Perucetus colossus substantially pushes the upper limit of skeletal mass in mammals, as well as in aquatic vertebrates in general. This early basilosaurid whale combines a gigantic size and the strongest degree of body mass index known to date. The ancient creature also potentially represents the heaviest animal ever described.
Reconstruction of Perucetus colossus in its coastal habitat; the associated sawfish, Pristis, was recovered from the same unit in the East Pisco Basin, the Yumaque Member of the Paracas Formation, Peru. Image credit: A. Gennari.
Perucetus colossus lived approximately 39 million years ago (Eocene period) in what is now Peru.
This whale was a type of basilosaurid, a family of extinct cetaceans that lived during the Eocene and are known from all continents, including Antarctica.
“Basilosaurids are unique in the sense that they acquired large sizes — up to around 20 m (66 feet) in body length — and body mass index (BMI),” said Staatliches Museum für Naturkunde Stuttgart paleontologist Eli Amson and colleagues.
“The degree of their BMI nevertheless did not match, up until now, that of some sirenians, for example, of which the whole rib cage is both strongly osteosclerotic and pachyostotic.”
The paleontologists unearthed the partial skeleton of Perucetus colossus — including 13 vertebrae, 4 ribs and 1 hip bone — in southern Peru in 2013.
“Each vertebra weighs well over 100 kg and the ribs of the prehistoric whale reach a length of 1.4 m (4.6 feet),” they said.
“At five to eight tons, the 20-m-long skeleton of the new species is two to three times heavier than the 25-m- (82-foot) long skeleton of a blue whale (Balaenoptera musculus) on display in the Hintze Hall of the Natural History Museum in London.”
The researchers estimated that Perucetus colossus had a body mass between 85 and 340 tons.
As the estimated body mass equals or exceeds the body mass of the blue whale, the new species would challenge the latter’s status as the heaviest animal ever to exist.
The findings indicate that cetaceans had reached peak body mass an estimated 30 million years before previously assumed, with the features of Perucetus colossus being fully adapted to an aquatic environment.
The buoyancy associated with the increase in bone mass is consistent with a shallow-water lifestyle, supporting the theory that basilosaurids were hyper-specialized for this type of coastal environment and shedding further light on this order of ancient animals.
“For us, one of the crucial findings of the work is that the transition to true gigantism in whales evolved much earlier in Earth’s history than we previously thought,” Dr. Amson said.
“Perucetus colossus combines gigantic size with extremely high skeletal weight and lived 39 million years ago.”
“This early whale drastically shifts the previously known upper limit of skeletal mass in mammals and aquatic vertebrates. It may also be the heaviest animal ever described.”
The findings were published in the journal Nature.
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G. Bianucci et al. A heavyweight early whale pushes the boundaries of vertebrate morphology. Nature, published online August 2, 2023; doi: 10.1038/s41586-023-06381-1
