恐竜にかみつく古代の哺乳類　極めて珍しい化石発見　研究

【パリＡＦＰ＝時事】中国とカナダの研究チームが１８日、科学誌「サイエンティフィック・リポーツ」に掲載された論文で、約１億２５００万年前の地層から見つかった、より体の大きな恐竜にかみつくアナグマのような哺乳類の化石についての研究結果を報告した。（写真はプシッタコサウルス〈恐竜〉にかみつくレペノマムス〈哺乳類〉の化石。カナダ自然史博物館提供）
　２匹の格闘シーンを捉えた化石は、中国北東部・遼寧省で２０１２年、白亜紀の火山灰層から発見された。
　化石には、オウムのようなくちばしを持つ体長１２０センチの植物食恐竜「プシッタコサウルス・ルジアトゥネンシス」を押さえつけ、胸部に歯を食い込ませる哺乳類「レペノマムス・ロブストゥス」の姿がはっきりと残されていた。プシッタコサウルスの体は、レペノマムスの３倍ほど大きかった。
　論文の共同執筆者を務めたカナダ自然史博物館の古生物学者ジョーダン・マロン氏は、これまで発見された化石の中で哺乳類と恐竜が争っているものは初めてだろうと語った。
　マロン氏は２匹の絡みつき方から、レペノマムスがプシッタコサウルスの死骸をあさっていたのではなく、攻撃していたことが分かると述べた。哺乳類が死骸をあさる場合によく見られるかみ痕が、プシッタコサウルスにはなかった。
　また哺乳類が自身よりはるかに大きな動物を捕食することは珍しいが、現代ではクズリ（クロアナグマ）が大きなカリブーを狩る様子が観察されたこともあると同氏は述べた。さらに、レペノマムスの狩りが単独によるものだったのか、群れによるものだったのかはこの化石だけでは不明だが、どちらの可能性もあると付け加えた。
　哺乳類が恐竜を捕食していたことを示唆する最初の化石は、２００５年に同じく中国の同地域で発見されている。この化石はレペノマムスの胃の中にプシッタコサウルスの子どもが入った状態で見つかった。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/07/20-17:23）

A badger-like mammal was sinking its teeth into the ribs of a dinosaur three times its size when they were buried in volcanic ash 125 million years ago, capturing the pair in a deadly embrace.
The fight scene, preserved in a fossil discovered in China, suggests that small mammals preyed on the dinosaurs that ruled Earth during the Cretaceous period more than previously thought, scientists said on Tuesday.
Jordan Mallon, a palaeontologist at the Canadian Museum of Nature, told AFP that when he first saw the fossil my eyes popped out of my head.
Mallon, a co-author of a new study led by Chinese researchers, said they believe the fossil is the first ever discovered that shows a mammal and dinosaur fighting each other.
Mammals were generally considered far too small to prey on the dinosaurs that dominated the world during the tens of millions of years they shared on Earth.
But the fossil shows a badger-sized Repenomamus robustus sitting on top of Psittacosaurus lujiatunensis, a plant-eating dinosaur that stood 120 centimetres (47 inches) tall and had a beak like a parrot.
The mammal -- one of the largest of its time but still a third the weight of the dinosaur -- is sinking his sharp teeth into the dinosaur's ribs and gripping onto its leg.
- 'Feisty mammals' -
The way the pair are intertwined shows that the mammal was not scavenging on a dead dinosaur, Mallon said.
The dinosaur has collapsed down and trapped the hind limb of the mammal in the fold of its knee, indicating it was an attack, he said.
The dinosaur also bears no bite marks, which mammals often leave on scavenged bodies.
While it is rare for mammals to prey on animals so much larger than them, Mallon said one example was how wolverines had been observed hunting far-larger caribou.
It was not possible to tell from the fossil if Repenomamus hunted solo or in a pack, he said, adding that either was possible.
The almost entirely complete skeletons were found in China's northeastern Liaoning province in 2012.
They were discovered at a site nicknamed Chinese Pompeii because of how many dinosaurs and other animals have been found preserved by volcanic debris there, similar to the ancient Roman city.
The first fossil suggesting that mammals ate dinosaurs was found at the Chinese site in 2005. It showed a baby Psittacosaurus in the stomach of a Repenomamus.
But the new fossil is the first piece of evidence that there were at least some feisty mammals around during the Cretaceous... capable of taking down an adult dinosaur, Mallon said.
The once-in-a-lifetime fossil is being exhibited at a museum attached to a primary school in the Chinese city of Weihai, he added.
The study was published in the journal Scientific Reports.