熱血または変温? 化学的手がかりは古生物学で最も古い謎の1つを解決します

研究の一環として調査された動物のサブセットの概略図。 代謝率とその結果生じる熱生理学的戦略は色分けされており、オレンジ色は温血と一致する高い代謝率を特徴づけ、青色は冷血と一致する低代謝率を特徴づけます。 左から右へ:プレシオサウルス、ステゴサウルス、ディプロドクス、アロサウルス、カリプテ(現代のハチドリ)。 クレジット:©J。Wiemann

古生物学者は、恐竜が現代の哺乳類や鳥のように温血動物であるのか、現代の爬虫類のように変温動物であるのかについて、何十年にもわたって議論してきました。 恐竜が温血か変温動物かを知ることで、恐竜の活動性と日常生活の様子を知ることができますが、以前の方法では、恐竜の温血動物と変温動物の代謝によって酸素がエネルギーに変わる速度を判断できました。決定的ではありませんでした。 しかし、ジャーナルに掲載された新しい論文では 自然、科学者たちは、恐竜の代謝率を研究するための新しい方法を発表しています。これは、個々の動物が人生の最後の1時間にどれだけ呼吸したかを示す手がかりを骨に使用しています。

「これは古生物学者として私たちにとって本当にエキサイティングです。恐竜が温血動物であるか冷血動物であるかという問題は古生物学で最も古い問題の1つであり、今ではほとんどの恐竜が温血動物であるというコンセンサスが得られていると思います」と述べています。論文の筆頭著者であり、カリフォルニア工科大学(Caltech)の博士研究員であるJasminaWiemann氏。

「JasminaWiemannによって開発された新しいプロキシを使用すると、絶滅した生物の代謝を直接推測できます。これは、ほんの数年前に夢見ていたことです。 また、さまざまなグループを特徴付けるさまざまな代謝率も見つかりました。これは、以前は他の方法に基づいて提案されていましたが、直接テストされたことはありませんでした」と、シカゴのフィールド博物館のポスドク研究員であり、研究の著者の1人であるMatteoFabbriは述べています。

代謝については、誰かが体調を整えるのがいかに簡単かという観点からよく言われますが、その核となるのは、「代謝とは、呼吸する酸素を、体に燃料を供給する化学エネルギーにどれだけ効果的に変換するかです」と語っています。エール大学とロサンゼルス郡自然史博物館と提携しています。

アロサウルスの骨、血管、細胞、およびマトリックス

タンパク質性骨基質の化石化産物の代謝シグナル(代謝架橋)について調査された恐竜標本の1つ(アロサウルス)の骨から抽出された軟組織の顕微鏡写真。 化石化は、代謝架橋と組み合わせて、骨細胞(暗く分岐構造)と血管(中央の管状構造)を所定の位置に保持する化石細胞外マトリックスの特徴的な茶色を生成する追加の架橋を導入します。 クレジット:©J。Wiemann

代謝率の高い動物は吸熱性または温血動物です。 鳥や哺乳類のような温血動物は、体温を維持して活動を続けるために、大量の酸素を摂取し、大量のカロリーを消費する必要があります。 爬虫類のような変温動物、または発熱性の動物は、呼吸が少なく、食べる量も少なくなります。 彼らのライフスタイルは、温血動物よりもエネルギー的に高価ではありませんが、代償が伴います。変温動物は、体が機能するのに適切な温度に保つために外界に依存しています(トカゲが日光浴をしているように)、そして、彼らは温血動物よりも活動的でない傾向があります。

鳥は温血動物であり、爬虫類は変温動物であり、恐竜は議論の真っ只中に捕らえられました。 鳥は、終わりに大量絶滅を生き延びた唯一の恐竜です[{” attribute=””>Cretaceous, but dinosaurs (and by extension, birds) are technically reptiles — outside of birds, their closest living relatives are crocodiles and alligators. So would that make dinosaurs warm-blooded, or cold-blooded?

“This is really exciting for us as paleontologists — the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus, that most dinosaurs were warm-blooded.” — Jasmina Wiemann

Scientists have tried to glean dinosaurs’ metabolic rates from chemical and osteohistological analyses of their bones. “In the past, people have looked at dinosaur bones with isotope geochemistry that basically works like a paleo-thermometer,” says Wiemann — researchers examine the minerals in a fossil and determine what temperatures those minerals would form in. “It’s a really cool approach and it was really revolutionary when it came out, and it continues to provide very exciting insights into the physiology of extinct animals. But we’ve realized that we don’t really understand yet how fossilization processes change the isotope signals that we pick up, so it is hard to unambiguously compare the data from fossils to modern animals.”

