Last year, the Blue Planet’s heaviest ever animal lost its titanic title to a pile of old bones.
Several researchers discovered 13 vertebrae, four ribs, and one hip bone from a 39-million-year-old animal called Perucetus colossus. Scant as they were, the remains convinced the group that the ancient animal — though only half as long as a blue whale — could be double its weight.
An artist’s impression of the extinct whale. Image: Alberto Gennari
It was no light-duty claim; blue whales (Balaenoptera musculus) can weigh 270 metric tons. The idea revolved around bone density, which the researchers measured with 3D scanning and drilling.
But not everyone agreed, and now one new research group has refuted their findings.
Ryosuke Motani, a paleobiologist at the UC Davis Department of Earth and Planetary Sciences, described his skepticism in a statement.
“It would have been a job for the whale to stay at the surface, or even to leave the sea bottom,” Motani said. “It would have required continuous swimming against gravity to do anything.”
Flawed assumptions
Motani and the Smithsonian Institute’s Nick Pyenson together poked holes in the narrative.
First, they examined the previous team’s choice to use the weight of the fossilized bones to extrapolate the weight of the entire animal. The assumption, according to Motani and Pyenson, is that skeletal and non-skeletal mass would scale at the same rate with increasing body size.
Unfortunately, this doesn’t track with all aquatic mammals. Manatees — plump as they are — are fairly light relative to their skeletal mass.
As Motani suggested, the whale’s lifestyle just wouldn’t have accommodated such a heavy body. He and Pyenson estimate that Perucetus colossus instead weighed a much more sustainable 60 metric tons.
“The new weight allows the whale to come to the surface and stay there while breathing and recovering from a dive like most whales do,” Motani said.
