As every schoolchild knows, the blue whale is not just the biggest animal alive today, but the biggest ever to have existed. Or is it? This stalwart of zoological record-breaker lists is being challenged for its title by a remarkable fossil unearthed in Peru.
When scientists came across the 13 vertebrae, four ribs and a single pelvic bone, there was doubt as to whether they were fossils at all. “The bones are so weird, so big and so dense that they look like pieces of marble,” says Eli Amson of the State Museum of Natural History in Stuttgart.
Only when Amson’s colleagues examined them microscopically did their origin become clear. “Bone fossilises extremely well right down to the cellular level, so the structure could be seen very clearly.”
The partial skeleton turned out to belong to a 39-million-year-old whale. After years of excavation, it was described in the journal Nature and named Perucetus colossus. It is a member of a family called the basilosaurids. “These were the first whales that were certainly fully aquatic,” says Amson, “because the hind limbs were extremely reduced compared to the rest of the body, so they were surely not able to get onto land.”
The biologists estimate that P. colossus would have been somewhat shorter than blue whales, which can reach about 30m in length. However, because of the density of its bones, it may have been significantly heavier, with a body mass of between 85 and 340 tonnes. The largest blue whales weigh just shy of 200 tonnes. Its lack of buoyancy would have restricted it to shallow waters, much like modern manatees and hippos.
“It’s a very interesting specimen, made all the more intriguing because of its incompleteness,” says Andrew Kitchener, principal curator of mammals at the National Museums of Scotland, who was not involved in the research. “The really fascinating part of the story is trying to work out what Perucetus fed on.”
Progress here is hampered by the lack of a skull and teeth. Modern whales can attain immense sizes, in part because they are able to filter vast quantities of tiny swimming prey using baleen plates, but there is no indication that basilosaurids could do this. Amson’s team speculates that Perucetus hoovered molluscs from the seabed. Kitchener agrees this is a possibility, but suggests it might also have been “some kind of ambush predator of things that move faster.”
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Main image: A view of fossilised remains of the colossus Perucetus colossus © Getty Images