Another method for studying metabolism is the growth rate. “If you look at a cross-section of dinosaur bone tissue, you can see a series of lines, like tree rings, that correspond to years of growth,” says Fabbri. “You can count the lines of growth and the space between them to see how fast the dinosaur grew. The limit relies on how you transform growth rate estimates into metabolism: growing faster or slower can have more to do with the animal’s stage in life than with its metabolism, like how we grow faster when we’re young and slower when we’re older.”

The new method proposed by Wiemann, Fabbri, and their colleagues doesn’t look at the minerals present in bone or how quickly the dinosaur grew. Instead, they look at one of the most basic hallmarks of metabolism: oxygen use. When animals breathe, side products form that react with proteins, sugars, and lipids, leaving behind molecular “waste.” This waste is extremely stable and water-insoluble, so it’s preserved during the fossilization process. It leaves behind a record of how much oxygen a dinosaur was breathing in, and thus, its metabolic rate.

“We are living in the sixth mass extinction, so it is important for us to understand how modern and extinct animals physiologically responded to previous climate change and environmental perturbations, so that the past can inform biodiversity conservation in the present and inform our future actions.” — Jasmina Wiemann

The researchers looked for these bits of molecular waste in dark-colored fossil femurs, because those dark colors indicate that lots of organic matter are preserved. They examined the fossils using Raman and Fourier-transform infrared spectroscopy — “these methods work like laser microscopes, we can basically quantify the abundance of these molecular markers that tell us about the metabolic rate,” says Wiemann. “It is a particularly attractive method to paleontologists, because it is non-destructive.”

The team analyzed the femurs of 55 different groups of animals, including dinosaurs, their flying cousins the pterosaurs, their more distant marine relatives the plesiosaurs, and modern birds, mammals, and lizards. They compared the amount of breathing-related molecular byproducts with the known metabolic rates of the living animals and used those data to infer the metabolic rates of the extinct ones.

The team found that dinosaurs’ metabolic rates were generally high. There are two big groups of dinosaurs, the saurischians and the ornithischians — lizard hips and bird hips. The bird-hipped dinosaurs, like Triceratops and Stegosaurus, had low metabolic rates comparable to those of cold-blooded modern animals. The lizard-hipped dinosaurs, including theropods and the sauropods — the two-legged, more bird-like predatory dinosaurs like Velociraptor and T. rex and the giant, long-necked herbivores like Brachiosaurus — were warm- or even hot-blooded. The researchers were surprised to find that some of these dinosaurs weren’t just warm-blooded — they had metabolic rates comparable to modern birds, much higher than mammals. These results complement previous independent observations that hinted at such trends but could not provide direct evidence, because of the lack of a direct proxy to infer metabolism.

These findings, the researchers say, can give us fundamentally new insights into what dinosaurs’ lives were like.

“Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures,” says Wiemann. “Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioral’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.”

On the other hand, she says, the hot-blooded dinosaurs would have been more active and would have needed to eat a lot. “The hot-blooded giant sauropods were herbivores, and it would take a lot of plant matter to feed this metabolic system. They had very efficient digestive systems, and since they were so big, it probably was more of a problem for them to cool down than to heat up.” Meanwhile, the theropod dinosaurs — the group that contains birds — developed high metabolisms even before some of their members evolved flight.

“Reconstructing the biology and physiology of extinct animals is one of the hardest things to do in paleontology. This new study adds a fundamental piece of the puzzle in understanding the evolution of physiology in deep time and complements previous proxies used to investigate these questions. We can now infer body temperature through isotopes, growth strategies through osteohistology, and metabolic rates through chemical proxies,” says Fabbri.

In addition to giving us insights into what dinosaurs were like, this study also helps us better understand the world around us today. Dinosaurs, with the exception of birds, died out in a mass extinction 65 million years ago when an asteroid struck the Earth. “Having a high metabolic rate has generally been suggested as one of the key advantages when it comes to surviving mass extinctions and successfully radiating afterward,” says Wiemann — some scientists have proposed that birds survived while the non-avian dinosaurs died because of the birds’ increased metabolic capacity. But this study, Wiemann says, helps to show that this isn’t true: many dinosaurs with bird-like, exceptional metabolic capacities went extinct.

“We are living in the sixth mass extinction,” says Wiemann, “so it is important for us to understand how modern and extinct animals physiologically responded to previous climate change and environmental perturbations, so that the past can inform biodiversity conservation in the present and inform our future actions.”

Reference: “Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur” by Jasmina Wiemann, Iris Menéndez, Jason M. Crawford, Matteo Fabbri, Jacques A. Gauthier, Pincelli M. Hull, Mark A. Norell and Derek E. G. Briggs, 25 May 2022, Nature.
DOI: 10.1038/s41586-022-04770-6

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