as whale vertebrae,” Boessenecker said. “The skeleton is so freaking weird… like what the hell is this?”
The dense bones signify a difference between the lifestyles of blue whales and P. colossus. They suggest that P. colossus dwelled on the seafloor in shallow, coastal water. These compact bones acted like a ballast, creating inertia to keep them steady in strong waves, Collareta said. By contrast, today’s whales have lighter bones filled with fat which helps them surface more efficiently from deep dives.
“[Bone mass increase] has happened many, many times in pinnipeds, sea otters, and virtually every single group of marine reptiles,” said Boessenecker. But “[P. colossus] is the king of bone mass increase.”
The estimated range of skeletal mass then allowed the team to predict the total body mass of P. colossus. In extant aquatic mammals, the skeleton makes up between 2.24 to 6.25% of the total body mass, from beaked whales to manatees, respectively. Bookending their estimates with these extremes, the researchers inferred that P. colossus would have weighed between 85 to 340 tonnes, putting it in fierce contention for the heaviest animal on record.
The method of inference provides “very reliable” estimates, even with a partial skeleton, Collareta explained. “I feel very comfortable with this range, which evokes Perucetus as the heaviest animal ever and well within the body mass range of the blue whale.”
The unknowns might feel paralyzing, but they also signify room for discovery…
It’s natural to wonder what P. colossus ate to support its massive body, but with no skull, it’s impossible to know. The tropical seafloor was teeming with nutrients, productivity, and food at the time, much like the upwellings that provide nutrients to modern-day whales. Maybe the ancient whale ate sessile invertebrates and slow-moving fishes from the seafloor, filter feeding them the way that gray whales do, the authors cautiously speculate. Boessenecker finds that plausible. Still, it’s not impossible that they were grazing seagrass like manatees or scavenging dead carcasses like sharks. “I want to see this thing’s skull… The skull will tell you a lot more about what it is eating,” he said.
Skull missing or not, Boessenecker is impressed by the skeleton and the research team’s thorough investigation of it. “They squeezed out every little bit of data that they could… They took this really puzzling specimen and told a fantastic story about it.”
Not only does the study show that whales might have reached peak size 30 million years earlier than previously thought; it also showed how opposite pressures drove the two separate instances of gigantism. “It’s such a different species from a blue whale,” Gough said. The present-day cetacean is streamlined, capable of bursts of acceleration and open ocean travel. “Meanwhile, this other animal just got really dense—just got super thick—and it’s living in shallow water like a giant manatee… Energetically, it seems they’ve taken the opposite route.”
The finding is a testament to how little we know about the history of life on Earth. The discovery itself, as monumental as it is, answers a rather elementary question: What species were around 40 million years ago? It can only speculate on the more interesting one: Why did this whale get so big?
“That’s a very good question. I’m not sure whether I can answer,” Collareta said. “This is a major question from an evolutionary point of view which we would like to pursue in the future,” noting that it would require more than one specimen and perhaps more fossils from close relatives.
Each eon in time that paleontologists travel back blurs the painting of the diversity of life, dappling it with broader impressionist brushstrokes. The unknowns might feel paralyzing, but they also signify room for discovery—room to wonder about a world so alien to ours, but one so recognizably Earth.
For Boessenecker at least, the finding is energizing. “Two words: F*ck yeah,” he thought after reading the study. “It really is a fantastic surprise. These are the surprises that motivate us to do what we do in paleontology.”
After all, if a 340-tonne whale could be hiding in the Peruvian desert, there’s no telling what else could be out there.