A whale from 40 million years ago, the heaviest animal that has inhabited the Earth

Today it is a sandy desert, but 39 million years ago the Ica Valley, in southeastern Peru, was all sea. And in that sea swam the heaviest animal on the planet. Only a few vertebrae, ribs and part of the pelvic bone have been found, but the former weigh more than 100 kilograms each and the latter measure 1.4 meters. The authors of the finding, published in the scientific journal Nature, estimate that the complete skeleton of this whale should weigh up to three times more than that of the blue whale, the largest animal known to date. Based on the ratio between bone mass and total mass of other whale species, they calculate that this cetacean could have weighed up to 340 tons. Blue whales rarely exceed 150 tons. They have named the new creature Perucetus colossus, from Peru and whale (cetus, in Latin). Colossus does not need to be translated.

The first find, that of a fossilized vertebra, occurred on a hill a few kilometers from the Samaca oasis and 15 kilometers from the current coastline. Its author was Mario Urbina, from the Natural History Museum of Lima, at the Universidad Nacional Mayor de San Marcos. Urbina had spent years searching the torrid desert for the remains of large marine vertebrates. It was difficult for them to excavate the first one, which was followed by another 12 vertebrae, all from what would be the lower part of the back and the lower back, four ribs and the right coxal bone, which would join the pelvis with a lower extremity that, as a marine animal, already , I did not have. Based on its position in the stratum, paleontologists estimate that this specimen of P. colossus lived and died between 39.8 and 37.8 million years ago. It would belong to the family of basilosaurids, the first exclusively marine cetaceans, mammals that changed the land for the sea about 50 million years ago.

As Giovanni Bianucci, from the University of Pisa (Italy) and one of the authors of the work published in Nature, recalls, P. colossus is not the largest animal discovered so far. “The largest are the blue whale (Balaenoptera musculus) among marine vertebrates and some extreme sauropods (such as Argentinosaurus) among terrestrial vertebrates,” he recalls. But it did have to be the most massive. Based on the size and weight of the bones found, scientists estimate that the complete skeleton should weigh between 5.3 and 7.6 tons. And with this range and knowing the skeletal mass of other cetaceans, which is between 2.2% and 5% of their total mass, they calculate that this colossus would weigh between a minimum of 80 tons and a maximum of 340 tons. The average value obtained in all the comparisons shows an average weight of 170 tons, thus surpassing the blue whale, which very rarely exceeds 150.

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The key to all these calculations is in the bones. It is not just that with them you can estimate the weight, shape and volume of the animal. It is that with only those few vertebrae and ribs it is possible to know key details of the life of this enormous whale. And it is that, as Bianucci says, they are not common bones: “No cetacean, alive or extinct, has such heavy and bulky bones.” All extant cetaceans, including the largest whales, share one characteristic: the processes, those pieces of bone that protrude from the vertebrae, are relatively thin. But the vertebral processes of P. colossus are comparatively huge, very thick. In medicine, this is called pachyostosis, but it is not a pathology, in this case it is part of the evolutionary design of the animal.

Part of the excavation team, revealing part of one of the gigantic vertebrae.Giovanni Bianucci.

Inside, the bones of this whale are also very different. Hans Thewissen of Northeastern Ohio Medical University in the United States is an expert on whale morphology. Not related to the new discovery, he has written an article in Nature analyzing the finding, in which he makes a comparison that helps to understand the relevance of the skeleton of the colossus: “The cross section of a mammalian bone resembles a baguette in that it having a hard, solid cortex (compact bone) surrounding a spongy interior (trabecular bone).” The aforementioned pachyostosis means that the compact part has grown at the expense of the trabecular part, with the consequent densification of the bone. The vertebrae and ribs of P. colossus have another peculiarity that in other animals (and humans) is a problem: osteosclerosis, where the increase in bone density is done at the expense of the marrow that they carry in the center.

The end result of this particular bone development is a very heavy skeleton. On land that would be a problem and only animals with aquatic habits have it, such as the hippopotamus. In the sea, there are only a few species of mammals with such large and heavy bones. They are the sirenians, distantly related to elephants. Only four species remain alive, three manatees and the dugong. Their habitat and way of swimming would be, according to the authors of the study, very similar to that of these animals, popularly called sea cows.

“No cetacean, living or extinct, has such heavy and bulky bones”

Giovanni Bianucci, paleontologist at the University of Pisa, Italy

“The skeleton of P. colossus shows a typical adaptation of diving animals in shallow coastal waters, such as increased bone mass,” says the researcher at the Stuttgart Natural History Museum (Germany) and senior author of the study. , Eli Amson. “This implies that the skeleton is heavier in this animal compared to its close relatives. We have the combination of an increase in compactness (all the internal cavities of the bones are filled with more bone tissue) and each bone also thickens, due to the additional layers of bone tissue deposited on the external surface of the bones. This gives the fossils this incredibly swollen appearance,” he adds. So unlike today’s large open-ocean whales, “the extra bone weight must have affected the animal’s overall buoyancy (like a divers’ lead belt), giving it the proper overall density to stay in shallow water.” deep”, ends Amson.

The discovery of this whale in the Peruvian desert also opens another avenue of investigation. Although from its beginnings in the sea there were large cetaceans, especially elongated ones, the gigantism of animals such as the sperm whale or the blue whale is relatively recent (about five million years ago). The new colossus, with its 20 meters and its possible 340 tons, means delaying the appearance of gigantism in the sea by almost 35 million. As for its diet, having not yet found its head, the mystery remains. The authors suggest that it could be a scavenger, catching whatever fell to the bottom. Asked by email, Thewissen concurs: “Since it must have spent a lot of time at the bottom of the ocean, its food was probably there. Maybe buried animals, shrimp and fish. Gray whales burrow to the bottom of the ocean in search of food.

Peruvian paleontologists have opened a financing campaign for Mario Urbina, who found the first vertebra of P. colossus, to finish tracking what is missing from this and other whales in the desert, in order to accommodate the heaviest animal as it deserves. of the world.

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