重さ３４０トン、ペルーで新種クジラの化石発見　史上最重量の生物か

【リマＡＦＰ＝時事】ペルーで見つかった４０００万年近く前に生息していたクジラの一種の部分化石を調査したところ、史上最大の生物であった可能性が浮上している。英科学誌ネイチャーに２日、ペルーと欧州の古生物学者チームが論文を掲載した。（写真はペルーで化石が見つかったクジラの仲間「Ｐｅｒｕｃｅｔｕｓ ｃｏｌｏｓｓｕｓ」の想像図。英ネイチャー・パブリッシング・グループ提供）
　この古生物は「Ｐｅｒｕｃｅｔｕｓ ｃｏｌｏｓｓｕｓ（ペルーの巨大クジラ）」と名付けられ、重さは最大３４０トンと推定されている。これまで史上最大の生物はシロナガスクジラとされ、ギネス世界記録には１９０トンの個体が登録されているが、それを大幅に上回ることになる。
　調査に携わった古生物学者らによると、体重は８５～３４０トン。平均で１８０トンとみられる。体長は２０メートル程度と推定されている。
　最初の化石は２０１０年、ペルー南部の海岸地帯の砂漠で数十年にわたって調査してきた古生物学者のマリオ・ウルビナ氏が発見した。これまでに脊椎骨１３個と、肋骨（ろっこつ）４本、寛骨（かんこつ）１個が見つかっている。脊椎骨の一つは重さ２００キロ近くあった。
　ウルビナ氏が見つけた化石は、首都リマの自然史博物館で初めて一般公開された。同氏らのチームは、化石をさらに収集して全体像を把握するには少なくとも数年を要するとしている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/08/03-17:15）

Look out, blue whale -- there's a new contender for your heavyweight title.
A newly discovered whale that lived nearly 40 million years ago could be the heaviest animal to have ever lived, based on a partial skeleton found in Peru, scientists said on Wednesday.
The modern blue whale has long been considered the largest and heaviest animal ever, beating out all the giant dinosaurs of the distant past.
But Perucetus colossus -- the colossal whale from Peru -- may have been even heavier, according to a study published in the journal Nature.
Extrapolating from some massive bones found in the Peruvian desert, an international team of researchers estimated that the animal had an average body mass of 180 tonnes.
That would not take the heavyweight title by itself. The biggest blue whale ever recorded weighed 190 tonnes, according to Guinness World Records.
But the researchers estimated the ancient whale's weight range was between 85 and 340 tonnes, meaning it could have been significantly larger.
The first fossil of the ancient whale was discovered back in 2010 by Mario Urbina, a palaeontologist who has spent decades searching the desert on the southern coast of Peru.
There is no record of the existence of an animal as large as this, it is the first, that's why nobody believed me when we discovered it, Urbina told AFP in Lima.
According to the researcher, this discovery is going to cause more questions than answers and give the rest of the palaeontologists a lot to talk about.
The remains were presented to the public for the first time during a press conference at the Natural History Museum in the Peruvian capital, where they are being displayed.
The researchers estimate that the animal reached about 20 meters (65 feet) in length.
- Rewriting cetacean history -
The researchers were careful not to declare the ancient whale had broken the record.
But there was also no reason to think that this specimen was the largest of its kind, study co-author Eli Amson told AFP.
I think there's a good chance that some of the individuals broke the record -- but the take-home message is that we are in the ballpark of the blue whale, said Amson, a palaeontologist at the State Museum of Natural History Stuttgart in Germany.
A total of 13 gigantic vertebrae -- one of which weighed nearly 200 kilograms (440 pounds) -- were found at the site, as well as four ribs and a hip bone.
It took years and multiple trips to collect and prepare the giant fossils, and even longer for the team of Peruvian and European researchers to confirm exactly what they had found.
On Wednesday, they revealed it is a new species of basilosaurid, an extinct family of cetaceans.
Today's cetaceans include dolphins, whales and porpoises, but their early ancestors lived on land, some resembling small deer.
Over time they moved into the water, and basilosaurids are believed to be the first cetaceans to have a fully aquatic lifestyle.
One of their adaptations at that time was gigantism -- they became very big.
But the new discovery indicates that cetaceans reached their peak body mass roughly 30 million years earlier than previously thought, the study said.
- Tiny head, heavy bones -
Like other basilosaurids, Perucetus colossus likely had a ridiculously small head compared to its body, Amson said -- though there were no available bones to confirm this.
Lacking any teeth, it was impossible to say for sure what they ate. But Amson speculated that scavenging off the seafloor was a strong possibility, partly because the animals could not swim quickly.
The researchers were confident that the animal lived in shallow waters in coastal environments, due to the strange heaviness of its bones.
Its whole skeleton was estimated to weigh between five and seven tonnes -- more than twice as heavy as the skeleton of a blue whale.
This is -- for sure -- the heaviest skeleton of any mammal known to date, as well as any aquatic animal, Amson said.
Perucetus colossus needed heavy bones to compensate for the huge amount of buoyant blubber -- and air in its lungs -- which could otherwise send it bobbing to the surface.
But just the right balance of bone density and blubber allowed the giant animal to stay in the middle of around 10 metres (33 feet) of water without moving a muscle, Amson explained.
Felix Marx, a marine mammal expert at the Museum of New Zealand Te Papa Tongarewa not involved in the study, told AFP that Perucetus colossus is very different from anything else we've ever found.
He cautioned that extinct sea cows had heavier bones than would be expected for their total body weight, potentially suggesting Perucetus colossus could be on the lower end of its estimated weight range.