Whale fossil may be the heaviest animal ever

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A colossal ancient whale discovered in Peru might be the heaviest animal on record, according to a new study.

At an estimated body mass of 85 to 340 metric tons (187,393 to 749,572 pounds), the heft of the now-extinct Perucetus colossus is equivalent to or exceeds that of the blue whale, which had indisputably been considered the animal with the greatest body mass until now, said Giovanni Bianucci, first author of the study published Wednesday in the journal Nature.

The partial skeleton of Perucetus, consisting of 13 vertebrae, four ribs and one hip bone, is estimated to be 17 to 20 meters (55.8 to 65.6 feet) long. The fossil specimen is shorter than that of a 25-meter long (82-foot-long) blue whale — but its skeletal mass still potentially exceeded that of any known mammal or sea vertebrate, including its gigantic relative, according to the study.

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What’s more, Perucetus likely weighed two to three times more than the blue whale, which today weighs a maximum of 149.6 metric tons (330,000 pounds).

“Perucetus could have weighed almost two blue whales, three Argentinosaurs (a giant sauropod dinosaur), over 30 African bush elephants and as many as 5,000 people,” Bianucci said via email. He is an associate professor of paleontology at the University of Pisa’s department of Earth sciences in Italy.

Perucetus likely swam slowly due to its enormous body mass and swimming style, which was undulatory and anguilliform, meaning its flexible body moved in curvy waves from head to tail.

The bones of Perucetus “are made up of extremely dense and compact bone,” Bianucci said. “This kind of thickening and heaviness of the skeleton — named pachyosteosclerosis — which Perucetus shares with sirenians is not found in any living cetacean.” Sirenians are large aquatic herbivorous mammals, such as manatees, sea cows and dugongs.

The weight and size of Perucetus could have been evolutionary adaptations to life in shallow and agitated coastal waters, he said, “where a particularly heavy skeleton acts as a ‘ballast’” for stability.

The discovery is the latest result of a diverse group of researchers’ “intense activity” that began in 2006 in the Ica (Valley) in southern Peru “in one of the most important fossil vertebrate assemblages of the Cenozoic Era” that occurred about 66 million years ago, Bianucci said.

Other specimens previously found in this area include “Peregocetus pacificus, the most ancient quadrupedal cetacean to have reached the Pacific Ocean, Mystacodon selenesis, the earliest ancestor of the modern baleen whales, and the enormous macropredatory sperm whale Livyatan melvillei,” he added.

“The extreme skeletal mass of Perucetus suggests that evolution can generate organisms with characteristics that go beyond our imagination,” Bianucci said.

The first Perucetus vertebra was discovered by Peruvian paleontologist Mario Urbina Schmitt more than 10 years ago, Bianucci said. Schmitt, a coauthor of the study, is a researcher and field collector in the department of vertebrate paleontology at the Natural History Museum of the National University of San Marcos in Lima.

Excavation from the silt-clay Paracas Formation “took several years due to the hard rock, the fact that the fossil was inside the core of the mountain, the extreme size of the bones and the hard environmental conditions of the Ica desert,” he explained.

At approximately 39 million years old, Perucetus colossus is new to the basilosaurid family within the order Cetacea, which includes whales, dolphins and porpoises. The gigantic creature’s name denotes its geographic origin, Peru; “cetus,” the Latin word for whale; and “kolossós,” which is ancient Greek for “large statue.”

“Discoveries of such extreme body forms are an opportunity to re-evaluate our understanding of animal evolution,” wrote J.G.M. Thewissen and David A. Waugh, who weren’t involved in the study, in a commentary on the research. Thewissen is the Ingalls-Brown Professor of Anatomy at Northeast Ohio Medical University; Waugh is a postdoctoral researcher in the department of anatomy and neurobiology at the same university.

“It seems that we are only dimly aware of how astonishing whale form and function can be,” they added.

The findings suggest that gigantism or peak body mass among cetaceans had been reached around 30 million years earlier than previously thought, according to the study.

Given the amount of heavy bone, Perucetus “must have had a lot of lighter tissues, too,” Thewissen and Waugh wrote. “This is a fundamental difference compared with animals living on land for which all tissues contribute to weight that needs to be supported by body parts such as limbs. By contrast, in water, heavier tissues can be offset by lighter tissues to acquire neutral buoyancy, and total mass is less important.”

The Perucetus specimen appeared to have reached sexual maturity, experts said, but might have still been growing, given the unfused ends of its vertebrae.

The authors didn’t have the animal’s skull or teeth, but its known characteristics indicate Perucetus likely fed near the bottom of the sea and wasn’t an active predator, Bianucci said.

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This feeding behavior is unusual compared with most whales that use their relatively light skeletons to chase fast-moving prey, Thewissen and Waugh wrote.

The study authors have three hypotheses about Perucetus’ diet, Bianucci said: The whale might have been a plant eater like a sea cow, but this herbivorous diet would be the only case among cetaceans. Secondly, the ancient creature could have fed on small mollusks and crustaceans in sandy bottoms like the contemporary gray whale does. And thirdly, maybe Perucetus was a scavenger of vertebrate carcasses.

“Surely we will continue to explore the desert of Ica to find other fossils which can allow us to tell even more details on the extraordinary evolutionary history of cetaceans,” Bianucci said.

Knowing more about Perucetus’ life history could help answer other questions, such as whether the fossil is a testament to the origin of blubber, Thewissen and Waugh wrote.

“That hypothesis is consistent with the fossil’s age of around 39 million years old, from a time when Earth and the oceans were cooling and insulating blubber might have been an advantage,” they added. “It is too early to tell, but such considerations demonstrate that the importance of this fossil goes beyond the documentation of a previously unknown life form.”