Dinosaurs started out hot, then some of them turned cold

Paleontologists have long wrestled with the question of dinosaur metabolisms – whether they were hot, as modern birds and mammals do, or resembled the slower metabolisms of modern reptiles. In a surprise, the answer seems to be both.

“While we had assumed that most dinosaurs were warm-blooded, there was simply no way to measure the underlying metabolic abilities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. With no dinosaurs available, she said, paleontologists grappling with questions about prehistoric metabolisms — whether a given beast was warm-blooded or cold-blooded, for example — had to rely on indirect evidence, such as isotopic evidence or growth rates from bone slices. .

Now, Dr. Wiemann and his colleagues have developed a new method to directly measure the metabolic rate of extinct animals. Their findings, published Wednesday in the journal Nature, confirmed that many dinosaurs as well as their winged relatives, the pterosaurs, were ancestrally warm-blooded. But in a twist, the research also suggests that some herbivorous dinosaurs spent tens of millions of years developing a cold-blooded metabolism more like that of contemporary and ancient reptiles.

The team analyzed more than 50 extinct and modern vertebrates from the collections of the Yale Peabody Museum of Natural History, 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 – a marker directly correlated to the amount of oxygen the animal breathes. This, in turn, is a direct indicator of its metabolism.

The team found that mammals and plesiosaurs – long-necked marine reptiles – independently developed their high metabolism. Pterosaurs and dinosaurs, which together form a group called Ornithodira, appear to descend from warm-blooded ancestors – a condition that persisted in long-necked sauropods, predatory theropods like Tyrannosaurus rex and their surviving feathered descendants, like the chickens.

Sauropods with high metabolisms are unexpected, says Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study. Researchers in the past had suggested that if any dinosaurs had lower metabolisms, it would be the giant, heavy herbivores.

“Just imagine the hundreds or thousands of pounds of plants they would have to eat every day to fuel such a fast metabolism,” Dr. Brusatte said.

The team’s findings around another group of dinosaurs – the diverse superfamily of herbivores called ornithischians – were even more startling. While ancestral ornithischians shared the warm-blooded metabolisms of other dinosaurs, Dr. Wiemann said, their larger descendants like Stegosaurus and Triceratops actually reduced their metabolism over time, ending up at higher metabolic rates. close to those of modern reptiles. And like modern reptiles, they might have needed to maintain their core temperature through behavior — basking in the sun or seasonally migrating to warmer climates.

“The pattern of decreased metabolic rates in some ornithischians is surprising, especially since the same is not true for giant sauropods,” said Jingmai O’Connor, associate curator of fossil reptiles at the Field. Chicago Museum, which also did not participate in the study. “This work will radically change the way we interpret the lifestyles and behaviors of these animals.”

Further research – and many more fossil samples – will be needed to take the temperature of all members of the ornithischian family tree. But they wouldn’t be the first members of the extended dinosaur family, the archosaurs, to potentially make the switch. Dr Wiemann said the growth rates of some groups of extinct crocodiles suggested they may also be warm-blooded, while their modern relatives developed slower metabolisms.

Now that they’ve demonstrated the technique’s potential, Dr. Wiemann said more detailed studies could help clarify why some dinosaur families gave up on high metabolisms.

“It seems counterintuitive because we cherish the hot blood within us like this great evolutionary innovation, which it was,” Dr. Brusatte said. But high metabolisms are expensive in terms of diet and energy, he notes, adding that what they needed to maintain it may have been “too troublesome for some dinosaurs”.

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