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Paleontologists have long grappled with the question of dinosaur metabolisms—do they run hot like modern birds and mammals or do they resemble the slower metabolisms of modern reptiles. Surprisingly, the answer seems to be both.
“Although we assumed that most dinosaurs were warm-blooded, there was no way to measure basic metabolic capacities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. In the absence of extant dinosaurs, he said, paleontologists grappling with questions about prehistoric metabolisms — for example, whether a particular beast was warm-blooded or cold-blooded — had to rely on indirect evidence, such as isotopic evidence or growth rates from bone slices. .
Now, Dr. Wiemann and colleagues pioneered a new method for directly measuring the metabolic rate of extinct animals. their results, Published Wednesday in the journal Natureconfirmed that the ancestors of many dinosaurs and their winged relatives, pterosaurs, were warm-blooded. But the research also suggests that some herbivorous dinosaurs spent tens of millions of years developing a cold-blooded metabolism more similar to that of modern and ancient reptiles.
The team analyzed more than 50 extinct and modern vertebrates from the Yale Peabody Museum of Natural History collections, including mammals, lizards, birds, and 11 different non-avian dinosaurs. Using laser microspectroscopy, they identified a specific molecular marker of metabolic stress in both fossils and modern bones—it’s directly related to how much oxygen the animal inhaled. This, in turn, is a direct indicator of its metabolism.
The team found that both mammals and plesiosaurs — long-necked marine reptiles — evolved their high metabolisms independently. Pterosaurs and dinosaurs, which together form a group called Ornithodira, seem to have descended from warm-blooded ancestors—a condition that persists in long-necked sauropods, predatory theropods. Tyrannosaurus rexand their surviving feathered descendants like chickens.
Stephen Brusatte, a University of Edinburgh paleontologist who was not involved in the study, says sauropods with high metabolisms are unexpected. Researchers in the past had suggested that if any dinosaurs had a lower metabolism, it would have been giant, clumsy herbivores.
Dr. “Imagine the hundreds or thousands of pounds of herbs they would have to eat every day to fuel such a fast metabolism,” Brusatte said.
Even more surprising was the team’s findings around another dinosaur group — a diverse herbivorous superfamily called ornithischians. Dr. While ancestral ornithischians shared the warm-blooded metabolisms of other dinosaurs, like their larger descendants Stegosaurus and Triceratops, their metabolisms decreased over time, eventually reaching metabolic rates close to those of modern reptiles, Wiemann said. And like modern reptiles, they may have needed to maintain core temperatures through behavior, basking in the sun or migrating seasonally to warmer climates.
“The evolution of decreased metabolic rates in some ornithischians is surprising, especially given that the same wasn’t true for giant sauropods,” said Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago. study. “This work will drastically change how we interpret the lifestyles and behavior of these animals.”
More research – and many more fossil specimens – will be required to measure the temperature of all limbs in the ornithischian family tree. However, they would not be the first members of the broader dinosaur family, archosaurs, to potentially transition. Dr. Wiemann said the growth rates of some extinct crocodile groups suggest that they too could have been warm-blooded, while their modern relatives develop slower metabolisms.
Stating that they have now demonstrated the potential of this technique, Dr. Wiemann said more detailed studies could help explain why some dinosaur families abandoned high metabolisms.
Dr. “This seems counterintuitive, because we nurture our own warmth as this great evolutionary innovation,” Brusatte said. But he added that high metabolisms are expensive in terms of diet and energy, adding that what they need to maintain it may be “too much responsibility for some dinosaurs.”
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