Warmblütig oder kaltblütig? Chemische Hinweise lösen eines der ältesten Rätsel der Paläontologie

Schematische Darstellung einer Untergruppe von Tieren, die im Rahmen der Forschung untersucht wurden. Stoffwechselraten und die daraus resultierenden thermophysiologischen Strategien sind farbcodiert, orangefarbene Farbtöne charakterisieren hohe Stoffwechselraten, die mit warmblütigen Blutungen zusammenfallen, und blaue Farbtöne charakterisieren niedrige Stoffwechselraten, die mit kaltblütigen Blutungen zusammenfallen. Von links nach rechts: Plesiosaurier, Stegosaurus, Diplodocus, Allosaurus, Calypt (moderner Kolibri). Bildnachweis: © J. Wiemann

Paläontologen streiten seit Jahrzehnten darüber, ob Dinosaurier warmblütig wie moderne Säugetiere und Vögel oder kaltblütig wie moderne Reptilien waren. Zu wissen, ob Dinosaurier warm- oder kaltblütig waren, kann uns Hinweise darauf geben, wie aktiv sie waren und wie ihr tägliches Leben war, aber frühere Methoden zur Bestimmung ihrer Warm- oder Kaltblüter – wie schnell ihr Stoffwechsel Sauerstoff in Energie umwandelt – waren es nicht schlüssig. Allerdings in einem neuen Artikel, der in der Zeitschrift veröffentlicht wurde Natur temperierenWissenschaftler zeigen eine neue Möglichkeit, die Stoffwechselraten von Dinosauriern zu untersuchen, indem sie Hinweise in ihren Knochen verwenden, die darauf hinweisen, wie viel einzelne Tiere in der letzten Stunde ihres Lebens geatmet haben.

„Das ist wirklich spannend für uns als Paläontologen – die Frage, ob Dinosaurier warmblütig oder kaltblütig waren, ist eine der ältesten Fragen in der Paläontologie, und jetzt glauben wir, dass wir einen Konsens darüber haben, dass die meisten Dinosaurier warmblütig waren“, Yasmina Wehmann, Autor des Artikels Principal and Postdoctoral Research Fellow am California Institute of Technology (Caltech).

„Der von Jasmina Wiemann entwickelte neue Wirkstoff erlaubt uns, direkt auf den Stoffwechsel ausgestorbener Organismen zu schließen, wovon wir seit einigen Jahren nur träumen. Wir haben auch unterschiedliche Stoffwechselraten gefunden, die verschiedene Gruppen charakterisieren, die zuvor mit anderen Methoden vorgeschlagen wurden , aber sie wurden nicht direkt getestet, sagt Matteo Fabri, Postdoktorand am Chicago Field Museum und einer der Autoren der Studie.

غالبًا ما يتحدث الناس عن التمثيل الغذائي من حيث مدى سهولة بقاء الشخص في حالة جيدة ، ولكن في جوهره ، “التمثيل الغذائي هو مدى فعالية تحويل الأكسجين الذي نتنفسه إلى طاقة كيميائية تغذي أجسامنا” ، كما يقول ويمان ، تابعة لجامعة ييل ومتحف التاريخ الطبيعي في مقاطعة Los Angeles.

Allosaurus-Knochen, Blutgefäße, Zellen und Matrix

Mikroskopische Ansicht von Weichgewebe, das aus den Knochen eines Allosaurus-Exemplars extrahiert wurde, das auf metabolische Signale (metabolische Querverbindungen) in den verknöcherten Auswüchsen der Protein-Knochenmatrix untersucht wurde. Die Ossifikation führt zusätzliche Vernetzungen ein, die in Kombination mit metabolischen Vernetzungen die charakteristische braune Farbe der fossilen extrazellulären Matrix erzeugen, die Osteoblasten (verzweigte dunkle Strukturen) und Blutgefäße (röhrenartige Struktur in der Mitte) an Ort und Stelle hält. Bildnachweis: © J. Wiemann

Tiere mit einer hohen Stoffwechselrate sind endotherm oder warmblütig. Warmblüter wie Vögel und Säugetiere verbrauchen viel Sauerstoff und müssen viele Kalorien verbrennen, um ihre Körpertemperatur zu halten und aktiv zu bleiben. Kaltblüter wie Reptilien atmen weniger und fressen weniger. Ihr Lebensstil ist weniger teuer als der von warmblütigen Tieren, aber es hat seinen Preis: Kaltblütige Tiere sind auf die Außenwelt angewiesen, um ihren Körper auf der richtigen Temperatur zu halten, um zu funktionieren (wie eine Eidechse, die sich in der Sonne sonnt), und Sie neigen dazu, weniger aktiv zu sein als warmblütige Kreaturen.

Da Vögel warmblütig und Reptilien kaltblütig sind, wurden Dinosaurier in die Debatte verwickelt. Vögel sind die einzigen Dinosaurier, die das Massensterben am Ende überlebt haben[{” 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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